Re: many sessions waiting DataFileRead and extend

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
> >
> >
>
>