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Use larger segment file names for pg_notify

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This avoids the wraparound in async.c and removes the corresponding code
complexity. The maximum amount of allocated SLRU pages for NOTIFY / LISTEN
queue is now determined by the max_notify_queue_pages GUC. The default
value is 1048576. It allows to consume up to 8 GB of disk space which is
exactly the limit we had previously.

Author: Maxim Orlov, Aleksander Alekseev, Alexander Korotkov, Teodor Sigaev
Author: Nikita Glukhov, Pavel Borisov, Yura Sokolov
Reviewed-by: Jacob Champion, Heikki Linnakangas, Alexander Korotkov
Reviewed-by: Japin Li, Pavel Borisov, Tom Lane, Peter Eisentraut, Andres Freund
Reviewed-by: Andrey Borodin, Dilip Kumar, Aleksander Alekseev
Discussion: https://postgr.es/m/CACG%3DezZe1NQSCnfHOr78AtAZxJZeCvxrts0ygrxYwe%3DpyyjVWA%40mail.gmail.com
Discussion: https://postgr.es/m/CAJ7c6TPDOYBYrnCAeyndkBktO0WG2xSdYduTF0nxq%2BvfkmTF5Q%40mail.gmail.com
This commit is contained in:
Alexander Korotkov 2023-11-29 01:41:48 +02:00
parent 4ed8f0913b
commit 2cdf131c46
7 changed files with 62 additions and 92 deletions
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16
doc/src/sgml/config.sgml
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@ -2151,6 +2151,22 @@ include_dir 'conf.d'
</listitem>
</varlistentry>
<varlistentry id="guc-max-notify-queue-pages" xreflabel="max_notify_queue_pages">
<term><varname>max_notify_queue_pages</varname> (<type>integer</type>)
<indexterm>
<primary><varname>max_notify_queue_pages</varname> configuration parameter</primary>
</indexterm>
</term>
<listitem>
<para>
Specifies the maximum amount of allocated pages for
<xref linkend="sql-notify"/> / <xref linkend="sql-listen"/> queue.
The default value is 1048576. For 8 KB pages it allows to consume
up to 8 GB of disk space.
</para>
</listitem>
</varlistentry>
</variablelist>
</sect2>

1
doc/src/sgml/ref/listen.sgml
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@ -148,6 +148,7 @@ Asynchronous notification "virtual" received from server process with PID 8448.
<simplelist type="inline">
<member><xref linkend="sql-notify"/></member>
<member><xref linkend="sql-unlisten"/></member>
<member><xref linkend="guc-max-notify-queue-pages"/></member>
</simplelist>
</refsect1>
</refentry>

1
doc/src/sgml/ref/notify.sgml
View File

@ -228,6 +228,7 @@ Asynchronous notification "foo" with payload "payload" received from server proc
<simplelist type="inline">
<member><xref linkend="sql-listen"/></member>
<member><xref linkend="sql-unlisten"/></member>
<member><xref linkend="guc-max-notify-queue-pages"/></member>
</simplelist>
</refsect1>
</refentry>

