Received: from malur.postgresql.org ([217.196.149.56]) by arkaria.postgresql.org with esmtps (TLS1.3) tls TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384 (Exim 4.94.2) (envelope-from ) id 1uUgP7-006yvA-Gs for pgsql-performance@arkaria.postgresql.org; Thu, 26 Jun 2025 06:48:13 +0000 Received: from localhost ([127.0.0.1] helo=malur.postgresql.org) by malur.postgresql.org with esmtp (Exim 4.94.2) (envelope-from ) id 1uUgP4-008owI-Hl for pgsql-performance@arkaria.postgresql.org; Thu, 26 Jun 2025 06:48:11 +0000 Received: from makus.postgresql.org ([2001:4800:3e1:1::229]) by malur.postgresql.org with esmtps (TLS1.3) tls TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384 (Exim 4.94.2) (envelope-from ) id 1uUgP3-008ow9-Vm for pgsql-performance@lists.postgresql.org; Thu, 26 Jun 2025 06:48:10 +0000 Received: from mail-yw1-x1130.google.com ([2607:f8b0:4864:20::1130]) by makus.postgresql.org with esmtps (TLS1.3) tls TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256 (Exim 4.96) (envelope-from ) id 1uUgP2-0043dt-0h for pgsql-performance@lists.postgresql.org; Thu, 26 Jun 2025 06:48:09 +0000 Received: by mail-yw1-x1130.google.com with SMTP id 00721157ae682-70a57a8ffc3so7337797b3.0 for ; Wed, 25 Jun 2025 23:48:08 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20230601; t=1750920487; x=1751525287; darn=lists.postgresql.org; h=cc:to:subject:message-id:date:from:in-reply-to:references :mime-version:from:to:cc:subject:date:message-id:reply-to; bh=vSg2DIfg1SLe1G2oGFvFkPGdupgqA1X/10CBUZHtCKs=; b=Qra+BlLMDEc/q0gYOLnJXDDGk9fAGEsnUObwI0NFaWxAUPBWRogJXfmlscdb7uZVoX TYXEZs1+aXKH/M4axLQAP0wockcz23hS+X2w6dape/TQmdHGNJxh4D/ACe6bvVOj3nQN N89KmNnBWZildBPOZU6YIuyQ20UAPX+bI5wg5aKMbE0S7kUsRFTHDjteigvlcglzbuwn 5LWjmKlRhB6QxchAci0eU0I9hiiJic6Q/NaOzW3supAPjB2sffiMao9kpAZNUVPxZoLm 2Ys9osqBPI/3IYIz3GOWjedq/E5LzXfmjLwUWvmdLPZ+LuWuOCQ09ZkwD4F/b0VVhHvn YR9g== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1750920487; x=1751525287; h=cc:to:subject:message-id:date:from:in-reply-to:references :mime-version:x-gm-message-state:from:to:cc:subject:date:message-id :reply-to; bh=vSg2DIfg1SLe1G2oGFvFkPGdupgqA1X/10CBUZHtCKs=; b=rIm6fd7hPG/lkZvJgyIJQafBv09JdPnykkvob1tE/JJCYHCM4HjT4T/8/slIPt7AeZ gyIOs2amWTi+vD7uPm8LiOEwMYr1+ZfKThFy4IkK4pKNRZrZwyRhrnpuYXhVuQcMuoSw 0jkfPDs+6aUA9yDBiauK2unB63f9o98bbxtAroPfbA5vf8IWMUIyUN5x59K2ckNi8Mzd gIZ5RLKmIbAeigMeXOylFyGP+ofOuhaMHZkoSiZor8MD3uHOoLH5M6ysFignIaLbHaUE ncjDfUG3ticdO53MgMhfvMk8bC5BhspPI+EsXJdri4Pz85lMTtLjnX8omzJS6hH2IxEE r15A== X-Forwarded-Encrypted: i=1; AJvYcCXMAb7pf9buRJ0bm9URcp4OKLtF0Olz6XIYMqWMybQNLGZW8CTgtY7j5kr7Gmy+GFErHRWOsCNIsUIoNEExEY+HgQ==@lists.postgresql.org X-Gm-Message-State: AOJu0Ywk2qkaPmxoJ8m+IzcgNFFFzArONHpn2EXMZm/P5BCIW5hdjYop gYn7v8yegKU3HxRbCR59vl71v5uGF9PE9gRgtOgfPWh+bu6AqMvYEcD3shAh8Yqud0y0qvXFv5A QhVDZF4dIoUhZtuf1QcsX50miphhj5G4= X-Gm-Gg: ASbGncvShEvFqbGNjojvG6YjutLHERz8zIVg1yd0Ta46C7zETiOXWOHdYVhQ4f1WD/k XMows9Wha9lBA1Y/7noIbg0WlXrMA3iTQc4RQSRf1V1quniNvaBQi9A7EEgetjA3PY7rdOZ5E+F