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postgresql/doc/TODO.detail/mmap
Bruce Momjian 6fdd71c133 Add to mmap discussion.
2003-03-18 01:36:01 +00:00

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From pgsql-hackers-owner+M5149@postgresql.org Mon Feb 26 03:32:49 2001
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From: ncm@zembu.com (Nathan Myers)
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On Sun, Feb 25, 2001 at 11:28:46PM -0500, Tom Lane wrote:
> Bruce Momjian <pgman@candle.pha.pa.us> writes:
> > It allows no backing store on disk.
I.e. it allows you to map memory without an associated inode; the memory
may still be swapped. Of course, there is no problem with mapping an
inode too, so that unrelated processes can join in. Solarix has a flag
to pin the shared pages in RAM so they can't be swapped out.
> > It is the BSD solution to SysV
> > share memory. Here are all the BSDi flags:
>
> > MAP_ANON Map anonymous memory not associated with any specific
> > file. The file descriptor used for creating MAP_ANON
> > must be -1. The offset parameter is ignored.
>
> Hmm. Now that I read down to the "nonstandard extensions" part of the
> HPUX man page for mmap(), I find
>
> If MAP_ANONYMOUS is set in flags:
>
> o A new memory region is created and initialized to all zeros.
> This memory region can be shared only with descendants of
> the current process.
This is supported on Linux and BSD, but not on Solarix 7. It's not
necessary; you can just map /dev/zero on SysV systems that don't
have MAP_ANON.
> While I've said before that I don't think it's really necessary for
> processes that aren't children of the postmaster to access the shared
> memory, I'm not sure that I want to go over to a mechanism that makes it
> *impossible* for that to be done. Especially not if the only motivation
> is to avoid having to configure the kernel's shared memory settings.
There are enormous advantages to avoiding the need to configure kernel
settings. It makes PG a better citizen. PG is much easier to drop in
and use if you don't need attention from the IT department.
But I don't know of any reason to avoid mapping an actual inode,
so using mmap doesn't necessarily mean giving up sharing among
unrelated processes.
> Besides, what makes you think there's not a limit on the size of shmem
> allocatable via mmap()?
I've never seen any mmap limit documented. Since mmap() is how
everybody implements shared libraries, such a limit would be equivalent
to a limit on how much/many shared libraries are used. mmap() with
MAP_ANONYMOUS (or its SysV /dev/zero equivalent) is a common, modern
way to get raw storage for malloc(), so such a limit would be a limit
on malloc() too.
The mmap architecture comes to us from the Mach microkernel memory
manager, backported into BSD and then copied widely. Since it was
the fundamental mechanism for all memory operations in Mach, arbitrary
limits would make no sense. That it worked so well is the reason it
was copied everywhere else, so adding arbitrary limits while copying
it would be silly. I don't think we'll see any systems like that.
Nathan Myers
ncm@zembu.com
From pgsql-hackers-owner+M6138@postgresql.org Mon Mar 19 07:57:59 2001
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From: Alfred Perlstein <bright@wintelcom.net>
To: Rod Taylor <rod.taylor@inquent.com>
Cc: Hackers List <pgsql-hackers@postgresql.org>
Subject: Re: [HACKERS] Fw: [vorbis-dev] ogg123: shared memory by mmap()
Message-ID: <20010319045500.T29888@fw.wintelcom.net>
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WOOT WOOT! DANGER WILL ROBINSON!
> ----- Original Message -----
> From: "Christian Weisgerber" <naddy@mips.inka.de>
> Newsgroups: list.vorbis.dev
> To: <vorbis-dev@xiph.org>
> Sent: Saturday, March 17, 2001 12:01 PM
> Subject: [vorbis-dev] ogg123: shared memory by mmap()
>
>
> > The patch below adds:
> >
> > - acinclude.m4: A new macro A_FUNC_SMMAP to check that sharing
> pages
> > through mmap() works. This is taken from Joerg Schilling's star.
> > - configure.in: A_FUNC_SMMAP
> > - ogg123/buffer.c: If we have a working mmap(), use it to create
> > a region of shared memory instead of using System V IPC.
> >
> > Works on BSD. Should also work on SVR4 and offspring (Solaris),
> > and Linux.
This is a really bad idea performance wise. Solaris has a special
code path for SYSV shared memory that doesn't require tons of swap
tracking structures per-page/per-process. FreeBSD also has this
optimization (it's off by default, but should work since FreeBSD
4.2 via the sysctl kern.ipc.shm_use_phys=1)
Both OS's use a trick of making the pages non-pageable, this allows
signifigant savings in kernel space required for each attached
process, as well as the use of large pages which reduce the amount
of TLB faults your processes will incurr.
Anyhow, if you could make this a runtime option it wouldn't be so
evil, but as a compile time option, it's a really bad idea for
Solaris and FreeBSD.
--
-Alfred Perlstein - [bright@wintelcom.net|alfred@freebsd.org]
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From pgsql-hackers-owner+M6255@postgresql.org Tue Mar 20 18:46:33 2001
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Date: Tue, 20 Mar 2001 15:44:10 -0800
From: Alfred Perlstein <bright@wintelcom.net>
To: Bruce Momjian <pgman@candle.pha.pa.us>
Cc: Rod Taylor <rod.taylor@inquent.com>,
Hackers List <pgsql-hackers@postgresql.org>
Subject: Re: [HACKERS] Fw: [vorbis-dev] ogg123: shared memory by mmap()
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* Bruce Momjian <pgman@candle.pha.pa.us> [010320 14:10] wrote:
> > > > The patch below adds:
> > > >
> > > > - acinclude.m4: A new macro A_FUNC_SMMAP to check that sharing
> > > pages
> > > > through mmap() works. This is taken from Joerg Schilling's star.
> > > > - configure.in: A_FUNC_SMMAP
> > > > - ogg123/buffer.c: If we have a working mmap(), use it to create
> > > > a region of shared memory instead of using System V IPC.
> > > >
> > > > Works on BSD. Should also work on SVR4 and offspring (Solaris),
> > > > and Linux.
> >
> > This is a really bad idea performance wise. Solaris has a special
> > code path for SYSV shared memory that doesn't require tons of swap
> > tracking structures per-page/per-process. FreeBSD also has this
> > optimization (it's off by default, but should work since FreeBSD
> > 4.2 via the sysctl kern.ipc.shm_use_phys=1)
>
> >
> > Both OS's use a trick of making the pages non-pageable, this allows
> > signifigant savings in kernel space required for each attached
> > process, as well as the use of large pages which reduce the amount
> > of TLB faults your processes will incurr.
>
> That is interesting. BSDi has SysV shared memory as non-pagable, and I
> always thought of that as a bug. Seems you are saying that having it
> pagable has a significant performance penalty. Interesting.
Yes, having it pageable is actually sort of bad.
It doesn't allow you to do several important optimizations.
--
-Alfred Perlstein - [bright@wintelcom.net|alfred@freebsd.org]
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From pgsql-general-owner+M14300@postgresql.org Mon Aug 27 13:07:32 2001
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Date: Tue, 28 Aug 2001 01:14:33 +1000
From: Martijn van Oosterhout <kleptog@svana.org>
To: Andrew Snow <andrew@modulus.org>
cc: pgsql-general@postgresql.org
Subject: Re: [GENERAL] raw partition
Message-ID: <20010828011433.E32309@svana.org>
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On Tue, Aug 28, 2001 at 12:02:08AM +1000, Andrew Snow wrote:
>
> What I think would be better would be moving postgresql to a system of
> using memory-mapped I/O. instead of the shared buffer cache, files
> would be directly memory-mapped and the OS would do the caching. I
> can't see this happening though because of platform dependancy, but I
> think its worth another look soon because many unix platforms support
> mmap(). I think it would improve the performance of disk-intensive
> tasks noticeably.
Well, this has other problems. Consider tables that are larger than your
system memory. You'd have to continuously map and unmap different sections.
