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.hoogl...@gmail.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 <jamespang...@gmail.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.hoogl...@gmail.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.a...@cybertec.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 >> > >> > >> >> >
