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Commit9e18eac

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Fix stats collector to recover nicely when system clock goes backwards.
Formerly, if the system clock went backwards, the stats collector wouldfail to update the stats file any more until the clock reading againexceeds whatever timestamp was last written into the stats file. Suchglitches in the clock's behavior are not terribly unlikely on machinesnot using NTP. Such a scenario has been observed to cause regression testfailures in the buildfarm, and it could have bad effects on the behaviorof autovacuum, so it seems prudent to install some defenses.We could directly detect the clock going backwards by addingGetCurrentTimestamp calls in the stats collector's main loop, but thatwould hurt performance on platforms where GetCurrentTimestamp is expensive.To minimize the performance hit in normal cases, adopt a more complicatedscheme wherein backends check for clock skew when reading the stats file,and if they see it, signal the stats collector by sending an extra statsinquiry message. The stats collector does an extra GetCurrentTimestamponly when it receives an inquiry with an apparently out-of-ordertimestamp.To avoid unnecessary GetCurrentTimestamp calls, expand the inquiry messagesto carry the backend's current clock reading as well as its stats cutofftime. The latter, being intentionally slightly in-the-past, would triggermore clock rechecks than we need if it were used for this purpose.We might want to backpatch this change at some point, but let's let itshake out in the buildfarm for awhile first.
1 parent920febd commit9e18eac

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2 files changed

+116
-32
lines changed

2 files changed

+116
-32
lines changed

‎src/backend/postmaster/pgstat.c‎

Lines changed: 114 additions & 31 deletions
Original file line numberDiff line numberDiff line change
@@ -1404,16 +1404,16 @@ pgstat_ping(void)
14041404
* pgstat_send_inquiry() -
14051405
*
14061406
*Notify collector that we need fresh data.
1407-
*ts specifies the minimum acceptable timestamp for the stats file.
14081407
* ----------
14091408
*/
14101409
staticvoid
1411-
pgstat_send_inquiry(TimestampTzts)
1410+
pgstat_send_inquiry(TimestampTzclock_time,TimestampTzcutoff_time)
14121411
{
14131412
PgStat_MsgInquirymsg;
14141413

14151414
pgstat_setheader(&msg.m_hdr,PGSTAT_MTYPE_INQUIRY);
1416-
msg.inquiry_time=ts;
1415+
msg.clock_time=clock_time;
1416+
msg.cutoff_time=cutoff_time;
14171417
pgstat_send(&msg,sizeof(msg));
14181418
}
14191419

@@ -3633,7 +3633,7 @@ pgstat_read_statsfile(Oid onlydb, bool permanent)
36333633

36343634
/*
36353635
* Set the current timestamp (will be kept only in case we can't load an
3636-
* existing statsfile.
3636+
* existing statsfile).
36373637
*/
36383638
globalStats.stat_reset_timestamp=GetCurrentTimestamp();
36393639

@@ -3922,53 +3922,98 @@ pgstat_read_statsfile_timestamp(bool permanent, TimestampTz *ts)
39223922
staticvoid
39233923
backend_read_statsfile(void)
39243924
{
3925-
TimestampTzcur_ts;
3926-
TimestampTzmin_ts;
3925+
TimestampTzmin_ts=0;
3926+
TimestampTzref_ts=0;
39273927
intcount;
39283928

39293929
/* already read it? */
39303930
if (pgStatDBHash)
39313931
return;
39323932
Assert(!pgStatRunningInCollector);
39333933