That can have odd side effects (witness mozilla on linux having 15,000
mapped areas or so...)
You would still however get the advantage that you wouldn't have to copy the
data from the disk buffers to user space, you simply get the disk buffer
mapped into your address space.
I think that for commonly used tables that are under 100K in size (most of
the system tables), this is quite a workable idea. If you don't mind keeping
them mapped the whole time.
--
Martijn van Oosterhout <kleptog@svana.org>
http://svana.org/kleptog/
> It would be nice if someone came up with a certification system that
> actually separated those who can barely regurgitate what they crammed over
> the last few weeks from those who command secret ninja networking powers.
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To: Martijn van Oosterhout <kleptog@svana.org>
cc: Andrew Snow <andrew@modulus.org>, pgsql-general@postgresql.org
Subject: Re: [GENERAL] raw partition
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Comments: In-reply-to Martijn van Oosterhout <kleptog@svana.org>
message dated "Tue, 28 Aug 2001 01:14:33 +1000"
Date: Mon, 27 Aug 2001 15:53:15 -0400
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From: Tom Lane <tgl@sss.pgh.pa.us>
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Martijn van Oosterhout <kleptog@svana.org> writes:
> You would still however get the advantage that you wouldn't have to copy the
> data from the disk buffers to user space, you simply get the disk buffer
> mapped into your address space.
AFAICS this would be the *only* advantage. While it's not negligible,
it's quite unclear that it's worth the bookkeeping and portability
headaches of managing lots of mmap'd areas, either.
Before I take this idea seriously at all, I'd want to see a design that
addresses a couple of critical issues:
1. Postgres' shared buffers are *shared*, potentially across many
processes. How will you deal with buffers for files that have been
mmap'd by only some of the processes? (Maybe this means that the
whole concept of shared buffers goes away, and each process does its
own buffer management based on its own mmaps. Not sure. That would be
a pretty radical restructuring though, and would completely invalidate
our present approach to page-level locking.)
2. How do you deal with extending a file? My system's mmap man page
says
If the size of the mapped file changes after the call to mmap(), the
effect of references to portions of the mapped region that correspond
to added or removed portions of the file is unspecified.
This suggests that the only portable way to cope is to issue a separate
mmap for every disk page. Will typical Unix systems perform well with
umpteen thousand small mmap requests?
3. How do you persuade the other backends to drop their mmaps of a table
you are deleting?
There are probably other gotchas, but without an understanding of how
to address these, I doubt it's worth looking further ...
regards, tom lane
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From: Janardhana Reddy <jana-reddy@mediaring.com.sg>
To: Bruce Momjian <pgman@candle.pha.pa.us>, Tom Lane <tgl@sss.pgh.pa.us>
cc: PostgreSQL-development <pgsql-hackers@postgresql.org>,
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Subject: Re: [HACKERS] PERFORMANCE IMPROVEMENT by mapping WAL FILES
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I have just completed the functional testing the WAL using mmap , it is
working fine, I have tested by commenting out the "CreateCheckPoint "
functionality so that
when i kill the postgres and restart it will redo all the records from the
WAL log file which
is updated using mmap.
Just i need to clean code and to do some stress testing.
By the end of this week i should able to complete the stress test and
generate the patch file .
As Tom Lane mentioned i see the problem in portability to all platforms,
what i propose is to use mmap for only WAL for some platforms like
linux,freebsd etc . For other platforms we can use the existing method by
slightly modifying the
write() routine to write only the modified part of the page.
Regards
jana
>
>
> OK, I have talked to Tom Lane about this on the phone and we have a few
> ideas.
>
> Historically, we have avoided mmap() because of portability problems,
> and because using mmap() to write to large tables could consume lots of
> address space with little benefit. However, I perhaps can see WAL as
> being a good use of mmap.
>
> First, there is the issue of using mmap(). For OS's that have the
> mmap() MAP_SHARED flag, different backends could mmap the same file and
> each see the changes. However, keep in mind we still have to fsync()
> WAL, so we need to use msync().
>
> So, looking at the benefits of using mmap(), we have overhead of
> different backends having to mmap something that now sits quite easily
> in shared memory. Now, I can see mmap reducing the copy from user to
> kernel, but there are other ways to fix that. We could modify the
> write() routines to write() 8k on first WAL page write and later write
> only the modified part of the page to the kernel buffers. The old
> kernel buffer is probably still around so it is unlikely to require a
> read from the file system to read in the rest of the page. This reduces
> the write from 8k to something probably less than 4k which is better
> than we can do with mmap.
>
> I will add a TODO item to this effect.
>
> As far as reducing the write to disk from 8k to 4k, if we have to
> fsync/msync, we have to wait for the disk to spin to the proper location
> and at that point writing 4k or 8k doesn't seem like much of a win.
>
> In summary, I think it would be nice to reduce the 8k transfer from user
> to kernel on secondary page writes to only the modified part of the
> page. I am uncertain if mmap() or anything else will help the physical
> write to the disk.
>
> --
> Bruce Momjian | http://candle.pha.pa.us
> pgman@candle.pha.pa.us | (610) 853-3000
> + If your life is a hard drive, | 830 Blythe Avenue
> + Christ can be your backup. | Drexel Hill, Pennsylvania 19026
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From pgsql-hackers-owner+M23388@postgresql.org Mon Jun 3 17:54:43 2002
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From: "Igor Kovalenko" <Igor.Kovalenko@motorola.com>
To: "Bruce Momjian" <pgman@candle.pha.pa.us>
cc: "Tom Lane" <tgl@sss.pgh.pa.us>, "mlw" <markw@mohawksoft.com>,
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Subject: Re: [HACKERS] HEADS UP: Win32/OS2/BeOS native ports
Date: Mon, 3 Jun 2002 16:53:51 -0500
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That's what Apache does. Note, on most platforms MAP_ANON is equivalent to
mmmap-ing /dev/zero. Solaris for example does not provide MAP_ANON but using
fd=open(/dev/zero)
mmap(fd, ...)
close(fd)
works just fine.
----- Original Message -----
From: "Bruce Momjian" <pgman@candle.pha.pa.us>
To: "Igor Kovalenko" <Igor.Kovalenko@motorola.com>
Cc: "Tom Lane" <tgl@sss.pgh.pa.us>; "mlw" <markw@mohawksoft.com>; "Marc G.
Fournier" <scrappy@hub.org>; <pgsql-hackers@postgresql.org>
Sent: Sunday, June 02, 2002 7:47 PM
Subject: Re: [HACKERS] HEADS UP: Win32/OS2/BeOS native ports
> Igor Kovalenko wrote:
> > It does not have to be anonymous. POSIX also defines shm_open(same
arguments
> > as open) API which will create named object in whatever location
corresponds
> > to shared memory storage on that platform (object is then grown to
needed
> > size by ftruncate() and the fd is then passed to mmap). The object will
> > exist in name space and can be detected by subsequent calls to
shm_open()
> > with same name. It is not really different from doing open(), but more
> > portable (mmap() on regular files may not be supported).
>
> Actually, I think the best shared memory implemention would be
> MAP_ANON | MAP_SHARED mmap(), which could be called from the postmaster
> and passed to child processes.
>
> While all our platforms have mmap(), many don't have MAP_ANON, but those
> that do could use it. You need MAP_ANON to prevent the shared memory
> from being written to a disk file.
>
> --
> Bruce Momjian | http://candle.pha.pa.us
> pgman@candle.pha.pa.us | (610) 853-3000
> + If your life is a hard drive, | 830 Blythe Avenue
> + Christ can be your backup. | Drexel Hill, Pennsylvania 19026
>
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From pgsql-hackers-owner+M24146@postgresql.org Tue Jun 25 02:27:29 2002
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Date: Tue, 25 Jun 2002 14:05:45 +0900 (JST)
From: Curt Sampson <cjs@cynic.net>
To: "J. R. Nield" <jrnield@usol.com>
cc: Bruce Momjian <pgman@candle.pha.pa.us>, Tom Lane <tgl@sss.pgh.pa.us>,
PostgreSQL Hacker <pgsql-hackers@postgresql.org>
Subject: [HACKERS] Buffer Management
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I'm splitting off this buffer mangement stuff into a separate thread.