iID2rtfPw6Eo71AaYUYrmIf/wa0IXnigxiHMtxUJIRkGU= X-Google-Smtp-Source: AGHT+IE5yHisBvRbEOSW/fTgc00uAzP6ji8AOLIjJH0kP9nIEmZ9MbK3v6u4gSjyNwwytT3OKmPbSksQIRk5y4S1A00= X-Received: by 2002:a05:690c:25c6:b0:70c:8f0c:f923 with SMTP id 00721157ae682-71406dc6c9bmr90252467b3.18.1750920487326; Wed, 25 Jun 2025 23:48:07 -0700 (PDT) MIME-Version: 1.0 References: <66879d8bd44148f2ef1dcde1eff056e6c671306e.camel@cybertec.at> In-Reply-To: From: James Pang Date: Thu, 26 Jun 2025 14:47:56 +0800 X-Gm-Features: Ac12FXwwPdhWWELnQZAd-Ppsv7ICHHy7EO0y6CKsvvpL22uRfcBoQ29eJ559YBk Message-ID: Subject: Re: many sessions waiting DataFileRead and extend To: Frits Hoogland Cc: Laurenz Albe , pgsql-performance@lists.postgresql.org Content-Type: multipart/alternative; boundary="000000000000ad59aa063873f126" List-Id: List-Help: List-Subscribe: List-Post: List-Owner: List-Archive: Archived-At: Precedence: bulk --000000000000ad59aa063873f126 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable 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 =E6=96=BC 2025=E5=B9=B46=E6=9C=88= 26=E6=97=A5=E9=80=B1=E5=9B=9B =E4=B8=8B=E5=8D=882:40=E5=AF=AB=E9=81=93=EF= =BC=9A > 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 the= n > 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 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 =E6=96=BC 2025=E5=B9=B46=E6=9C= =8825=E6=97=A5=E9=80=B1=E4=B8=89 =E4=B8=8B=E5=8D=8810:27=E5=AF=AB=E9=81=93= =EF=BC=9A > >> >> >> > On 25 Jun 2025, at 07:59, Laurenz Albe >> 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 u= sed >> 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 fro= m >> idle. This means that if there is no, or too little, idle time, iowait t= hat >> 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 abo= ut >> 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 waitin= g >> on "extend" >> >> and "DataFileRead", there are triggers in this table "After delete" t= o >> 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 ma= kes >> 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 spen= t >> cpu (system/kernel) time. >> >> What sounds suspicious is that you indicate you indicate there is you se= e >> 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 ha= ve >> 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 >> > >> > >> >> > --000000000000ad59aa063873f126 Content-Type: text/html; charset="UTF-8" Content-Transfer-Encoding: quoted-printable
=C2=A0 =C2=A0we faced this issue 3 times this week, each t= ime last only 2 seconds, so not easy to run perf in peak business time to c= apture that, anyway, I will try. before that, I want to understand if "= ;os page cache" or "pg buffer cache" can contribute to the w= ait_event time "extend" and "DataFileRead", or bgwriter= ,checkpoint flushing data to disk can impact that too ?=C2=A0 we enable bg= writer , and we see checkpointer get scheduled by "wal" during th= e time, so I just increased max_wal_size to make checkpoint scheduled in lo= nger time.=C2=A0