3934-
/*
3935-
* We set the minimum acceptable timestamp to PGSTAT_STAT_INTERVAL msec
3936-
* before now.This indirectly ensures that the collector needn't write
3937-
* the file more often than PGSTAT_STAT_INTERVAL. In an autovacuum
3938-
* worker, however, we want a lower delay to avoid using stale data, so we
3939-
* use PGSTAT_RETRY_DELAY (since the number of worker is low, this
3940-
* shouldn't be a problem).
3941-
*
3942-
* Note that we don't recompute min_ts after sleeping; so we might end up
3943-
* accepting a file a bit older than PGSTAT_STAT_INTERVAL.In practice
3944-
* that shouldn't happen, though, as long as the sleep time is less than
3945-
* PGSTAT_STAT_INTERVAL; and we don't want to lie to the collector about
3946-
* what our cutoff time really is.
3947-
*/
3948-
cur_ts=GetCurrentTimestamp();
3949-
if (IsAutoVacuumWorkerProcess())
3950-
min_ts=TimestampTzPlusMilliseconds(cur_ts,-PGSTAT_RETRY_DELAY);
3951-
else
3952-
min_ts=TimestampTzPlusMilliseconds(cur_ts,-PGSTAT_STAT_INTERVAL);
3953-
39543934
/*
39553935
* Loop until fresh enough stats file is available or we ran out of time.
39563936
* The stats inquiry message is sent repeatedly in case collector drops
39573937
* it; but not every single time, as that just swamps the collector.
39583938
*/
39593939
for (count=0;count<PGSTAT_POLL_LOOP_COUNT;count++)
39603940
{
3941+
boolok;
39613942
TimestampTzfile_ts=0;
3943+
TimestampTzcur_ts;
39623944

39633945
CHECK_FOR_INTERRUPTS();
39643946

3965-
if (pgstat_read_statsfile_timestamp(false,&file_ts)&&
3966-
file_ts >=min_ts)
3947+
ok=pgstat_read_statsfile_timestamp(false,&file_ts);
3948+
3949+
cur_ts=GetCurrentTimestamp();
3950+
/* Calculate min acceptable timestamp, if we didn't already */
3951+
if (count==0||cur_ts<ref_ts)
3952+
{
3953+
/*
3954+
* We set the minimum acceptable timestamp to PGSTAT_STAT_INTERVAL
3955+
* msec before now. This indirectly ensures that the collector
3956+
* needn't write the file more often than PGSTAT_STAT_INTERVAL.
3957+
* In an autovacuum worker, however, we want a lower delay to
3958+
* avoid using stale data, so we use PGSTAT_RETRY_DELAY (since the
3959+
* number of workers is low, this shouldn't be a problem).
3960+
*
3961+
* We don't recompute min_ts after sleeping, except in the
3962+
* unlikely case that cur_ts went backwards. So we might end up
3963+
* accepting a file a bit older than PGSTAT_STAT_INTERVAL. In
3964+
* practice that shouldn't happen, though, as long as the sleep
3965+
* time is less than PGSTAT_STAT_INTERVAL; and we don't want to
3966+
* tell the collector that our cutoff time is less than what we'd
3967+
* actually accept.
3968+
*/
3969+
ref_ts=cur_ts;
3970+
if (IsAutoVacuumWorkerProcess())
3971+
min_ts=TimestampTzPlusMilliseconds(ref_ts,
3972+
-PGSTAT_RETRY_DELAY);
3973+
else
3974+
min_ts=TimestampTzPlusMilliseconds(ref_ts,
3975+
-PGSTAT_STAT_INTERVAL);
3976+
}
3977+
3978+
/*
3979+
* If the file timestamp is actually newer than cur_ts, we must have
3980+
* had a clock glitch (system time went backwards) or there is clock
3981+
* skew between our processor and the stats collector's processor.
3982+
* Accept the file, but send an inquiry message anyway to make
3983+
* pgstat_recv_inquiry do a sanity check on the collector's time.
3984+
*/
3985+
if (ok&&file_ts>cur_ts)
3986+
{
3987+
/*
3988+
* A small amount of clock skew between processors isn't terribly
3989+
* surprising, but a large difference is worth logging. We
3990+
* arbitrarily define "large" as 1000 msec.
3991+
*/
3992+
if (file_ts >=TimestampTzPlusMilliseconds(cur_ts,1000))
3993+
{
3994+
char*filetime;
3995+
char*mytime;
3996+
3997+
/* Copy because timestamptz_to_str returns a static buffer */
3998+
filetime=pstrdup(timestamptz_to_str(file_ts));
3999+
mytime=pstrdup(timestamptz_to_str(cur_ts));
4000+
elog(LOG,"stats collector's time %s is later than backend local time %s",
4001+
filetime,mytime);
4002+
pfree(filetime);
4003+
pfree(mytime);
4004+
}
4005+
4006+
pgstat_send_inquiry(cur_ts,min_ts);
4007+
break;
4008+
}
4009+
4010+
/* Normal acceptance case: file is not older than cutoff time */
4011+
if (ok&&file_ts >=min_ts)
39674012
break;
39684013