On 24 Jun 2002, J. R. Nield wrote:
> I'll back off on that. I don't know if we want to use the OS buffer
> manager, but shouldn't we try to have our buffer manager group writes
> together by files, and pro-actively get them out to disk?
The only way the postgres buffer manager can "get [data] out to disk"
is to do an fsync(). For data files (as opposed to log files), this can
only slow down overall system throughput, as this would only disrupt the
OS's write management.
> Right now, it
> looks like all our write requests are delayed as long as possible and
> the order in which they are written is pretty-much random, as is the
> backend that writes the block, so there is no locality of reference even
> when the blocks are adjacent on disk, and the write calls are spread-out
> over all the backends.
It doesn't matter. The OS will introduce locality of reference with its
write algorithms. Take a look at
http://www.cs.wisc.edu/~solomon/cs537/disksched.html
for an example. Most OSes use the elevator or one-way elevator
algorithm. So it doesn't matter whether it's one back-end or many
writing, and it doesn't matter in what order they do the write.
> Would it not be the case that things like read-ahead, grouping writes,
> and caching written data are probably best done by PostgreSQL, because
> only our buffer manager can understand when they will be useful or when
> they will thrash the cache?
Operating systems these days are not too bad at guessing guessing what
you're doing. Pretty much every OS I've seen will do read-ahead when
it detects you're doing sequential reads, at least in the forward
direction. And Solaris is even smart enough to mark the pages you've
read as "not needed" so that they quickly get flushed from the cache,
rather than blowing out your entire cache if you go through a large
file.
> Would O_DSYNC|O_RSYNC turn off the cache?
No. I suppose there's nothing to stop it doing so, in some
implementations, but the interface is not designed for direct I/O.
> Since you know a lot about NetBSD internals, I'd be interested in
> hearing about what postgresql looks like to the NetBSD buffer manager.
Well, looks like pretty much any program, or group of programs,
doing a lot of I/O. :-)
> Am I right that strings of successive writes get randomized?
No; as I pointed out, they in fact get de-randomized as much as
possible. The more proceses you have throwing out requests, the better
the throughput will be in fact.
> What do our cache-hit percentages look like? I'm going to do some
> experimenting with this.
Well, that depends on how much memory you have and what your working
set is. :-)
cjs
--
Curt Sampson <cjs@cynic.net> +81 90 7737 2974 http://www.netbsd.org
Don't you know, in this new Dark Age, we're all light. --XTC
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From cjs@cynic.net Tue Jun 25 09:52:23 2002
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Date: Tue, 25 Jun 2002 22:52:14 +0900 (JST)
From: Curt Sampson <cjs@cynic.net>
To: "J. R. Nield" <jrnield@usol.com>
cc: Bruce Momjian <pgman@candle.pha.pa.us>, Tom Lane <tgl@sss.pgh.pa.us>,
PostgreSQL Hacker <pgsql-hackers@postgresql.org>
Subject: Re: [HACKERS] Buffer Management
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Status: OR
So, while we're at it, what's the current state of people's thinking
on using mmap rather than shared memory for data file buffers? I
see some pretty powerful advantages to this approach, and I'm not
(yet :-)) convinced that the disadvantages are as bad as people think.
I think I can address most of the concerns in doc/TODO.detail/mmap.
Is this worth pursuing a bit? (I.e., should I spend an hour or two
writing up the advantages and thoughts on how to get around the
problems?) Anybody got objections that aren't in doc/TODO.detail/mmap?
cjs
--
Curt Sampson <cjs@cynic.net> +81 90 7737 2974 http://www.netbsd.org
Don't you know, in this new Dark Age, we're all light. --XTC
From tgl@sss.pgh.pa.us Tue Jun 25 10:09:07 2002
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To: Curt Sampson <cjs@cynic.net>
cc: "J. R. Nield" <jrnield@usol.com>, Bruce Momjian <pgman@candle.pha.pa.us>,
PostgreSQL Hacker <pgsql-hackers@postgresql.org>
Subject: Re: [HACKERS] Buffer Management
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Comments: In-reply-to Curt Sampson <cjs@cynic.net>
message dated "Tue, 25 Jun 2002 22:52:14 +0900"
Date: Tue, 25 Jun 2002 10:09:02 -0400
Message-ID: <7298.1025014142@sss.pgh.pa.us>
From: Tom Lane <tgl@sss.pgh.pa.us>
Status: ORr
Curt Sampson <cjs@cynic.net> writes:
> So, while we're at it, what's the current state of people's thinking
> on using mmap rather than shared memory for data file buffers?
There seem to be a couple of different threads in doc/TODO.detail/mmap.
One envisions mmap as a one-for-one replacement for our current use of
SysV shared memory, the main selling point being to get out from under
kernels that don't have SysV support or have it configured too small.
This might be worth doing, and I think it'd be relatively easy to do
now that the shared memory support is isolated in one file and there's
provisions for selecting a shmem implementation at configure time.
The only thing you'd really have to think about is how to replace the
current behavior that uses shmem attach counts to discover whether any
old backends are left over from a previous crashed postmaster. I dunno
if mmap offers any comparable facility.
The other discussion seemed to be considering how to mmap individual
data files right into backends' address space. I do not believe this
can possibly work, because of loss of control over visibility of data
changes to other backends, timing of write-backs, etc.
But as long as you stay away from interpretation #2 and go with
mmap-as-a-shmget-substitute, it might be worthwhile.
(Hey Marc, can one do mmap in a BSD jail?)
regards, tom lane
From pgsql-hackers-owner+M24158@postgresql.org Tue Jun 25 10:20:42 2002
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Date: Tue, 25 Jun 2002 23:20:15 +0900 (JST)
From: Curt Sampson <cjs@cynic.net>
To: Tom Lane <tgl@sss.pgh.pa.us>
cc: "J. R. Nield" <jrnield@usol.com>, Bruce Momjian <pgman@candle.pha.pa.us>,
PostgreSQL Hacker <pgsql-hackers@postgresql.org>
Subject: Re: [HACKERS] Buffer Management
In-Reply-To: <7298.1025014142@sss.pgh.pa.us>
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On Tue, 25 Jun 2002, Tom Lane wrote:
> The only thing you'd really have to think about is how to replace the
> current behavior that uses shmem attach counts to discover whether any
> old backends are left over from a previous crashed postmaster. I dunno
> if mmap offers any comparable facility.
Sure. Just mmap a file, and it will be persistent.
> The other discussion seemed to be considering how to mmap individual
> data files right into backends' address space. I do not believe this
> can possibly work, because of loss of control over visibility of data
> changes to other backends, timing of write-backs, etc.
I don't understand why there would be any loss of visibility of changes.
If two backends mmap the same block of a file, and it's shared, that's
the same block of physical memory that they're accessing. Changes don't
even need to "propagate," because the memory is truly shared. You'd keep
your locks in the page itself as well, of course.
Can you describe the problem in more detail?
> But as long as you stay away from interpretation #2 and go with
> mmap-as-a-shmget-substitute, it might be worthwhile.