122
src/backend/commands/async.c
View File

@ -103,12 +103,11 @@
* until we reach either a notification from an uncommitted transaction or
* the head pointer's position.
*
* 6. To avoid SLRU wraparound and limit disk space consumption, the tail
* pointer needs to be advanced so that old pages can be truncated.
* This is relatively expensive (notably, it requires an exclusive lock),
* so we don't want to do it often. We make sending backends do this work
* if they advanced the queue head into a new page, but only once every
* QUEUE_CLEANUP_DELAY pages.
* 6. To limit disk space consumption, the tail pointer needs to be advanced
* so that old pages can be truncated. This is relatively expensive
* (notably, it requires an exclusive lock), so we don't want to do it
* often. We make sending backends do this work if they advanced the queue
* head into a new page, but only once every QUEUE_CLEANUP_DELAY pages.
*
* An application that listens on the same channel it notifies will get
* NOTIFY messages for its own NOTIFYs. These can be ignored, if not useful,
@ -120,7 +119,7 @@
* The amount of shared memory used for notify management (NUM_NOTIFY_BUFFERS)
* can be varied without affecting anything but performance. The maximum
* amount of notification data that can be queued at one time is determined
* by slru.c's wraparound limit; see QUEUE_MAX_PAGE below.
* by max_notify_queue_pages GUC.
*-------------------------------------------------------------------------
*/
@ -312,23 +311,8 @@ static SlruCtlData NotifyCtlData;
#define NotifyCtl (&NotifyCtlData)
#define QUEUE_PAGESIZE BLCKSZ
#define QUEUE_FULL_WARN_INTERVAL 5000 /* warn at most once every 5s */
/*
* Use segments 0000 through FFFF. Each contains SLRU_PAGES_PER_SEGMENT pages
* which gives us the pages from 0 to SLRU_PAGES_PER_SEGMENT * 0x10000 - 1.
* We could use as many segments as SlruScanDirectory() allows, but this gives
* us so much space already that it doesn't seem worth the trouble.
*
* The most data we can have in the queue at a time is QUEUE_MAX_PAGE/2
* pages, because more than that would confuse slru.c into thinking there
* was a wraparound condition. With the default BLCKSZ this means there
* can be up to 8GB of queued-and-not-read data.
*
* Note: it's possible to redefine QUEUE_MAX_PAGE with a smaller multiple of
* SLRU_PAGES_PER_SEGMENT, for easier testing of queue-full behaviour.
*/
#define QUEUE_MAX_PAGE (SLRU_PAGES_PER_SEGMENT * 0x10000 - 1)
#define QUEUE_FULL_WARN_INTERVAL 5000 /* warn at most once every 5s */
/*
* listenChannels identifies the channels we are actually listening to
@ -439,12 +423,15 @@ static bool amRegisteredListener = false;
/* have we advanced to a page that's a multiple of QUEUE_CLEANUP_DELAY? */
static bool tryAdvanceTail = false;
/* GUC parameter */
/* GUC parameters */
bool Trace_notify = false;
/* For 8 KB pages this gives 8 GB of disk space */
int max_notify_queue_pages = 1048576;
/* local function prototypes */
static int64 asyncQueuePageDiff(int64 p, int64 q);
static bool asyncQueuePagePrecedes(int64 p, int64 q);
static inline int64 asyncQueuePageDiff(int64 p, int64 q);
static inline bool asyncQueuePagePrecedes(int64 p, int64 q);
static void queue_listen(ListenActionKind action, const char *channel);
static void Async_UnlistenOnExit(int code, Datum arg);
static void Exec_ListenPreCommit(void);
@ -474,39 +461,23 @@ static int notification_match(const void *key1, const void *key2, Size keysize);
static void ClearPendingActionsAndNotifies(void);
/*
* Compute the difference between two queue page numbers (i.e., p - q),
* accounting for wraparound.
* Compute the difference between two queue page numbers.
* Previously this function accounted for a wraparound.
*/
static int64
static inline int64
asyncQueuePageDiff(int64 p, int64 q)
{
int64 diff;
/*
* We have to compare modulo (QUEUE_MAX_PAGE+1)/2. Both inputs should be
* in the range 0..QUEUE_MAX_PAGE.
*/
Assert(p >= 0 && p <= QUEUE_MAX_PAGE);
Assert(q >= 0 && q <= QUEUE_MAX_PAGE);
diff = p - q;
if (diff >= ((QUEUE_MAX_PAGE + 1) / 2))
diff -= QUEUE_MAX_PAGE + 1;
else if (diff < -((QUEUE_MAX_PAGE + 1) / 2))
diff += QUEUE_MAX_PAGE + 1;
return diff;
return p - q;
}
/*
* Is p < q, accounting for wraparound?
*
* Since asyncQueueIsFull() blocks creation of a page that could precede any
* extant page, we need not assess entries within a page.
* Determines whether p precedes q.
* Previously this function accounted for a wraparound.
*/
static bool
static inline bool
asyncQueuePagePrecedes(int64 p, int64 q)
{
return asyncQueuePageDiff(p, q) < 0;
return p < q;
}
/*
@ -566,12 +537,13 @@ AsyncShmemInit(void)
}
/*
* Set up SLRU management of the pg_notify data.
* Set up SLRU management of the pg_notify data. Note that long segment
* names are used in order to avoid wraparound.
*/
NotifyCtl->PagePrecedes = asyncQueuePagePrecedes;
SimpleLruInit(NotifyCtl, "Notify", NUM_NOTIFY_BUFFERS, 0,
NotifySLRULock, "pg_notify", LWTRANCHE_NOTIFY_BUFFER,
SYNC_HANDLER_NONE, false);
SYNC_HANDLER_NONE, true);
if (!found)
{
@ -1305,27 +1277,11 @@ asyncQueueUnregister(void)
static bool
asyncQueueIsFull(void)
{
int nexthead;
int boundary;
int headPage = QUEUE_POS_PAGE(QUEUE_HEAD);
int tailPage = QUEUE_POS_PAGE(QUEUE_TAIL);
int occupied = headPage - tailPage;
/*
* The queue is full if creating a new head page would create a page that
* logically precedes the current global tail pointer, ie, the head
* pointer would wrap around compared to the tail. We cannot create such
* a head page for fear of confusing slru.c. For safety we round the tail
* pointer back to a segment boundary (truncation logic in
* asyncQueueAdvanceTail does not do this, so doing it here is optional).
*
* Note that this test is *not* dependent on how much space there is on
* the current head page. This is necessary because asyncQueueAddEntries
* might try to create the next head page in any case.
*/
nexthead = QUEUE_POS_PAGE(QUEUE_HEAD) + 1;
if (nexthead > QUEUE_MAX_PAGE)
nexthead = 0; /* wrap around */
boundary = QUEUE_STOP_PAGE;
boundary -= boundary % SLRU_PAGES_PER_SEGMENT;
return asyncQueuePagePrecedes(nexthead, boundary);
return occupied >= max_notify_queue_pages;
}
/*
@ -1355,8 +1311,6 @@ asyncQueueAdvance(volatile QueuePosition *position, int entryLength)
if (offset + QUEUEALIGN(AsyncQueueEntryEmptySize) > QUEUE_PAGESIZE)
{
pageno++;
if (pageno > QUEUE_MAX_PAGE)
pageno = 0; /* wrap around */
offset = 0;
pageJump = true;
}
@ -1433,9 +1387,6 @@ asyncQueueAddEntries(ListCell *nextNotify)
* If this is the first write since the postmaster started, we need to
* initialize the first page of the async SLRU. Otherwise, the current
* page should be initialized already, so just fetch it.
*
* (We could also take the first path when the SLRU position has just
* wrapped around, but re-zeroing the page is harmless in that case.)
*/
pageno = QUEUE_POS_PAGE(queue_head);
if (QUEUE_POS_IS_ZERO(queue_head))
@ -1548,20 +1499,12 @@ asyncQueueUsage(void)
{
int headPage = QUEUE_POS_PAGE(QUEUE_HEAD);
int tailPage = QUEUE_POS_PAGE(QUEUE_TAIL);
int occupied;
occupied = headPage - tailPage;
int occupied = headPage - tailPage;
if (occupied == 0)
return (double) 0; /* fast exit for common case */
if (occupied < 0)
{
/* head has wrapped around, tail not yet */
occupied += QUEUE_MAX_PAGE + 1;
}
return (double) occupied / (double) ((QUEUE_MAX_PAGE + 1) / 2);
return (double) occupied / (double) max_notify_queue_pages;
}
/*
@ -2209,11 +2152,6 @@ asyncQueueAdvanceTail(void)
*/
SimpleLruTruncate(NotifyCtl, newtailpage);
/*
* Update QUEUE_STOP_PAGE. This changes asyncQueueIsFull()'s verdict
* for the segment immediately prior to the old tail, allowing fresh
* data into that segment.
*/
LWLockAcquire(NotifyQueueLock, LW_EXCLUSIVE);
QUEUE_STOP_PAGE = newtailpage;
LWLockRelease(NotifyQueueLock);