Thanks,

James= =C2=A0


Frits Hoogland <frits.hoogland@gmail.com> = =E6=96=BC 2025=E5=B9=B46=E6=9C=8826=E6=97=A5=E9=80=B1=E5=9B=9B =E4=B8=8B=E5= =8D=882:40=E5=AF=AB=E9=81=93=EF=BC=9A
Okay. So it's a situation that is reproducab= le.
And like was mentioned, the system time (percentage) is very high.<= /div>
Is this a physical machine, or a virtual machine?

<= /div>
The next thing to do, is use perf to record about 20 seconds or s= o during a period of time when you see this behavior (perf record -g, takin= g the backtrace with it).
This records (samples) the backtraces o= f on cpu tasks, from which you then can derive what they are doing, for whi= ch you should see lots of tasks in kernel space, and what that is, using pe= rf report.

Frits Hoogland

<= /div>


On 26 Jun 2025, at 04:32, James Pan= g <jamespang= 886@gmail.com> wrote:

thans for you s= uggestions, we have iowait from sar command too, copy here, checking with i= nfra team not found abnormal IO activities either.=C2=A0=C2=A0
02:00:01= PM =C2=A0CPU =C2=A0 =C2=A0%usr =C2=A0 %nice =C2=A0 =C2=A0%sys %iowait =C2= =A0 =C2=A0%irq =C2=A0 %soft =C2=A0%steal =C2=A0%guest =C2=A0%gnice =C2=A0 %= idle
02:00:03 PM =C2=A0all=C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A015.92 =C2=A0= =C2=A00.00 =C2=A0 43.02 =C2=A0 =C2=A00.65 =C2=A0 =C2=A00.76 =C2=A0 =C2=A02= .56 =C2=A0 =C2=A00.00 =C2=A0 =C2=A00.00 =C2=A0 =C2=A00.00 =C2=A0 37.09
0= 2:00:03 PM =C2=A0 =C2=A00=C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A017.59 =C2=A0 =C2= =A00.00 =C2=A0 46.73 =C2=A0 =C2=A01.01 =C2=A0 =C2=A00.50 =C2=A0 =C2=A00.50 = =C2=A0 =C2=A00.00 =C2=A0 =C2=A00.00 =C2=A0 =C2=A00.00 =C2=A0 33.67
02:00= :03 PM =C2=A0 =C2=A01=C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 9.50=C2=A0 =C2=A0 = =C2=A00.00 =C2=A0 61.50 =C2=A0 =C2=A00.50 =C2=A0 =C2=A00.50 =C2=A0 =C2=A01.= 00 =C2=A0 =C2=A00.00 =C2=A0 =C2=A00.00 =C2=A0 =C2=A00.00 =C2=A0 27.00
02= :00:03 PM =C2=A0 =C2=A02=C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 20.71 =C2=A0 =C2= =A00.00 =C2=A0 44.44 =C2=A0 =C2=A01.01 =C2=A0 =C2=A00.51 =C2=A0 =C2=A00.51 = =C2=A0 =C2=A00.00 =C2=A0 =C2=A00.00 =C2=A0 =C2=A00.00 =C2=A0 32.83
02:00= :03 PM =C2=A0 =C2=A03=C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A014.00 =C2=A0 =C2=A00= .00 =C2=A0 51.50 =C2=A0 =C2=A02.00 =C2=A0 =C2=A01.00 =C2=A0 =C2=A01.00 =C2= =A0 =C2=A00.00 =C2=A0 =C2=A00.00 =C2=A0 =C2=A00.00 =C2=A0 30.50
02:00:03= PM =C2=A0 =C2=A04=C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 6.57=C2=A0 =C2=A0 =C2= =A00.00 =C2=A0 52.53 =C2=A0 =C2=A00.51 =C2=A0 =C2=A00.51 =C2=A0 =C2=A03.54 = =C2=A0 =C2=A00.00 =C2=A0 =C2=A00.00 =C2=A0 =C2=A00.00 =C2=A0 36.36
02:00= :03 PM =C2=A0 =C2=A05=C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 10.20 =C2=A0 =C2=A0= 0.00 =C2=A0 49.49 =C2=A0 =C2=A01.02 =C2=A0 =C2=A01.53 =C2=A0 =C2=A00.00 =C2= =A0 =C2=A00.00 =C2=A0 =C2=A00.00 =C2=A0 =C2=A00.00 =C2=A0 37.76