It's #2 that I was really looking at. :-)
cjs
--
Curt Sampson <cjs@cynic.net> +81 90 7737 2974 http://www.netbsd.org
Don't you know, in this new Dark Age, we're all light. --XTC
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From pgsql-hackers-owner+M24159@postgresql.org Tue Jun 25 10:25:21 2002
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From: Bruce Momjian <pgman@candle.pha.pa.us>
Message-ID: <200206251420.g5PEKT310222@candle.pha.pa.us>
Subject: Re: [HACKERS] Buffer Management
In-Reply-To: <7298.1025014142@sss.pgh.pa.us>
To: Tom Lane <tgl@sss.pgh.pa.us>
Date: Tue, 25 Jun 2002 10:20:29 -0400 (EDT)
cc: Curt Sampson <cjs@cynic.net>, "J. R. Nield" <jrnield@usol.com>,
PostgreSQL Hacker <pgsql-hackers@postgresql.org>
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Tom Lane wrote:
> Curt Sampson <cjs@cynic.net> writes:
> > So, while we're at it, what's the current state of people's thinking
> > on using mmap rather than shared memory for data file buffers?
>
> There seem to be a couple of different threads in doc/TODO.detail/mmap.
>
> One envisions mmap as a one-for-one replacement for our current use of
> SysV shared memory, the main selling point being to get out from under
> kernels that don't have SysV support or have it configured too small.
> This might be worth doing, and I think it'd be relatively easy to do
> now that the shared memory support is isolated in one file and there's
> provisions for selecting a shmem implementation at configure time.
> The only thing you'd really have to think about is how to replace the
> current behavior that uses shmem attach counts to discover whether any
> old backends are left over from a previous crashed postmaster. I dunno
> if mmap offers any comparable facility.
>
> The other discussion seemed to be considering how to mmap individual
> data files right into backends' address space. I do not believe this
> can possibly work, because of loss of control over visibility of data
> changes to other backends, timing of write-backs, etc.
Agreed. Also, there was in intresting thread that mmap'ing /dev/zero is
the same as anonmap for OS's that don't have anonmap. That should cover
most of them. The only downside I can see is that SysV shared memory is
locked into RAM on some/most OS's while mmap anon probably isn't.
Locking in RAM is good in most cases, bad in others.
This will also work well when we have non-SysV semaphore support, like
Posix semaphores, so we would be able to run with no SysV stuff.
--
Bruce Momjian | http://candle.pha.pa.us
pgman@candle.pha.pa.us | (610) 853-3000
+ If your life is a hard drive, | 830 Blythe Avenue
+ Christ can be your backup. | Drexel Hill, Pennsylvania 19026
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From: Lee Kindness <lkindness@csl.co.uk>
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Date: Tue, 25 Jun 2002 15:20:49 +0100
To: Tom Lane <tgl@sss.pgh.pa.us>
Subject: Re: [HACKERS] Buffer Management
In-Reply-To: <7298.1025014142@sss.pgh.pa.us>
References: <Pine.NEB.4.43.0206252239230.670-100000@angelic.cynic.net>
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Tom Lane writes:
> There seem to be a couple of different threads in
> doc/TODO.detail/mmap.
> [ snip ]
A place where mmap could be easily used and would offer a good
performance increase is for COPY FROM.
Lee.
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From cjs@cynic.net Tue Jun 25 10:24:49 2002
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Date: Tue, 25 Jun 2002 23:24:44 +0900 (JST)
From: Curt Sampson <cjs@cynic.net>
To: Bruce Momjian <pgman@candle.pha.pa.us>
cc: Tom Lane <tgl@sss.pgh.pa.us>, "J. R. Nield" <jrnield@usol.com>,
PostgreSQL Hacker <pgsql-hackers@postgresql.org>
Subject: Re: [HACKERS] Buffer Management
In-Reply-To: <200206251420.g5PEKT310222@candle.pha.pa.us>
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On Tue, 25 Jun 2002, Bruce Momjian wrote:
> The only downside I can see is that SysV shared memory is
> locked into RAM on some/most OS's while mmap anon probably isn't.
It is if you mlock() it. :-)
cjs
--
Curt Sampson <cjs@cynic.net> +81 90 7737 2974 http://www.netbsd.org
Don't you know, in this new Dark Age, we're all light. --XTC
From tgl@sss.pgh.pa.us Tue Jun 25 10:29:53 2002
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To: Curt Sampson <cjs@cynic.net>
cc: "J. R. Nield" <jrnield@usol.com>, Bruce Momjian <pgman@candle.pha.pa.us>,
PostgreSQL Hacker <pgsql-hackers@postgresql.org>
Subject: Re: [HACKERS] Buffer Management
In-Reply-To: <Pine.NEB.4.43.0206252318020.670-100000@angelic.cynic.net>
References: <Pine.NEB.4.43.0206252318020.670-100000@angelic.cynic.net>
Comments: In-reply-to Curt Sampson <cjs@cynic.net>
message dated "Tue, 25 Jun 2002 23:20:15 +0900"
Date: Tue, 25 Jun 2002 10:29:49 -0400
Message-ID: <7498.1025015389@sss.pgh.pa.us>
From: Tom Lane <tgl@sss.pgh.pa.us>
Status: ORr
Curt Sampson <cjs@cynic.net> writes:
> On Tue, 25 Jun 2002, Tom Lane wrote:
>> The other discussion seemed to be considering how to mmap individual
>> data files right into backends' address space. I do not believe this
>> can possibly work, because of loss of control over visibility of data
>> changes to other backends, timing of write-backs, etc.
> I don't understand why there would be any loss of visibility of changes.
> If two backends mmap the same block of a file, and it's shared, that's
> the same block of physical memory that they're accessing.
Is it? You have a mighty narrow conception of the range of
implementations that's possible for mmap.
But the main problem is that mmap doesn't let us control when changes to
the memory buffer will get reflected back to disk --- AFAICT, the OS is
free to do the write-back at any instant after you dirty the page, and
that completely breaks the WAL algorithm. (WAL = write AHEAD log;
the log entry describing a change must hit disk before the data page
change itself does.)
regards, tom lane
From pgsql-hackers-owner+M24164@postgresql.org Tue Jun 25 10:44:39 2002
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To: Bruce Momjian <pgman@candle.pha.pa.us>
cc: Curt Sampson <cjs@cynic.net>, "J. R. Nield" <jrnield@usol.com>,
PostgreSQL Hacker <pgsql-hackers@postgresql.org>
Subject: Re: [HACKERS] Buffer Management
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Comments: In-reply-to Bruce Momjian <pgman@candle.pha.pa.us>
message dated "Tue, 25 Jun 2002 10:20:29 -0400"
Date: Tue, 25 Jun 2002 10:32:10 -0400
Message-ID: <7524.1025015530@sss.pgh.pa.us>
From: Tom Lane <tgl@sss.pgh.pa.us>
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Bruce Momjian <pgman@candle.pha.pa.us> writes:
> This will also work well when we have non-SysV semaphore support, like
> Posix semaphores, so we would be able to run with no SysV stuff.
You do realize that we can use Posix semaphores today? The Darwin (OS X)
port uses 'em now. That's one reason I am more interested in mmap as
a shmget substitute than I used to be.
regards, tom lane
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From pgsql-hackers-owner+M24167@postgresql.org Tue Jun 25 11:02:20 2002
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From: Bruce Momjian <pgman@candle.pha.pa.us>
Message-ID: <200206251455.g5PEtst15464@candle.pha.pa.us>
Subject: Re: [HACKERS] Buffer Management
In-Reply-To: <7524.1025015530@sss.pgh.pa.us>
To: Tom Lane <tgl@sss.pgh.pa.us>
Date: Tue, 25 Jun 2002 10:55:54 -0400 (EDT)
cc: Curt Sampson <cjs@cynic.net>, "J. R. Nield" <jrnield@usol.com>,
PostgreSQL Hacker <pgsql-hackers@postgresql.org>
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Tom Lane wrote:
> Bruce Momjian <pgman@candle.pha.pa.us> writes:
> > This will also work well when we have non-SysV semaphore support, like
> > Posix semaphores, so we would be able to run with no SysV stuff.
>
> You do realize that we can use Posix semaphores today? The Darwin (OS X)
> port uses 'em now. That's one reason I am more interested in mmap as
No, I didn't realize we had gotten that far.
