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Commit7fd89f4

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Fix async.c to not register any SLRU stats counts in the postmaster.
Previously, AsyncShmemInit forcibly initialized the first page of theasync SLRU, to save dealing with that case in asyncQueueAddEntries.But this is a poor tradeoff, since many installations do not ever useNOTIFY; for them, expending those cycles in AsyncShmemInit is acomplete waste. Besides, this only saves a couple of instructionsin asyncQueueAddEntries, which hardly seems likely to be measurable.The real reason to change this now, though, is that now that we trackSLRU access stats, the existing code is causing the postmaster toaccumulate some access counts, which then get inherited into childprocesses by fork(), messing up the statistics. Delaying theinitialization into the first child that does a NOTIFY fixes that.Hence, we can revertf3d23d8, which was an incorrect attempt atfixing that issue. Also, add an Assert to pgstat.c that shouldcatch any future errors of the same sort.Discussion:https://postgr.es/m/8367.1589391884@sss.pgh.pa.us
1 parentd82a505 commit7fd89f4

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2 files changed

+24
-14
lines changed

2 files changed

+24
-14
lines changed

‎src/backend/commands/async.c

Lines changed: 18 additions & 11 deletions
Original file line numberDiff line numberDiff line change
@@ -200,7 +200,10 @@ typedef struct QueuePosition
200200
} while (0)
201201

202202
#defineQUEUE_POS_EQUAL(x,y) \
203-
((x).page == (y).page && (x).offset == (y).offset)
203+
((x).page == (y).page && (x).offset == (y).offset)
204+
205+
#defineQUEUE_POS_IS_ZERO(x) \
206+
((x).page == 0 && (x).offset == 0)
204207

205208
/* choose logically smaller QueuePosition */
206209
#defineQUEUE_POS_MIN(x,y) \
@@ -515,7 +518,6 @@ void
515518
AsyncShmemInit(void)
516519
{
517520
boolfound;
518-
intslotno;
519521
Sizesize;
520522

521523
/*
@@ -562,13 +564,6 @@ AsyncShmemInit(void)
562564
* During start or reboot, clean out the pg_notify directory.
563565
*/
564566
(void)SlruScanDirectory(AsyncCtl,SlruScanDirCbDeleteAll,NULL);
565-
566-
/* Now initialize page zero to empty */
567-
LWLockAcquire(AsyncCtlLock,LW_EXCLUSIVE);
568-
slotno=SimpleLruZeroPage(AsyncCtl,QUEUE_POS_PAGE(QUEUE_HEAD));
569-
/* This write is just to verify that pg_notify/ is writable */
570-
SimpleLruWritePage(AsyncCtl,slotno);
571-
LWLockRelease(AsyncCtlLock);
572567
}
573568
}
574569

@@ -1470,9 +1465,21 @@ asyncQueueAddEntries(ListCell *nextNotify)
14701465
*/
14711466
queue_head=QUEUE_HEAD;
14721467

1473-
/* Fetch the current page */
1468+
/*
1469+
* If this is the first write since the postmaster started, we need to
1470+
* initialize the first page of the async SLRU. Otherwise, the current
1471+
* page should be initialized already, so just fetch it.
1472+
*
1473+
* (We could also take the first path when the SLRU position has just
1474+
* wrapped around, but re-zeroing the page is harmless in that case.)
1475+
*/
14741476
pageno=QUEUE_POS_PAGE(queue_head);
1475-
slotno=SimpleLruReadPage(AsyncCtl,pageno, true,InvalidTransactionId);
1477+
if (QUEUE_POS_IS_ZERO(queue_head))
1478+
slotno=SimpleLruZeroPage(AsyncCtl,pageno);
1479+
else
1480+
slotno=SimpleLruReadPage(AsyncCtl,pageno, true,
1481+
InvalidTransactionId);
1482+
14761483
/* Note we mark the page dirty before writing in it */
14771484
AsyncCtl->shared->page_dirty[slotno]= true;
14781485

‎src/backend/postmaster/pgstat.c

Lines changed: 6 additions & 3 deletions
Original file line numberDiff line numberDiff line change
@@ -2906,9 +2906,6 @@ pgstat_initialize(void)
29062906
MyBEEntry=&BackendStatusArray[MaxBackends+MyAuxProcType];
29072907
}
29082908

2909-
/* Initialize SLRU statistics to zero */
2910-
memset(&SLRUStats,0,sizeof(SLRUStats));
2911-
29122909
/* Set up a process-exit hook to clean up */
29132910
on_shmem_exit(pgstat_beshutdown_hook,0);
29142911
}
@@ -6727,6 +6724,12 @@ pgstat_slru_name(int slru_idx)
67276724
staticinlinePgStat_MsgSLRU*
67286725
slru_entry(intslru_idx)
67296726
{
6727+
/*
6728+
* The postmaster should never register any SLRU statistics counts; if it
6729+
* did, the counts would be duplicated into child processes via fork().
6730+
*/
6731+
Assert(IsUnderPostmaster|| !IsPostmasterEnvironment);
6732+
67306733
Assert((slru_idx >=0)&& (slru_idx<SLRU_NUM_ELEMENTS));
67316734

67326735
return&SLRUStats[slru_idx];

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