Re: many sessions waiting DataFileRead and extend

we faced this issue 3 times this week, each time last only 2 seconds, so
not easy to run perf in peak business time to capture that, anyway, I will
try. before that, I want to understand if "os page cache" or "pg buffer
cache" can contribute to the wait_event time "extend" and "DataFileRead",
or bgwriter ,checkpoint flushing data to disk can impact that too ? we
enable bgwriter , and we see checkpointer get scheduled by "wal" during the
time, so I just increased max_wal_size to make checkpoint scheduled in
longer time.

Thanks,

James

Frits Hoogland <frits(dot)hoogland(at)gmail(dot)com> 於 2025年6月26日週四 下午2:40寫道：

> Okay. So it's a situation that is reproducable.
> And like was mentioned, the system time (percentage) is very high.
> Is this a physical machine, or a virtual machine?
>
> The next thing to do, is use perf to record about 20 seconds or so during
> a period of time when you see this behavior (perf record -g, taking the
> backtrace with it).
> This records (samples) the backtraces of on cpu tasks, from which you then
> can derive what they are doing, for which you should see lots of tasks in
> kernel space, and what that is, using perf report.
>
> *Frits Hoogland*
>
>
>
>
> On 26 Jun 2025, at 04:32, James Pang <jamespang886(at)gmail(dot)com> wrote:
>
> thans for you suggestions, we have iowait from sar command too, copy here,
> checking with infra team not found abnormal IO activities either.
> 02:00:01 PM CPU %usr %nice %sys %iowait %irq %soft %steal
> %guest %gnice %idle
> 02:00:03 PM all 15.92 0.00 43.02 0.65 0.76 2.56
> 0.00 0.00 0.00 37.09
> 02:00:03 PM 0 17.59 0.00 46.73 1.01 0.50 0.50
> 0.00 0.00 0.00 33.67
> 02:00:03 PM 1 9.50 0.00 61.50 0.50 0.50 1.00
> 0.00 0.00 0.00 27.00
> 02:00:03 PM 2 20.71 0.00 44.44 1.01 0.51 0.51
> 0.00 0.00 0.00 32.83
> 02:00:03 PM 3 14.00 0.00 51.50 2.00 1.00 1.00
> 0.00 0.00 0.00 30.50
> 02:00:03 PM 4 6.57 0.00 52.53 0.51 0.51 3.54
> 0.00 0.00 0.00 36.36
> 02:00:03 PM 5 10.20 0.00 49.49 1.02 1.53 0.00
> 0.00 0.00 0.00 37.76
> 02:00:03 PM 6 27.64 0.00 41.21 0.50 0.50 0.50
> 0.00 0.00 0.00 29.65
> 02:00:03 PM 7 9.05 0.00 50.75 0.50 1.01 0.50
> 0.00 0.00 0.00 38.19
> 02:00:03 PM 8 12.18 0.00 49.75 0.51 0.51 0.51
> 0.00 0.00 0.00 36.55
> 02:00:03 PM 9 13.00 0.00 9.50 0.50 1.50 15.50
> 0.00 0.00 0.00 60.00
> 02:00:03 PM 10 15.58 0.00 46.23 0.00 0.50 0.50
> 0.00 0.00 0.00 37.19
> 02:00:03 PM 11 20.71 0.00 10.10 0.00 1.01 14.14
> 0.00 0.00 0.00 54.04
> 02:00:03 PM 12 21.00 0.00 37.00 0.50 1.00 1.00
> 0.00 0.00 0.00 39.50
> 02:00:03 PM 13 13.57 0.00 45.73 1.01 1.01 1.01
> 0.00 0.00 0.00 37.69
> 02:00:03 PM 14 18.18 0.00 39.39 1.01 0.51 0.51 0.00
> 0.00 0.00 40.40
> 02:00:03 PM 15 14.00 0.00 49.50 0.50 0.50 3.50 0.00
> 0.00 0.00 32.00
> 02:00:03 PM 16 19.39 0.00 39.80 1.02 1.53 0.51 0.00
> 0.00 0.00 37.76
> 02:00:03 PM 17 16.75 0.00 45.18 1.52 1.02 2.54 0.00
> 0.00 0.00 32.99
> 02:00:03 PM 18 12.63 0.00 50.00 0.00 1.01 0.00 0.00
> 0.00 0.00 36.36
> 02:00:03 PM 19 5.56 0.00 82.32 0.00 0.00 0.00 0.00
> 0.00 0.00 12.12
> 02:00:03 PM 20 15.08 0.00 48.24 0.50 0.50 3.52 0.00
> 0.00 0.00 32.16
> 02:00:03 PM 21 17.68 0.00 9.09 0.51 1.52 13.64 0.00
> 0.00 0.00 57.58
> 02:00:03 PM 22 13.13 0.00 43.94 0.51 0.51 0.51 0.00
> 0.00 0.00 41.41
> 02:00:03 PM 23 14.07 0.00 42.71 0.50 0.50 0.50 0.00
> 0.00 0.00 41.71
> 02:00:03 PM 24 13.13 0.00 41.92 1.01 0.51 0.51 0.00
> 0.00 0.00 42.93
> 02:00:03 PM 25 16.58 0.00 47.74 0.50 1.01 0.50 0.00
> 0.00 0.00 33.67
> 02:00:03 PM 26 16.58 0.00 46.73 0.50 1.01 0.50 0.00
> 0.00 0.00 34.67
> 02:00:03 PM 27 45.50 0.00 54.50 0.00 0.00 0.00 0.00
> 0.00 0.00 0.00
> 02:00:03 PM 28 6.06 0.00 32.32 0.00 0.51 13.13 0.00
> 0.00 0.00 47.98
> 02:00:03 PM 29 13.93 0.00 44.78 1.00 1.00 0.50 0.00
> 0.00 0.00 38.81
> 02:00:03 PM 30 11.56 0.00 57.79 0.00 0.50 1.01 0.00
> 0.00 0.00 29.15
> 02:00:03 PM 31 33.85 0.00 9.23 0.51 1.54 0.51 0.00
> 0.00 0.00 54.36
> 02:00:03 PM 32 30.15 0.00 41.71 0.50 0.50 1.51 0.00
> 0.00 0.00 25.63
>
> Thanks,
>
> James
>
> Frits Hoogland <frits(dot)hoogland(at)gmail(dot)com> 於 2025年6月25日週三 下午10:27寫道：
>
>>
>>
>> > On 25 Jun 2025, at 07:59, Laurenz Albe <laurenz(dot)albe(at)cybertec(dot)at>
>> wrote:
>> >
>> > On Wed, 2025-06-25 at 11:15 +0800, James Pang wrote:
>> >> pgv14, RHEL8, xfs , we suddenly see tens of sessions waiting on
>> "DataFileRead" and