--
Bruce Momjian | http://candle.pha.pa.us
pgman@candle.pha.pa.us | (610) 853-3000
+ If your life is a hard drive, | 830 Blythe Avenue
+ Christ can be your backup. | Drexel Hill, Pennsylvania 19026
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From pgsql-hackers-owner+M24168@postgresql.org Tue Jun 25 11:05:13 2002
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Tue, 25 Jun 2002 10:56:58 -0400 (EDT)
From: Bruce Momjian <pgman@candle.pha.pa.us>
Message-ID: <200206251456.g5PEuwO15564@candle.pha.pa.us>
Subject: Re: [HACKERS] Buffer Management
In-Reply-To: <7498.1025015389@sss.pgh.pa.us>
To: Tom Lane <tgl@sss.pgh.pa.us>
Date: Tue, 25 Jun 2002 10:56:58 -0400 (EDT)
cc: Curt Sampson <cjs@cynic.net>, "J. R. Nield" <jrnield@usol.com>,
PostgreSQL Hacker <pgsql-hackers@postgresql.org>
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Tom Lane wrote:
> Curt Sampson <cjs@cynic.net> writes:
> > On Tue, 25 Jun 2002, Tom Lane wrote:
> >> The other discussion seemed to be considering how to mmap individual
> >> data files right into backends' address space. I do not believe this
> >> can possibly work, because of loss of control over visibility of data
> >> changes to other backends, timing of write-backs, etc.
>
> > I don't understand why there would be any loss of visibility of changes.
> > If two backends mmap the same block of a file, and it's shared, that's
> > the same block of physical memory that they're accessing.
>
> Is it? You have a mighty narrow conception of the range of
> implementations that's possible for mmap.
>
> But the main problem is that mmap doesn't let us control when changes to
> the memory buffer will get reflected back to disk --- AFAICT, the OS is
> free to do the write-back at any instant after you dirty the page, and
> that completely breaks the WAL algorithm. (WAL = write AHEAD log;
> the log entry describing a change must hit disk before the data page
> change itself does.)
Can we mmap WAL without problems? Not sure if there is any gain to it
because we just write it and rarely read from it.
--
Bruce Momjian | http://candle.pha.pa.us
pgman@candle.pha.pa.us | (610) 853-3000
+ If your life is a hard drive, | 830 Blythe Avenue
+ Christ can be your backup. | Drexel Hill, Pennsylvania 19026
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From tgl@sss.pgh.pa.us Tue Jun 25 11:00:20 2002
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To: Bruce Momjian <pgman@candle.pha.pa.us>
cc: Curt Sampson <cjs@cynic.net>, "J. R. Nield" <jrnield@usol.com>,
PostgreSQL Hacker <pgsql-hackers@postgresql.org>
Subject: Re: [HACKERS] Buffer Management
In-Reply-To: <200206251456.g5PEuwO15564@candle.pha.pa.us>
References: <200206251456.g5PEuwO15564@candle.pha.pa.us>
Comments: In-reply-to Bruce Momjian <pgman@candle.pha.pa.us>
message dated "Tue, 25 Jun 2002 10:56:58 -0400"
Date: Tue, 25 Jun 2002 11:00:19 -0400
Message-ID: <7805.1025017219@sss.pgh.pa.us>
From: Tom Lane <tgl@sss.pgh.pa.us>
Status: ORr
Bruce Momjian <pgman@candle.pha.pa.us> writes:
> Can we mmap WAL without problems? Not sure if there is any gain to it
> because we just write it and rarely read from it.
Perhaps, but I don't see any point to it.
regards, tom lane
From pgsql-hackers-owner+M24171@postgresql.org Tue Jun 25 11:14:23 2002
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From: Bruce Momjian <pgman@candle.pha.pa.us>
Message-ID: <200206251502.g5PF25r16113@candle.pha.pa.us>
Subject: Re: [HACKERS] Buffer Management
In-Reply-To: <7805.1025017219@sss.pgh.pa.us>
To: Tom Lane <tgl@sss.pgh.pa.us>
Date: Tue, 25 Jun 2002 11:02:05 -0400 (EDT)
cc: Curt Sampson <cjs@cynic.net>, "J. R. Nield" <jrnield@usol.com>,
PostgreSQL Hacker <pgsql-hackers@postgresql.org>
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Tom Lane wrote:
> Bruce Momjian <pgman@candle.pha.pa.us> writes:
> > Can we mmap WAL without problems? Not sure if there is any gain to it
> > because we just write it and rarely read from it.
>
> Perhaps, but I don't see any point to it.
Agreed. I have been poking around google looking for an article I read
months ago saying that mmap of files is slighly faster in low memory
usage situations, but much slower in high memory usage situations
because the kernel doesn't know as much about the file access in mmap as
it does with stdio. I will find it. :-)
--
Bruce Momjian | http://candle.pha.pa.us
pgman@candle.pha.pa.us | (610) 853-3000
+ If your life is a hard drive, | 830 Blythe Avenue
+ Christ can be your backup. | Drexel Hill, Pennsylvania 19026
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From pgsql-hackers-owner+M24179@postgresql.org Tue Jun 25 12:13:40 2002
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Date: Tue, 25 Jun 2002 12:12:45 -0400
From: Bradley McLean <brad@bradm.net>
To: Tom Lane <tgl@sss.pgh.pa.us>
cc: Mario Weilguni <mario.weilguni@icomedias.com>,
Curt Sampson <cjs@cynic.net>, "J. R. Nield" <jrnield@usol.com>,
Bruce Momjian <pgman@candle.pha.pa.us>,
PostgreSQL Hacker <pgsql-hackers@postgresql.org>
Subject: Re: [HACKERS] Buffer Management
Message-ID: <20020625121245.A14762@nia.bradm.net>
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* Tom Lane (tgl@sss.pgh.pa.us) [020625 11:00]:
>
> msync can force not-yet-written changes down to disk. It does not
> prevent the OS from choosing to write changes *before* you invoke msync.
>
> Our problem is that we want to enforce the write ordering "WAL before
> data file". To do that, we write and fsync (or DSYNC, or something)
> a WAL entry before we issue the write() against the data file. We
> don't really care if the kernel delays the data file write beyond that
> point, but we can be certain that the data file write did not occur
> too early.
>
> msync is designed to ensure exactly the opposite constraint: it can
> guarantee that no changes remain unwritten after time T, but it can't
> guarantee that changes aren't written before time T.
Okay, so instead of looking for constraints from the OS on the data file,
use the constraints on the WAL file. It would work at the cost of a buffer
copy? Er, maybe two:
mmap the data file and WAL separately.
Copy the data file page to the WAL mmap area.
Modify the page.
msync() the WAL.
Copy the page to the data file mmap area.
msync() or not the data file.
(This is half baked, just thought I'd see if it stirred further thought).
As another approach, how expensive is re-MMAPing portions of the files
compared to the copies.
-Brad
>
> regards, tom lane
>
>
>
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From cjs@cynic.net Wed Jun 26 00:13:45 2002
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Date: Wed, 26 Jun 2002 13:13:42 +0900 (JST)
From: Curt Sampson <cjs@cynic.net>
To: Tom Lane <tgl@sss.pgh.pa.us>
cc: "J. R. Nield" <jrnield@usol.com>, Bruce Momjian <pgman@candle.pha.pa.us>,
PostgreSQL Hacker <pgsql-hackers@postgresql.org>
Subject: Re: [HACKERS] Buffer Management
In-Reply-To: <7498.1025015389@sss.pgh.pa.us>
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On Tue, 25 Jun 2002, Tom Lane wrote:
> Curt Sampson <cjs@cynic.net> writes:
>
> > I don't understand why there would be any loss of visibility of changes.
> > If two backends mmap the same block of a file, and it's shared, that's
> > the same block of physical memory that they're accessing.
>
> Is it? You have a mighty narrow conception of the range of
> implementations that's possible for mmap.