10
src/backend/utils/misc/guc_tables.c
View File

@ -2687,6 +2687,16 @@ struct config_int ConfigureNamesInt[] =
NULL, NULL, NULL
},
{
{"max_notify_queue_pages", PGC_POSTMASTER, RESOURCES_DISK,
gettext_noop("Sets the maximum number of allocated pages for NOTIFY / LISTEN queue."),
NULL,
},
&max_notify_queue_pages,
1048576, 64, INT_MAX,
NULL, NULL, NULL
},
{
{"wal_decode_buffer_size", PGC_POSTMASTER, WAL_RECOVERY,
gettext_noop("Buffer size for reading ahead in the WAL during recovery."),

3
src/backend/utils/misc/postgresql.conf.sample
View File

@ -166,6 +166,9 @@
#temp_file_limit = -1 # limits per-process temp file space
# in kilobytes, or -1 for no limit
#max_notify_queue_pages = 1048576 # limits the number of SLRU pages allocated
# for NOTIFY / LISTEN queue
# - Kernel Resources -
#max_files_per_process = 1000 # min 64

1
src/include/commands/async.h
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@ -21,6 +21,7 @@
#define NUM_NOTIFY_BUFFERS 8
extern PGDLLIMPORT bool Trace_notify;
extern PGDLLIMPORT int max_notify_queue_pages;
extern PGDLLIMPORT volatile sig_atomic_t notifyInterruptPending;
extern Size AsyncShmemSize(void);