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Commite880df2

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Allow multiple bgworkers to be launched per postmaster iteration.
Previously, maybe_start_bgworker() would launch at most one bgworkerprocess per call, on the grounds that the postmaster might otherwiseneglect its other duties for too long. However, that seems overlyconservative, especially since bad effects only become obvious whenmany hundreds of bgworkers need to be launched at once. On the otherside of the coin is that the existing logic could result in substantialdelay of bgworker launches, because ServerLoop isn't guaranteed toiterate immediately after a signal arrives. (My attempt to fix thatby using pselect(2) encountered too many portability question marks,and in any case could not help on platforms without pselect().)One could also question the wisdom of using an O(N^2) processingmethod if the system is intended to support so many bgworkers.As a compromise, allow that function to launch up to 100 bgworkersper call (and in consequence, rename it to maybe_start_bgworkers).This will allow any normal parallel-query request for workersto be satisfied immediately during sigusr1_handler, avoiding thequestion of whether ServerLoop will be able to launch more promptly.There is talk of rewriting the postmaster to use a WaitEventSet toavoid the signal-response-delay problem, but I'd argue that this changeshould be kept even after that happens (if it ever does).Backpatch to 9.6 where parallel query was added. The issue existsbefore that, but previous uses of bgworkers typically aren't assensitive to how quickly they get launched.Discussion:https://postgr.es/m/4707.1493221358@sss.pgh.pa.us
1 parent86e640a commite880df2

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1 file changed

+22
-17
lines changed

1 file changed

+22
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‎src/backend/postmaster/postmaster.c

Lines changed: 22 additions & 17 deletions
Original file line numberDiff line numberDiff line change
@@ -413,7 +413,7 @@ static void TerminateChildren(int signal);
413413

414414
staticintCountChildren(inttarget);
415415
staticboolassign_backendlist_entry(RegisteredBgWorker*rw);
416-
staticvoidmaybe_start_bgworker(void);
416+
staticvoidmaybe_start_bgworkers(void);
417417
staticboolCreateOptsFile(intargc,char*argv[],char*fullprogname);
418418
staticpid_tStartChildProcess(AuxProcTypetype);
419419
staticvoidStartAutovacuumWorker(void);
@@ -1318,7 +1318,7 @@ PostmasterMain(int argc, char *argv[])
13181318
pmState=PM_STARTUP;
13191319

13201320
/* Some workers may be scheduled to start now */
1321-
maybe_start_bgworker();
1321+
maybe_start_bgworkers();
13221322

13231323
status=ServerLoop();
13241324

@@ -1785,7 +1785,7 @@ ServerLoop(void)
17851785

17861786
/* Get other worker processes running, if needed */
17871787
if (StartWorkerNeeded||HaveCrashedWorker)
1788-
maybe_start_bgworker();
1788+
maybe_start_bgworkers();
17891789

17901790
#ifdefHAVE_PTHREAD_IS_THREADED_NP
17911791

@@ -2823,7 +2823,7 @@ reaper(SIGNAL_ARGS)
28232823
PgStatPID=pgstat_start();
28242824

28252825
/* workers may be scheduled to start now */
2826-
maybe_start_bgworker();
2826+
maybe_start_bgworkers();
28272827

28282828
/* at this point we are really open for business */
28292829
ereport(LOG,
@@ -4970,7 +4970,7 @@ sigusr1_handler(SIGNAL_ARGS)
49704970
}
49714971

49724972
if (StartWorkerNeeded||HaveCrashedWorker)
4973-
maybe_start_bgworker();
4973+
maybe_start_bgworkers();
49744974

49754975
if (CheckPostmasterSignal(PMSIGNAL_WAKEN_ARCHIVER)&&
49764976
PgArchPID!=0)
@@ -5679,22 +5679,23 @@ assign_backendlist_entry(RegisteredBgWorker *rw)
56795679
}
56805680

56815681
/*
5682-
* If the time is right, startonebackground worker.
5682+
* If the time is right, start background worker(s).
56835683
*
5684-
* As a side effect, the bgworker control variables are set or reset whenever
5685-
* there are more workers to start after this one, and whenever the overall
5686-
* system state requires it.
5684+
* As a side effect, the bgworker control variables are set or reset
5685+
* depending on whether more workers may need to be started.
56875686
*
5688-
*The reason we start at most one workerper call is to avoid consuming the
5687+
*We limit the number of workers startedper call, to avoid consuming the
56895688
* postmaster's attention for too long when many such requests are pending.
56905689
* As long as StartWorkerNeeded is true, ServerLoop will not block and will
56915690
* call this function again after dealing with any other issues.
56925691
*/
56935692
staticvoid
5694-
maybe_start_bgworker(void)
5693+
maybe_start_bgworkers(void)
56955694
{
5696-
slist_mutable_iteriter;
5695+
#defineMAX_BGWORKERS_TO_LAUNCH 100
5696+
intnum_launched=0;
56975697
TimestampTznow=0;
5698+
slist_mutable_iteriter;
56985699

56995700
/*
57005701
* During crash recovery, we have no need to be called until the state
@@ -5779,12 +5780,16 @@ maybe_start_bgworker(void)
57795780
}
57805781

57815782
/*
5782-
* Quit, but have ServerLoop call us again to look for additional
5783-
* ready-to-run workers. There might not be any, but we'll find
5784-
* out the next time we run.
5783+
* If we've launched as many workers as allowed, quit, but have
5784+
* ServerLoop call us again to look for additional ready-to-run
5785+
* workers. There might not be any, but we'll find out the next
5786+
* time we run.
57855787
*/
5786-
StartWorkerNeeded= true;
5787-
return;
5788+
if (++num_launched >=MAX_BGWORKERS_TO_LAUNCH)
5789+
{
5790+
StartWorkerNeeded= true;
5791+
return;
5792+
}
57885793
}
57895794
}
57905795
}

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