It's certainly possible to implement something that you call mmap
that is not. But if you are using the posix-defined MAP_SHARED flag,
the behaviour above is what you see. It might be implemented slightly
differently internally, but that's no concern of postgres. And I find
it pretty unlikely that it would be implemented otherwise without good
reason.
Note that your proposal of using mmap to replace sysv shared memory
relies on the behaviour I've described too. As well, if you're replacing
sysv shared memory with an mmap'd file, you may end up doing excessive
disk I/O on systems without the MAP_NOSYNC option. (Without this option,
the update thread/daemon may ensure that every buffer is flushed to the
backing store on disk every 30 seconds or so. You might be able to get
around this by using a small file-backed area for things that need to
persist after a crash, and a larger anonymous area for things that don't
need to persist after a crash.)
> But the main problem is that mmap doesn't let us control when changes to
> the memory buffer will get reflected back to disk --- AFAICT, the OS is
> free to do the write-back at any instant after you dirty the page, and
> that completely breaks the WAL algorithm. (WAL = write AHEAD log;
> the log entry describing a change must hit disk before the data page
> change itself does.)
Hm. Well ,we could try not to write the data to the page until
after we receive notification that our WAL data is committed to
stable storage. However, new the data has to be availble to all of
the backends at the exact time that the commit happens. Perhaps a
shared list of pending writes?
Another option would be to just let it write, but on startup, scan
all of the data blocks in the database for tuples that have a
transaction ID later than the last one we updated to, and remove
them. That could pretty darn expensive on a large database, though.
cjs
--
Curt Sampson <cjs@cynic.net> +81 90 7737 2974 http://www.netbsd.org
Don't you know, in this new Dark Age, we're all light. --XTC
From tgl@sss.pgh.pa.us Wed Jun 26 09:22:05 2002
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To: Curt Sampson <cjs@cynic.net>
cc: "J. R. Nield" <jrnield@usol.com>, Bruce Momjian <pgman@candle.pha.pa.us>,
PostgreSQL Hacker <pgsql-hackers@postgresql.org>
Subject: Re: [HACKERS] Buffer Management
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Comments: In-reply-to Curt Sampson <cjs@cynic.net>
message dated "Wed, 26 Jun 2002 13:13:42 +0900"
Date: Wed, 26 Jun 2002 09:21:59 -0400
Message-ID: <1696.1025097719@sss.pgh.pa.us>
From: Tom Lane <tgl@sss.pgh.pa.us>
Status: ORr
Curt Sampson <cjs@cynic.net> writes:
> Note that your proposal of using mmap to replace sysv shared memory
> relies on the behaviour I've described too.
True, but I was not envisioning mapping an actual file --- at least
on HPUX, the only way to generate an arbitrary-sized shared memory
region is to use MAP_ANONYMOUS and not have the mmap'd area connected
to any file at all. It's not farfetched to think that this aspect
of mmap might work differently from mapping pieces of actual files.
In practice of course we'd have to restrict use of any such
implementation to platforms where mmap behaves reasonably ... according
to our definition of "reasonably".
regards, tom lane
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From: Bruce Momjian <pgman@candle.pha.pa.us>
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Subject: Re: [HACKERS] Buffer Management
In-Reply-To: <1696.1025097719@sss.pgh.pa.us>
To: Tom Lane <tgl@sss.pgh.pa.us>
Date: Wed, 26 Jun 2002 13:11:19 -0400 (EDT)
cc: Curt Sampson <cjs@cynic.net>, "J. R. Nield" <jrnield@usol.com>,
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Tom Lane wrote:
> Curt Sampson <cjs@cynic.net> writes:
> > Note that your proposal of using mmap to replace sysv shared memory
> > relies on the behaviour I've described too.
>
> True, but I was not envisioning mapping an actual file --- at least
> on HPUX, the only way to generate an arbitrary-sized shared memory
> region is to use MAP_ANONYMOUS and not have the mmap'd area connected
> to any file at all. It's not farfetched to think that this aspect
> of mmap might work differently from mapping pieces of actual files.
>
> In practice of course we'd have to restrict use of any such
> implementation to platforms where mmap behaves reasonably ... according
> to our definition of "reasonably".
Yes, I am told mapping /dev/zero is the same as the anon map.
--
Bruce Momjian | http://candle.pha.pa.us
pgman@candle.pha.pa.us | (610) 853-3000
+ If your life is a hard drive, | 830 Blythe Avenue
+ Christ can be your backup. | Drexel Hill, Pennsylvania 19026
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From: Curt Sampson <cjs@cynic.net>
To: Tom Lane <tgl@sss.pgh.pa.us>
cc: "J. R. Nield" <jrnield@usol.com>, Bruce Momjian <pgman@candle.pha.pa.us>,
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Subject: Re: [HACKERS] Buffer Management
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On Wed, 26 Jun 2002, Tom Lane wrote:
> Curt Sampson <cjs@cynic.net> writes:
> > Note that your proposal of using mmap to replace sysv shared memory
> > relies on the behaviour I've described too.
>
> True, but I was not envisioning mapping an actual file --- at least
> on HPUX, the only way to generate an arbitrary-sized shared memory
> region is to use MAP_ANONYMOUS and not have the mmap'd area connected
> to any file at all. It's not farfetched to think that this aspect
> of mmap might work differently from mapping pieces of actual files.
I find it somewhat farfetched, for a couple of reasons:
1. Memory mapped with the MAP_SHARED flag is shared memory,
anonymous or not. POSIX is pretty explicit about how this works,
and the "standard" for mmap that predates POSIX is the same.
Anonymous memory does not behave differently.
You could just as well say that some systems might exist such
that one process can write() a block to a file, and then another
might read() it afterwards but not see the changes. Postgres
should not try to deal with hypothetical systems that are so
completely broken.
2. Mmap is implemented as part of a unified buffer cache system
on all of today's operating systems that I know of. The memory
is backed by swap space when anonymous, and by a specified file
when not anonymous; but the way these two are handled is
*exactly* the same internally.
Even on older systems without unified buffer cache, the behaviour
is the same between anonymous and file-backed mmap'd memory.
And there would be no point in making it otherwise. Mmap is
designed to let you share memory; why make a broken implementation
under certain circumstances?
> In practice of course we'd have to restrict use of any such
> implementation to platforms where mmap behaves reasonably ... according
> to our definition of "reasonably".
Of course. As we do already with regular I/O.
cjs
--
Curt Sampson <cjs@cynic.net> +81 90 7737 2974 http://www.netbsd.org
Don't you know, in this new Dark Age, we're all light. --XTC
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[moving to -performance, please drop -committers from replies]
> > I've toyed with the idea of adding this because it is monstrously more
> > efficient than select()/poll() in basically every way, shape, and
> > form.
>=20
> From what I've looked at, kqueue only wins when you are watching a
> large number of file descriptors at the same time; which is an
> operation done nowhere in Postgres. I think the above would be a
> complete waste of effort.
It scales very well to many thousands of descriptors, but it also
works well on small numbers as well. kqueue is about 5x faster than
select() or poll() on the low end of number of fd's. As I said
earlier, I don't think there is _much_ to gain in this regard, but I
do think that it would be a speed improvement but only to one OS
supported by PostgreSQL. I think that there are bigger speed
improvements to be had elsewhere in the code.
> > Is this one of the areas of PostgreSQL that just needs to get
> > slowly migrated to use mmap() or are there any gaping reasons why
> > to not use the family of system calls?
>=20
> There has been much speculation on this, and no proof that it
> actually buys us anything to justify the portability hit.
Actually, I think that it wouldn't be that big of a portability hit
because you still would read() and write() as always, but in
performance sensitive areas, an #ifdef HAVE_MMAP section would have
the appropriate mmap() calls. If the system doesn't have mmap(),
there isn't much to loose and we're in the same position we're in now.