02:00:03= PM =C2=A0 =C2=A06=C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 27.64 =C2=A0 =C2=A00.0= 0 =C2=A0 41.21 =C2=A0 =C2=A00.50 =C2=A0 =C2=A00.50 =C2=A0 =C2=A00.50 =C2=A0= =C2=A00.00 =C2=A0 =C2=A00.00 =C2=A0 =C2=A00.00 =C2=A0 29.65
02:00:03 PM= =C2=A0 =C2=A07=C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 9.05 =C2=A0 =C2=A00.00 = =C2=A0 50.75 =C2=A0 =C2=A00.50 =C2=A0 =C2=A01.01 =C2=A0 =C2=A00.50 =C2=A0 = =C2=A00.00 =C2=A0 =C2=A00.00 =C2=A0 =C2=A00.00 =C2=A0 38.19
02:00:03 PM = =C2=A0 =C2=A08=C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 12.18 =C2=A0 =C2=A00.00 = =C2=A0 49.75 =C2=A0 =C2=A00.51 =C2=A0 =C2=A00.51 =C2=A0 =C2=A00.51 =C2=A0 = =C2=A00.00 =C2=A0 =C2=A00.00 =C2=A0 =C2=A00.00 =C2=A0 36.55
02:00:03 PM = =C2=A0 =C2=A09=C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 13.00 =C2=A0 =C2=A00.00 = =C2=A0 =C2=A09.50 =C2=A0 =C2=A00.50 =C2=A0 =C2=A01.50 =C2=A0 15.50 =C2=A0 = =C2=A00.00 =C2=A0 =C2=A00.00 =C2=A0 =C2=A00.00 =C2=A0 60.00
02:00:03 PM = =C2=A0 10=C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A015.58 =C2=A0 =C2=A00.00 =C2=A0 4= 6.23 =C2=A0 =C2=A00.00 =C2=A0 =C2=A00.50 =C2=A0 =C2=A00.50 =C2=A0 =C2=A00.0= 0 =C2=A0 =C2=A00.00 =C2=A0 =C2=A00.00 =C2=A0 37.19
02:00:03 PM =C2=A0 11= =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 20.71 =C2=A0 =C2=A00.00 =C2=A0 10.10 =C2= =A0 =C2=A00.00 =C2=A0 =C2=A01.01 =C2=A0 14.14 =C2=A0 =C2=A00.00 =C2=A0 =C2= =A00.00 =C2=A0 =C2=A00.00 =C2=A0 54.04
02:00:03 PM =C2=A0 12=C2=A0 =C2= =A0 =C2=A0 =C2=A0 =C2=A021.00 =C2=A0 =C2=A00.00 =C2=A0 37.00 =C2=A0 =C2=A00= .50 =C2=A0 =C2=A01.00 =C2=A0 =C2=A01.00 =C2=A0 =C2=A00.00 =C2=A0 =C2=A00.00= =C2=A0 =C2=A00.00 =C2=A0 39.50
02:00:03 PM =C2=A0 13=C2=A0 =C2=A0 =C2= =A0 =C2=A0 =C2=A013.57 =C2=A0 =C2=A00.00 =C2=A0 45.73 =C2=A0 =C2=A01.01 =C2= =A0 =C2=A01.01 =C2=A0 =C2=A01.01 =C2=A0 =C2=A00.00 =C2=A0 =C2=A00.00 =C2=A0= =C2=A00.00 =C2=A0 37.69
02:00:03 PM =C2=A0 14 =C2=A0 18.18 =C2=A0 =C2= =A00.00 =C2=A0 39.39 =C2=A0 =C2=A01.01 =C2=A0 =C2=A00.51 =C2=A0 =C2=A00.51 = =C2=A0 =C2=A00.00 =C2=A0 =C2=A00.00 =C2=A0 =C2=A00.00 =C2=A0 40.40
02:00= :03 PM =C2=A0 15 =C2=A0 14.00 =C2=A0 =C2=A00.00 =C2=A0 49.50 =C2=A0 =C2=A00= .50 =C2=A0 =C2=A00.50 =C2=A0 =C2=A03.50 =C2=A0 =C2=A00.00 =C2=A0 =C2=A00.00= =C2=A0 =C2=A00.00 =C2=A0 32.00
02:00:03 PM =C2=A0 16 =C2=A0 19.39 =C2= =A0 =C2=A00.00 =C2=A0 39.80 =C2=A0 =C2=A01.02 =C2=A0 =C2=A01.53 =C2=A0 =C2= =A00.51 =C2=A0 =C2=A00.00 =C2=A0 =C2=A00.00 =C2=A0 =C2=A00.00 =C2=A0 37.76<= br>02:00:03 PM =C2=A0 17 =C2=A0 16.75 =C2=A0 =C2=A00.00 =C2=A0 45.18 =C2=A0= =C2=A01.52 =C2=A0 =C2=A01.02 =C2=A0 =C2=A02.54 =C2=A0 =C2=A00.00 =C2=A0 = =C2=A00.00 =C2=A0 =C2=A00.00 =C2=A0 32.99
