in addition to "DataFileRead", actually we have more session waiting on "extend", and we enabled log_lock, for example 2025-06-24 18:00:11.368 xxxx:[1865315]:[4-1]:mbsLOG: process 1865315 still waiting for ExclusiveLock on extension of relation 14658239 of database 16384 after 1000.161 ms 2025-06-24 18:00:11.368 xxxx:mbs@pgaaweb:[1865315]:[5-1]:mbsDETAIL: Process holding the lock: 1748100. Wait queue: 232658, 606235, 1865315, 1939296, 1368901, 2472806, 2072266, 963893, 1304751, 1084094, 875968, 451652, 672314, 1240983, 2085501, 851401, 2043926, 2522611, 889816, 191663, 1090944, 536057, 1810868, 1240510, 1195564, 1067298.
see both "DataFileRead" and "extend" , each time last 2 seconds and automatically recoverred. for this "extend" wait_event, in addtion IO, OS page_cache and PG buffer cache, or bgwriter,checkpointer could be impact that too? Thanks, James James Pang <jamespang...@gmail.com> 於 2025年6月26日週四 下午2:47寫道： > 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 >>> > >>> > >>> >>> >>