> There would be some nontrivial problems to solve, such as the
> mechanics of accessing a large number of files from a large number
> of backends without running out of virtual memory. Also, is it
> guaranteed that multiple backends mmap'ing the same block will
> access the very same physical buffer, and not multiple copies?
> Multiple copies would be fatal. See the acrhives for more
> discussion.
Have read through the archives. Making a call to madvise() will speed
up access to the pages as it gives hints to the VM about what order
the pages are accessed/used. Here are a few bits from the BSD mmap()
and madvise() man pages:
mmap(2):
MAP_NOSYNC Causes data dirtied via this VM map to be flushed to
physical media only when necessary (usually by the
pager) rather then gratuitously. Typically this pre-
vents the update daemons from flushing pages dirtied
through such maps and thus allows efficient sharing =
of
memory across unassociated processes using a file-
backed shared memory map. Without this option any VM
pages you dirty may be flushed to disk every so often
(every 30-60 seconds usually) which can create perfo=
r-
mance problems if you do not need that to occur (such
as when you are using shared file-backed mmap regions
for IPC purposes). Note that VM/filesystem coherency
is maintained whether you use MAP_NOSYNC or not. Th=
is
option is not portable across UNIX platforms (yet),
though some may implement the same behavior by defau=
lt.
WARNING! Extending a file with ftruncate(2), thus c=
re-
ating a big hole, and then filling the hole by modif=
y-
ing a shared mmap() can lead to severe file fragment=
a-
tion. In order to avoid such fragmentation you shou=
ld
always pre-allocate the file's backing store by
write()ing zero's into the newly extended area prior=
to
modifying the area via your mmap(). The fragmentati=
on
problem is especially sensitive to MAP_NOSYNC pages,
because pages may be flushed to disk in a totally ra=
n-
dom order.
The same applies when using MAP_NOSYNC to implement a
file-based shared memory store. It is recommended t=
hat
you create the backing store by write()ing zero's to
the backing file rather then ftruncate()ing it. You
can test file fragmentation by observing the KB/t
(kilobytes per transfer) results from an ``iostat 1''
while reading a large file sequentially, e.g. using
``dd if=3Dfilename of=3D/dev/null bs=3D32k''.
The fsync(2) function will flush all dirty data and
metadata associated with a file, including dirty NOS=
YNC
VM data, to physical media. The sync(8) command and
sync(2) system call generally do not flush dirty NOS=
YNC
VM data. The msync(2) system call is obsolete since
BSD implements a coherent filesystem buffer cache.
However, it may be used to associate dirty VM pages
with filesystem buffers and thus cause them to be
flushed to physical media sooner rather then later.
madvise(2):
MADV_NORMAL Tells the system to revert to the default paging beha=
v-
ior.
MADV_RANDOM Is a hint that pages will be accessed randomly, and
prefetching is likely not advantageous.
MADV_SEQUENTIAL Causes the VM system to depress the priority of pages
immediately preceding a given page when it is faulted
in.
mprotect(2):
The mprotect() system call changes the specified pages to have protect=
ion
prot. Not all implementations will guarantee protection on a page bas=
is;
the granularity of protection changes may be as large as an entire
region. A region is the virtual address space defined by the start and
end addresses of a struct vm_map_entry.
Currently these protection bits are known, which can be combined, OR'd
together:
PROT_NONE No permissions at all.
PROT_READ The pages can be read.
PROT_WRITE The pages can be written.
PROT_EXEC The pages can be executed.
msync(2):
The msync() system call writes any modified pages back to the filesyst=
em
and updates the file modification time. If len is 0, all modified pag=
es
within the region containing addr will be flushed; if len is non-zero,
only those pages containing addr and len-1 succeeding locations will be
examined. The flags argument may be specified as follows:
MS_ASYNC Return immediately
MS_SYNC Perform synchronous writes
MS_INVALIDATE Invalidate all cached data
A few thoughts come to mind:
1) backends could share buffers by mmap()'ing shared regions of data.
While I haven't seen any numbers to reflect this, I'd wager that
mmap() is a faster interface than ipc.
2) It looks like while there are various file IO schemes scattered all
over the place, the bulk of the critical routines that would need
to be updated are in backend/storage/file/fd.c, more specifically:
*) fileNameOpenFile() would need the appropriate mmap() call made
to it.
*) FileTruncate() would need some attention to avoid fragmentation.
*) a new "sync" GUC would have to be introduced to handle msync
(affects only pg_fsync() and pg_fdatasync()).
3) There's a bit of code in pgsql/src/backend/storage/smgr that could
be gutted/removed. Which of those storage types are even used any
more? There's a reference in the code to PostgreSQL 3.0. :)
And I think that'd be it. The LRU code could be used if necessary to
help manage the amount of mmap()'ed in the VM at any one time, at the
very least that could be a handled by a shm var that various backends
would increment/decrement as files are open()'ed/close()'ed.
I didn't spend too long looking at this, but I _think_ that'd cover
80% of PostgreSQL's disk access needs. The next bit to possibly add
would be passing a flag on FileOpen operations that'd act as a hint to
madvise() that way the VM could proactively react to PostgreSQL's
needs.
I don't have my copy of Steven's handy (it's some 700mi away atm
otherwise I'd cite it), but if Tom or someone else has it handy, look
up the example re: the performance gain from read()'ing an mmap()'ed
file versus a non-mmap()'ed file. The difference is non-trivial and
_WELL_ worth the time given the speed increase. The same speed
benefit held true for writes as well, iirc. It's been a while, but I
think it was around page 330. The index has it listed and it's not
that hard of an example to find. -sc
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From: Sean Chittenden <sean@chittenden.org>
To: Neil Conway <neilc@samurai.com>
cc: Tom Lane <tgl@sss.pgh.pa.us>,
Christopher Kings-Lynne <chriskl@familyhealth.com.au>,
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> > I don't have my copy of Steven's handy (it's some 700mi away atm
> > otherwise I'd cite it), but if Tom or someone else has it handy, look
> > up the example re: the performance gain from read()'ing an mmap()'ed
> > file versus a non-mmap()'ed file. The difference is non-trivial and
> > _WELL_ worth the time given the speed increase.
>=20
> Can anyone confirm this? If so, one easy step we could take in this
> direction would be adapting COPY FROM to use mmap().
Weeee! Alright, so I got to have some fun writing out some simple
tests with mmap() and friends tonight. Are the results interesting?
Absolutely! Is this a simple benchmark? Yup. Do I think it
simulates PostgreSQL? Eh, not particularly. Does it demonstrate that
mmap() is a win and something worth implementing? I sure hope so. Is
this a test program to demonstrate the ideal use of mmap() in
PostgreSQL? No. Is it a place to start a factual discussion? I hope
so.
I have here four tests that are conditionalized by cpp.