02:00:03 PM =C2=A0 18 =C2=A0 1= 2.63 =C2=A0 =C2=A00.00 =C2=A0 50.00 =C2=A0 =C2=A00.00 =C2=A0 =C2=A01.01 =C2= =A0 =C2=A00.00 =C2=A0 =C2=A00.00 =C2=A0 =C2=A00.00 =C2=A0 =C2=A00.00 =C2=A0= 36.36
02:00:03 PM =C2=A0 19 =C2=A0 =C2=A05.56 =C2=A0 =C2=A00.00 =C2=A0 = 82.32 =C2=A0 =C2=A00.00 =C2=A0 =C2=A00.00 =C2=A0 =C2=A00.00 =C2=A0 =C2=A00.= 00 =C2=A0 =C2=A00.00 =C2=A0 =C2=A00.00 =C2=A0 12.12
02:00:03 PM =C2=A0 2= 0 =C2=A0 15.08 =C2=A0 =C2=A00.00 =C2=A0 48.24 =C2=A0 =C2=A00.50 =C2=A0 =C2= =A00.50 =C2=A0 =C2=A03.52 =C2=A0 =C2=A00.00 =C2=A0 =C2=A00.00 =C2=A0 =C2=A0= 0.00 =C2=A0 32.16
02:00:03 PM =C2=A0 21 =C2=A0 17.68 =C2=A0 =C2=A00.00 = =C2=A0 =C2=A09.09 =C2=A0 =C2=A00.51 =C2=A0 =C2=A01.52 =C2=A0 13.64 =C2=A0 = =C2=A00.00 =C2=A0 =C2=A00.00 =C2=A0 =C2=A00.00 =C2=A0 57.58
02:00:03 PM = =C2=A0 22 =C2=A0 13.13 =C2=A0 =C2=A00.00 =C2=A0 43.94 =C2=A0 =C2=A00.51 =C2= =A0 =C2=A00.51 =C2=A0 =C2=A00.51 =C2=A0 =C2=A00.00 =C2=A0 =C2=A00.00 =C2=A0= =C2=A00.00 =C2=A0 41.41
02:00:03 PM =C2=A0 23 =C2=A0 14.07 =C2=A0 =C2= =A00.00 =C2=A0 42.71 =C2=A0 =C2=A00.50 =C2=A0 =C2=A00.50 =C2=A0 =C2=A00.50 = =C2=A0 =C2=A00.00 =C2=A0 =C2=A00.00 =C2=A0 =C2=A00.00 =C2=A0 41.71
02:00= :03 PM =C2=A0 24 =C2=A0 13.13 =C2=A0 =C2=A00.00 =C2=A0 41.92 =C2=A0 =C2=A01= .01 =C2=A0 =C2=A00.51 =C2=A0 =C2=A00.51 =C2=A0 =C2=A00.00 =C2=A0 =C2=A00.00= =C2=A0 =C2=A00.00 =C2=A0 42.93
02:00:03 PM =C2=A0 25 =C2=A0 16.58 =C2= =A0 =C2=A00.00 =C2=A0 47.74 =C2=A0 =C2=A00.50 =C2=A0 =C2=A01.01 =C2=A0 =C2= =A00.50 =C2=A0 =C2=A00.00 =C2=A0 =C2=A00.00 =C2=A0 =C2=A00.00 =C2=A0 33.67<= br>02:00:03 PM =C2=A0 26 =C2=A0 16.58 =C2=A0 =C2=A00.00 =C2=A0 46.73 =C2=A0= =C2=A00.50 =C2=A0 =C2=A01.01 =C2=A0 =C2=A00.50 =C2=A0 =C2=A00.00 =C2=A0 = =C2=A00.00 =C2=A0 =C2=A00.00 =C2=A0 34.67
02:00:03 PM =C2=A0 27 =C2=A0 4= 5.50 =C2=A0 =C2=A00.00 =C2=A0 54.50 =C2=A0 =C2=A00.00 =C2=A0 =C2=A00.00 =C2= =A0 =C2=A00.00 =C2=A0 =C2=A00.00 =C2=A0 =C2=A00.00 =C2=A0 =C2=A00.00 =C2=A0= =C2=A00.00
02:00:03 PM =C2=A0 28 =C2=A0 =C2=A06.06 =C2=A0 =C2=A00.00 = =C2=A0 32.32 =C2=A0 =C2=A00.00 =C2=A0 =C2=A00.51 =C2=A0 13.13 =C2=A0 =C2=A0= 0.00 =C2=A0 =C2=A00.00 =C2=A0 =C2=A00.00 =C2=A0 47.98
02:00:03 PM =C2=A0= 29 =C2=A0 13.93 =C2=A0 =C2=A00.00 =C2=A0 44.78 =C2=A0 =C2=A01.00 =C2=A0 = =C2=A01.00 =C2=A0 =C2=A00.50 =C2=A0 =C2=A00.00 =C2=A0 =C2=A00.00 =C2=A0 =C2= =A00.00 =C2=A0 38.81
02:00:03 PM =C2=A0 30 =C2=A0 11.56 =C2=A0 =C2=A00.0= 0 =C2=A0 57.79 =C2=A0 =C2=A00.00 =C2=A0 =C2=A00.50 =C2=A0 =C2=A01.01 =C2=A0= =C2=A00.00 =C2=A0 =C2=A00.00 =C2=A0 =C2=A00.00 =C2=A0 29.15
02:00:03 PM= =C2=A0 31 =C2=A0 33.85 =C2=A0 =C2=A00.00 =C2=A0 =C2=A09.23 =C2=A0 =C2=A00.= 51 =C2=A0 =C2=A01.54 =C2=A0 =C2=A00.51 =C2=A0 =C2=A00.00 =C2=A0 =C2=A00.00 = =C2=A0 =C2=A00.00 =C2=A0 54.36
02:00:03 PM =C2=A0 32 =C2=A0 30.15 =C2=A0= =C2=A00.00 =C2=A0 41.71 =C2=A0 =C2=A00.50 =C2=A0 =C2=A00.50 =C2=A0 =C2=A01= .51 =C2=A0 =C2=A00.00 =C2=A0 =C2=A00.00 =C2=A0 =C2=A00.00 =C2=A0 25.63
<= /div>

