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Add more about LRU-2.
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Bruce Momjian
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@ -1059,7 +1059,7 @@ From owner-pgsql-hackers@hub.org Thu Jan 20 18:45:32 2000
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@ -1919,4 +1919,3 @@ Best Regards,
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Tiago
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PS - again: I'm starting, so, some of my comments can be completly dumb.
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@ -454,7 +454,7 @@ From owner-pgsql-hackers@hub.org Tue Oct 19 21:25:30 1999
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@ -552,3 +552,146 @@ Inoue@tpf.co.jp
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************
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From pgsql-hackers-owner+M6267@hub.org Sun Aug 27 21:46:37 2000
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To: pgsql-hackers@postgreSQL.org
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Subject: [HACKERS] Possible performance improvement: buffer replacement policy
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Date: Sun, 27 Aug 2000 20:05:29 -0400
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Message-ID: <1601.967421129@sss.pgh.pa.us>
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From: Tom Lane <tgl@sss.pgh.pa.us>
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Status: ORr
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Those of you with long memories may recall a benchmark that Edmund Mergl
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drew our attention to back in May '99. That test showed extremely slow
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performance for updating a table with many indexes (about 20). At the
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time, it seemed the problem was due to bad performance of btree with
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many equal keys, so I thought I'd go back and retry the benchmark after
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this latest round of btree hackery.
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The good news is that btree itself seems to be pretty well fixed; the
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bad news is that the benchmark is still slow for large numbers of rows.
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The problem is I/O: the CPU mostly sits idle waiting for the disk.
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As best I can tell, the difficulty is that the working set of pages
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needed to update this many indexes is too large compared to the number
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of disk buffers Postgres is using. (I was running with -B 1000 and
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looking at behavior for a 100000-row test table. This gave me a table
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size of 3876 pages, plus 11526 pages in 20 indexes.)
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Of course, there's only so much we can do when the number of buffers
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is too small, but I still started to wonder if we are using the buffers
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as effectively as we can. Some tracing showed that most of the pages
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of the indexes were being read and written multiple times within a
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single UPDATE query, while most of the pages of the table proper were
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fetched and written only once. That says we're not using the buffers
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as well as we could; the index pages are not being kept in memory when
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they should be. In a query like this, we should displace main-table
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pages sooner to allow keeping more index pages in cache --- but with
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the simple LRU replacement method we use, once a page has been loaded
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it will stay in cache for at least the next NBuffers (-B) page
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references, no matter what. With a large NBuffers that's a long time.
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I've come across an interesting article:
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The LRU-K Page Replacement Algorithm For Database Disk Buffering
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Elizabeth J. O'Neil, Patrick E. O'Neil, Gerhard Weikum
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Proceedings of the 1993 ACM SIGMOD international conference
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on Management of Data, May 1993
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(If you subscribe to the ACM digital library, you can get a PDF of this
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from there.) This article argues that standard LRU buffer management is
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inherently not great for database caches, and that it's much better to
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replace pages on the basis of time since the K'th most recent reference,
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not just time since the most recent one. K=2 is enough to get most of
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the benefit. The big win is that you are measuring an actual page
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interreference time (between the last two references) and not just
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dealing with a lower-bound guess on the interreference time. Frequently
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used pages are thus much more likely to stay in cache.
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It looks like it wouldn't take too much work to replace shared buffers
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on the basis of LRU-2 instead of LRU, so I'm thinking about trying it.
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Has anyone looked into this area? Is there a better method to try?
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regards, tom lane
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From prlw1@newn.cam.ac.uk Fri Jan 19 12:54:45 2001
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Date: Fri, 19 Jan 2001 17:53:28 +0000
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From: Patrick Welche <prlw1@newn.cam.ac.uk>
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To: Bruce Momjian <pgman@candle.pha.pa.us>
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Cc: Tom Lane <tgl@sss.pgh.pa.us>, pgsql-hackers@postgreSQL.org
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Subject: Re: [HACKERS] Possible performance improvement: buffer replacement policy
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Message-ID: <20010119175328.A6223@quartz.newn.cam.ac.uk>
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References: <1601.967421129@sss.pgh.pa.us> <200101191703.MAA25873@candle.pha.pa.us>
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In-Reply-To: <200101191703.MAA25873@candle.pha.pa.us>; from pgman@candle.pha.pa.us on Fri, Jan 19, 2001 at 12:03:58PM -0500
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Status: OR
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On Fri, Jan 19, 2001 at 12:03:58PM -0500, Bruce Momjian wrote:
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>
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> Tom, did we ever test this? I think we did and found that it was the
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> same or worse, right?
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(Funnily enough, I just read that message:)
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To: Bruce Momjian <pgman@candle.pha.pa.us>
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cc: pgsql-hackers@postgreSQL.org
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Subject: Re: [HACKERS] Possible performance improvement: buffer replacement policy
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In-reply-to: <200010161541.LAA06653@candle.pha.pa.us>
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References: <200010161541.LAA06653@candle.pha.pa.us>
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Comments: In-reply-to Bruce Momjian <pgman@candle.pha.pa.us>
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message dated "Mon, 16 Oct 2000 11:41:41 -0400"
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Date: Mon, 16 Oct 2000 11:49:52 -0400
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Message-ID: <26100.971711392@sss.pgh.pa.us>
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From: Tom Lane <tgl@sss.pgh.pa.us>
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Bruce Momjian <pgman@candle.pha.pa.us> writes:
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>> It looks like it wouldn't take too much work to replace shared buffers
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>> on the basis of LRU-2 instead of LRU, so I'm thinking about trying it.
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>>
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>> Has anyone looked into this area? Is there a better method to try?
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> Sounds like a perfect idea. Good luck. :-)
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Actually, the idea went down in flames :-(, but I neglected to report
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back to pghackers about it. I did do some code to manage buffers as
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LRU-2. I didn't have any good performance test cases to try it with,
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but Richard Brosnahan was kind enough to re-run the TPC tests previously
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published by Great Bridge with that code in place. Wasn't any faster,
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in fact possibly a little slower, likely due to the extra CPU time spent
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on buffer freelist management. It's possible that other scenarios might
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show a better result, but right now I feel pretty discouraged about the
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LRU-2 idea and am not pursuing it.
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regards, tom lane
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