# The first one uses read() and write() but with the buffer size set
# to the same size as the file.
gcc -O3 -finline-functions -fkeep-inline-functions -funroll-loops -o test-=
mmap test-mmap.c
/usr/bin/time ./test-mmap > /dev/null
Beginning tests with file: services
Page size: 4096
File read size is the same as the file size
Number of iterations: 100000
Start time: 1047013002.412516
Time: 82.88178
Completed tests
82.09 real 2.13 user 68.98 sys
# The second one uses read() and write() with the default buffer size:
# 65536
gcc -O3 -finline-functions -fkeep-inline-functions -funroll-loops -DDEFAUL=
T_READSIZE=3D1 -o test-mmap test-mmap.c
/usr/bin/time ./test-mmap > /dev/null
Beginning tests with file: services
Page size: 4096
File read size is default read size: 65536
Number of iterations: 100000
Start time: 1047013085.16204
Time: 18.155511
Completed tests
18.16 real 0.90 user 14.79 sys
# Please note this is significantly faster, but that's expected
# The third test uses mmap() + madvise() + write()
gcc -O3 -finline-functions -fkeep-inline-functions -funroll-loops -DDEFAUL=
T_READSIZE=3D1 -DDO_MMAP=3D1 -o test-mmap test-mmap.c
/usr/bin/time ./test-mmap > /dev/null
Beginning tests with file: services
Page size: 4096
File read size is the same as the file size
Number of iterations: 100000
Start time: 1047013103.859818
Time: 8.4294203644
Completed tests
7.24 real 0.41 user 5.92 sys
# Faster still, and twice as fast as the normal read() case
# The last test only calls mmap()'s once when the file is opened and
# only msync()'s, munmap()'s, close()'s the file once at exit.
gcc -O3 -finline-functions -fkeep-inline-functions -funroll-loops -DDEFAUL=
T_READSIZE=3D1 -DDO_MMAP=3D1 -DDO_MMAP_ONCE=3D1 -o test-mmap test-mmap.c
/usr/bin/time ./test-mmap > /dev/null
Beginning tests with file: services
Page size: 4096
File read size is the same as the file size
Number of iterations: 100000
Start time: 1047013111.623712
Time: 1.174076
Completed tests
1.18 real 0.09 user 0.92 sys
# Substantially faster
Obviously this isn't perfect, but reading and writing data is faster
(specifically moving pages through the VM/OS). Doing partial writes
from mmap()'ed data should be faster along with scanning through
mmap()'ed portions of - or completely mmap()'ed - files because the
pages are already loaded in the VM. PostgreSQL's LRU file descriptor
cache could easily be adjusted to add mmap()'ing of frequently
accessed files (specifically, system catalogs come to mind). It's not
hard to figure out how often particular files are accessed and to
either _avoid_ mmap()'ing a file that isn't accessed often, or to
mmap() files that _are_ accessed often. mmap() does have a cost, but
I'd wager that mmap()'ing the same file a second or third time from a
different process would be more efficient. The speedup of searching
through an mmap()'ed file may be worth it, however, to mmap() all
files if the system is under a tunable resource limit
(max_mmaped_bytes?).
If someone is so inclined or there's enough interest, I can reverse
this test case so that data is written to an mmap()'ed file, but the
same performance difference should hold true (assuming this isn't a
write to a tape drive ::grin::).
The URL for the program used to generate the above tests is at:
http://people.freebsd.org/~seanc/mmap_test/
Please ask if you have questions. -sc
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To: Tom Lane <tgl@sss.pgh.pa.us>
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> > Absolutely! Is this a simple benchmark? Yup. Do I think it
> > simulates PostgreSQL? Eh, not particularly.
I think quite a few of these Q's would have been answered by reading
the code/Makefile....
> This would be on what OS?
FreeBSD, but it shouldn't matter. Any reasonably written VM should
have similar numbers (though BSD is generally regarded as having the
best VM, which, I think Linux poached not that long ago, iirc
::grimace::).
> What hardware?
My ultra-pathetic laptop with some fine - overly-noisy and can hardly
buildworld - IDE drives.
> What size test file?
In this case, only 72K. I've just updated the test program to use an
array of files though.
> Do the "iterations" mean so many reads of the entire file, or so
> many buffer-sized read requests?
In some cases, yes. With the file mmap()'ed, sorta. One of the test
cases (the one that did it in ~8s), mmap()'ed and munmap()'ed the file
every iteration and was twice as fast as the vanilla read() call.
> Did the mmap case actually *read* anything, or just map and unmap
> the file?
Nope, read it and wrote it out to stdout (which was redirected to
/dev/null).
> Also, what did you do to normalize for the effects of the test file
> being already in kernel disk cache after the first test?
That honestly doesn't matter too much since I wasn't testing the rate
of reading in files from my hard drive, only the OS's ability to
read/write pages of data around. In any case, I've updated my test
case to iterate through an array of files instead of just reading in a
copy of /etc/services. My laptop is generally a poor benchmark for
disk read performance given it takes 8hrs to buildworld, over 12hrs to
build mozilla, 18 for KDE, and about 48hrs for Open Office. :)
Someone with faster disks may want to try this and report back, but it
doesn't matter much in terms of relevancy for considering the benefits
of mmap(). The point is that there are calls that can be used that
substantially speed up read()'s and write()'s by allowing the VM to
align pages of data and give hints about its usage. For the sake of
argument re: the previously done tests, I'll reverse the order in
which I ran them and I bet dime to dollar that the times will be
identical.
% make =
~/open_source/mmap_test
cp -f /etc/services ./services
gcc -O3 -finline-functions -fkeep-inline-functions -funroll-loops -DDEFAUL=
T_READSIZE=3D1 -DDO_MMAP=3D1 -DDO_MMAP_ONCE=3D1 -o mmap-test mmap-test.c
/usr/bin/time ./mmap-test > /dev/null
Beginning tests with file: services
Page size: 4096
File read size is the same as the file size
Number of iterations: 100000
Start time: 1047064672.276544
Time: 1.281477
Completed tests
1.29 real 0.10 user 0.92 sys
gcc -O3 -finline-functions -fkeep-inline-functions -funroll-loops -DDEFAUL=
T_READSIZE=3D1 -DDO_MMAP=3D1 -o mmap-test mmap-test.c
/usr/bin/time ./mmap-test > /dev/null
Beginning tests with file: services
Page size: 4096
File read size is the same as the file size
Number of iterations: 100000
Start time: 1047064674.266191
Time: 7.486622
Completed tests
7.49 real 0.41 user 6.01 sys
gcc -O3 -finline-functions -fkeep-inline-functions -funroll-loops -DDEFAUL=
T_READSIZE=3D1 -o mmap-test mmap-test.c
/usr/bin/time ./mmap-test > /dev/null
Beginning tests with file: services
Page size: 4096
File read size is default read size: 65536
Number of iterations: 100000
Start time: 1047064682.288637
Time: 19.35214
Completed tests
19.04 real 0.88 user 15.43 sys
gcc -O3 -finline-functions -fkeep-inline-functions -funroll-loops -o mmap-=
test mmap-test.c
/usr/bin/time ./mmap-test > /dev/null
Beginning tests with file: services
Page size: 4096
File read size is the same as the file size
Number of iterations: 100000
Start time: 1047064701.867031
Time: 82.4294540875
Completed tests
81.57 real 2.10 user 69.55 sys
Here's the updated test that iterates through. Ooh! One better, the
files I've used are actual data files from ~pgsql. The new benchmark
iterates through the list of files and and calls bench() once for each
file and restarts at the first file after reaching the end of its
list (ARGV).
Whoa, if these tests are even close to real world, then we at the very
least should be mmap()'ing the file every time we read it (assuming
we're reading more than just a handful of bytes):
find /usr/local/pgsql/data -type f | /usr/bin/xargs /usr/bin/time ./mmap-te=
st > /dev/null
Page size: 4096
File read size is the same as the file size
Number of iterations: 100000
Start time: 1047071143.463360
Time: 12.109530
Completed tests
12.11 real 0.36 user 6.80 sys
find /usr/local/pgsql/data -type f | /usr/bin/xargs /usr/bin/time ./mmap-te=
st > /dev/null
Page size: 4096
File read size is default read size: 65536
Number of iterations: 100000
.... [been waiting here for >40min now....]
Ah well, if these tests finish this century, I'll post the results in
a bit, but it's pretty clearly a win. In terms of the data that I'm
copying, I'm copying ~700MB of data from my test DB on my laptop. I
only have 256MB of RAM so I can pretty much promise you that the data
isn't in my system buffers. If anyone else would like to run the
tests or look at the results, please check it out:
o1 and o2 should be the only targets used if FILES is bigger than the
RAM on the system. o3's by far and away the fastest, but only in rare
cases will a DBA have more RAM than data. But, as mentioned earlier,
the LRU cache could easily be modified to munmap() infrequently
accessed files to keep the size of mmap()'ed data down to a reasonable
level.
The updated test programs are at:
http://people.FreeBSD.org/~seanc/mmap_test/
-sc
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