Thanks,

James=C2=A0
=

= Frits Hoogland <frits.hoogland@gmail.com> =E6=96=BC 2025=E5=B9=B46=E6=9C=8825= =E6=97=A5=E9=80=B1=E4=B8=89 =E4=B8=8B=E5=8D=8810:27=E5=AF=AB=E9=81=93=EF=BC= =9A


> On 25 Jun 2025, at 07:59, Laurenz Albe <laurenz.albe@cybertec.at> wrote:<= br> >
> On Wed, 2025-06-25 at 11:15 +0800, James Pang wrote:
>> pgv14, RHEL8, xfs , we suddenly see tens of sessions waiting on &q= uot;DataFileRead" and
>> "extend", it last about 2 seconds(based on pg_stat_activ= ity query) , during the
>> waiting time, "%sys" cpu increased to 80% , but from &qu= ot;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 fr= om idle. This means that if there is no, or too little, idle time, iowait t= hat should be there is gone.
Second, the calculation to transfer idle time to iowait is done for synchro= nous IO calls only. Which currently is not a problem for postgres because i= t 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:
>
>=C2=A0 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 pa= rallel waiting on "extend"
>> and "DataFileRead", there are triggers in this table &qu= ot;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 connect= ion pool.
This is true if the limits of the IO device, or anything towards to IO devi= ce or devices are hit.
And in general, high "%sys", alias lots of time spent in kernel m= ode alias system time indicates lots of time spent in system calls, which i= s what the read and write calls in postgres are.
Therefore these figures suggest blocking for IO, for which Laurenz' adv= ise to lower the amount of concurrent sessions doing IO in general makes se= nse.
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 spe= nt cpu (system/kernel) time.

What sounds suspicious is that you indicate you indicate there is you see n= o signficant change in the amount of IO in iostat.

In order to understand this, you will have to first carefully find the actu= al IO physical IO devices that you are using for postgres IO.
In current linux this can be tricky, depending on how the hardware or virtu= al 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 pe= r second -> bandwdith).

There is an additional thing to realize, which makes this really tricky: po= stgres for common IO uses buffered IO.
Buffered IO means any read or write will use the linux buffercache, and rea= d 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 an= d write IOs served from the cache, which can make an enormous amounts of di= fference 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 eve= nts.

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 perf= orming.


>
> Yours,
> Laurenz Albe
>
>


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