39694014
/* Not there or too old, so kick the collector and wait a bit */
39704015
if ((count %PGSTAT_INQ_LOOP_COUNT)==0)
3971-
pgstat_send_inquiry(min_ts);
4016+
pgstat_send_inquiry(cur_ts,min_ts);
39724017

39734018
pg_usleep(PGSTAT_RETRY_DELAY*1000L);
39744019
}
@@ -4036,8 +4081,46 @@ pgstat_clear_snapshot(void)
40364081
staticvoid
40374082
pgstat_recv_inquiry(PgStat_MsgInquiry*msg,intlen)
40384083
{
4039-
if (msg->inquiry_time>last_statrequest)
4040-
last_statrequest=msg->inquiry_time;
4084+
/*
4085+
* Advance last_statrequest if this requestor has a newer cutoff time
4086+
* than any previous request.
4087+
*/
4088+
if (msg->cutoff_time>last_statrequest)
4089+
last_statrequest=msg->cutoff_time;
4090+
4091+
/*
4092+
* If the requestor's local clock time is older than last_statwrite, we
4093+
* should suspect a clock glitch, ie system time going backwards; though
4094+
* the more likely explanation is just delayed message receipt. It is
4095+
* worth expending a GetCurrentTimestamp call to be sure, since a large
4096+
* retreat in the system clock reading could otherwise cause us to neglect
4097+
* to update the stats file for a long time.
4098+
*/
4099+
if (msg->clock_time<last_statwrite)
4100+
{
4101+
TimestampTzcur_ts=GetCurrentTimestamp();
4102+
4103+
if (cur_ts<last_statwrite)
4104+
{
4105+
/*
4106+
* Sure enough, time went backwards. Force a new stats file write
4107+
* to get back in sync; but first, log a complaint.
4108+
*/
4109+
char*writetime;
4110+
char*mytime;
4111+
4112+
/* Copy because timestamptz_to_str returns a static buffer */
4113+
writetime=pstrdup(timestamptz_to_str(last_statwrite));
4114+
mytime=pstrdup(timestamptz_to_str(cur_ts));
4115+
elog(LOG,"last_statwrite %s is later than collector's time %s",
4116+
writetime,mytime);
4117+
pfree(writetime);
4118+
pfree(mytime);
4119+
4120+
last_statrequest=cur_ts;
4121+
last_statwrite=last_statrequest-1;
4122+
}
4123+
}
40414124
}
40424125

40434126

‎src/include/pgstat.h‎

Lines changed: 2 additions & 1 deletion
Original file line numberDiff line numberDiff line change
@@ -203,7 +203,8 @@ typedef struct PgStat_MsgDummy
203203
typedefstructPgStat_MsgInquiry
204204
{
205205
PgStat_MsgHdrm_hdr;
206-
TimestampTzinquiry_time;/* minimum acceptable file timestamp */
206+
TimestampTzclock_time;/* observed local clock time */
207+
TimestampTzcutoff_time;/* minimum acceptable file timestamp */
207208
}PgStat_MsgInquiry;
208209

209210

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