>> >> "extend", it last about 2 seconds(based on pg_stat_activity query) ,
>> during the
>> >> waiting time, "%sys" cpu increased to 80% , but from "iostat" , no
>> high iops and
>> >> io read/write latency increased either.
>> >
>> > Run "sar -P all 1" and see if "%iowait" is high.
>> I would (strongly) advise against the use of iowait as an indicator. It
>> is a kernel approximation of time spent in IO from which cannot be use used
>> in any sensible way other than possibly you're doing IO.
>> First of all, iowait is not a kernel state, and therefore it's taken from
>> idle. This means that if there is no, or too little, idle time, iowait that
>> should be there is gone.
>> Second, the calculation to transfer idle time to iowait is done for
>> synchronous IO calls only. Which currently is not a problem for postgres
>> because it uses exactly that, but in the future it might.
>> Very roughly put, what the kernel does is keep a counter of tasks
>> currently in certain system IO calls, and then try to express that using
>> iowait. The time in IO wait can't be used calculate any IO facts.
>>
>> In that sense, it puts it in the same area as the load figure:
>> indicative, but mostly useless because it doesn't give you any facts about
>> what it is expressing.
>> >
>> > Check if you have transparent hugepages enabled:
>> >
>> > cat /sys/kernel/mm/transparent_hugepage/enabled
>> >
>> > If they are enabled, disable them and see if it makes a difference.
>> >
>> > I am only guessing here.
>> Absolutely. Anything that is using signficant amounts of memory and is
>> not created to take advantage of transparent hugepages will probably
>> experience more downsides from THP than it helps.
>> >
>> >> many sessions were running same "DELETE FROM xxxx" in parallel waiting
>> on "extend"
>> >> and "DataFileRead", there are triggers in this table "After delete" to
>> insert/delete
>> >> other tables in the tigger.
>> >
>> > One thing that almost certainly would improve your situation is to run
>> fewer
>> > concurrent statements, for example by using a reasonably sized
>> connection pool.
>> This is true if the limits of the IO device, or anything towards to IO
>> device or devices are hit.
>> And in general, high "%sys", alias lots of time spent in kernel mode
>> alias system time indicates lots of time spent in system calls, which is
>> what the read and write calls in postgres are.
>> Therefore these figures suggest blocking for IO, for which Laurenz'
>> advise to lower the amount of concurrent sessions doing IO in general makes
>> sense.
>> A more nuanced analysis: if IO requests get queued, these will wait in
>> 'D' state in linux, which by definition is off cpu, and thus do not spent
>> cpu (system/kernel) time.
>>
>> What sounds suspicious is that you indicate you indicate there is you see
>> no signficant change in the amount of IO in iostat.
>>
>> In order to understand this, you will have to first carefully find the
>> actual IO physical IO devices that you are using for postgres IO.
>> In current linux this can be tricky, depending on how the hardware or
>> virtual machine looks like, and how the disks are arranged in linux.
>> What you need to determine is which actual disk devices are used, and
>> what their limits are.
>> Limits for any disk are IOPS (operations per second) and MBPS (megabytes
>> per second -> bandwdith).
>>
>> There is an additional thing to realize, which makes this really tricky:
>> postgres for common IO uses buffered IO.
>> Buffered IO means any read or write will use the linux buffercache, and
>> read or writes can be served from the buffercache if possible.
>>
>> So in your case, if you managed to make the database perform identical
>> read or write requests, this could result in a difference of amounts of
>> read and write IOs served from the cache, which can make an enormous
>> amounts of difference for how fast these requests are served. If somehow
>> you managed to make the operating system choose to use the physical IO
>> path, you will see significant amounts time spent on that, which will have
>> IO related wait events.
>>
>> Not a simple answer, but this is how it works.
>>
>> So I would suggest checking the difference between the situation of when
>> it's doing the same which is considered well performing versus badly
>> performing.
>>
>>
>> >
>> > Yours,
>> > Laurenz Albe
>> >
>> >
>>
>>
>