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Commitaa1351f

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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 parentfda4fec commitaa1351f

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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
@@ -421,7 +421,7 @@ static void TerminateChildren(int signal);
421421

422422
staticintCountChildren(inttarget);
423423
staticboolassign_backendlist_entry(RegisteredBgWorker*rw);
424-
staticvoidmaybe_start_bgworker(void);
424+
staticvoidmaybe_start_bgworkers(void);
425425
staticboolCreateOptsFile(intargc,char*argv[],char*fullprogname);
426426
staticpid_tStartChildProcess(AuxProcTypetype);
427427
staticvoidStartAutovacuumWorker(void);
@@ -1346,7 +1346,7 @@ PostmasterMain(int argc, char *argv[])
13461346
pmState=PM_STARTUP;
13471347

13481348
/* Some workers may be scheduled to start now */
1349-
maybe_start_bgworker();
1349+
maybe_start_bgworkers();
13501350

13511351
status=ServerLoop();
13521352

@@ -1813,7 +1813,7 @@ ServerLoop(void)
18131813

18141814
/* Get other worker processes running, if needed */
18151815
if (StartWorkerNeeded||HaveCrashedWorker)
1816-
maybe_start_bgworker();
1816+
maybe_start_bgworkers();
18171817

18181818
#ifdefHAVE_PTHREAD_IS_THREADED_NP
18191819

@@ -2859,7 +2859,7 @@ reaper(SIGNAL_ARGS)
28592859
PgStatPID=pgstat_start();
28602860

28612861
/* workers may be scheduled to start now */
2862-
maybe_start_bgworker();
2862+
maybe_start_bgworkers();
28632863

28642864
/* at this point we are really open for business */
28652865
ereport(LOG,
@@ -5026,7 +5026,7 @@ sigusr1_handler(SIGNAL_ARGS)
50265026
}
50275027

50285028
if (StartWorkerNeeded||HaveCrashedWorker)
5029-
maybe_start_bgworker();
5029+
maybe_start_bgworkers();
50305030

50315031
if (CheckPostmasterSignal(PMSIGNAL_WAKEN_ARCHIVER)&&
50325032
PgArchPID!=0)
@@ -5726,22 +5726,23 @@ assign_backendlist_entry(RegisteredBgWorker *rw)
57265726
}
57275727

57285728
/*
5729-
* If the time is right, startonebackground worker.
5729+
* If the time is right, start background worker(s).
57305730
*
5731-
* As a side effect, the bgworker control variables are set or reset whenever
5732-
* there are more workers to start after this one, and whenever the overall
5733-
* system state requires it.
5731+
* As a side effect, the bgworker control variables are set or reset
5732+
* depending on whether more workers may need to be started.
57345733
*
5735-
*The reason we start at most one workerper call is to avoid consuming the
5734+
*We limit the number of workers startedper call, to avoid consuming the
57365735
* postmaster's attention for too long when many such requests are pending.
57375736
* As long as StartWorkerNeeded is true, ServerLoop will not block and will
57385737
* call this function again after dealing with any other issues.
57395738
*/
57405739
staticvoid
5741-
maybe_start_bgworker(void)
5740+
maybe_start_bgworkers(void)
57425741
{
5743-
slist_mutable_iteriter;
5742+
#defineMAX_BGWORKERS_TO_LAUNCH 100
5743+
intnum_launched=0;
57445744
TimestampTznow=0;
5745+
slist_mutable_iteriter;
57455746

57465747
/*
57475748
* During crash recovery, we have no need to be called until the state
@@ -5826,12 +5827,16 @@ maybe_start_bgworker(void)
58265827
}
58275828

58285829
/*
5829-
* Quit, but have ServerLoop call us again to look for additional
5830-
* ready-to-run workers. There might not be any, but we'll find
5831-
* out the next time we run.
5830+
* If we've launched as many workers as allowed, quit, but have
5831+
* ServerLoop call us again to look for additional ready-to-run
5832+
* workers. There might not be any, but we'll find out the next
5833+
* time we run.
58325834
*/
5833-
StartWorkerNeeded= true;
5834-
return;
5835+
if (++num_launched >=MAX_BGWORKERS_TO_LAUNCH)
5836+
{
5837+
StartWorkerNeeded= true;
5838+
return;
5839+
}
58355840
}
58365841
}
58375842
}

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