Date: Mon, 6 May 2024 13:47:01 +0200
From: Paul Menzel <pmenzel@...gen.mpg.de>
To: Holger Kiehl <Holger.Kiehl@....de>
Cc: linux-kernel@...r.kernel.org, linux-raid@...r.kernel.org,
 linux-block@...r.kernel.org, Jens Axboe <axboe@...nel.dk>,
 linux-ext4@...r.kernel.org, Theodore Ts'o <tytso@....edu>
Subject: Re: Massive slowdown in kernels as of 6.x

Dear Holger,


Thank you for your report.

Am 06.05.24 um 13:31 schrieb Holger Kiehl:

> on a 4 socket file server distributing ~90 million files with
> ~130TiB of data daily, I see a massive slowdown of IO operation
> after some time (sometimes in less then a day). This slowdown
> only started as of Kernel 6.x and does not happen with 5.15.x.
> Have so far tried, 6.0.9, 6.1.27 and 6.6.30 kernel and they all
> have this same slowdown effect after some time. If the load is
> taken away from the server and it is nearly idle, it still has
> this slowdown effect and only recovers after some hours by itself.
> During this slowdown and idle time I had a look at an rsync
> process with strace that was uploading some small files to the
> server and could see the slowdown here was in the rename() system
> call, all other system calls (read(), write(), newfstatat(),
> openat(), fchmod(), etc) where not effected:
> 
>     rename(".27095571.iXVMMT", "27095571")  = 0 <18.305817>
>     rename(".272629ef.22gv2x", "272629ef")  = 0 <18.325222>
>     rename(".275fbacf.UBj6J5", "275fbacf")  = 0 <18.317571>
>     rename(".277ab7da.K5y144", "277ab7da")  = 0 <18.312568>
>     rename(".27873039.ZQ4Lum", "27873039")  = 0 <18.310120>
>     rename(".27ebf01f.t1FKeU", "27ebf01f")  = 0 <18.376816>
>     rename(".27f97e6a.kJqqfL", "27f97e6a")  = 0 <18.290618>
>     rename(".28078cd9.rV7JdN", "28078cd9")  = 0 <18.315415>
>     rename(".28105bb4.gljiDk", "28105bb4")  = 0 <18.325392>
>     rename(".282209b1.Cy3Wt2", "282209b1")  = 0 <30.188303>
>     rename(".28888272.aUCxRj", "28888272")  = 0 <18.263236>
>     rename(".288d8408.XjfGbH", "288d8408")  = 0 <18.312444>
>     rename(".2897f455.hm3FG6", "2897f455")  = 0 <18.281729>
>     rename(".28d7d7e8.pzMMF6", "28d7d7e8")  = 0 <18.281402>
>     rename(".28d9a820.KQuaM0", "28d9a820")  = 0 <32.620562>
>     rename(".294ae845.8Y6vYR", "294ae845")  = 0 <18.289532>
>     rename(".294fee3f.eccu2p", "294fee3f")  = 0 <18.260564>
>     rename(".29581b50.zPTjTh", "29581b50")  = 0 <18.314536>
>     rename(".2975d45f.l5FUYX", "2975d45f")  = 0 <18.293864>
>     rename(".29b3770a.tlNMvb", "29b3770a")  = 0 <0.000062>
>     rename(".29c5e6ee.EexCwZ", "29c5e6ee")  = 0 <18.268144>
>     rename(".29d23183.sLqxpd", "29d23183")  = 0 <18.344478>
>     rename(".29d4f65.oyjRWj", "29d4f65")    = 0 <18.553610>
>     rename(".29dcfab1.Y47Z1B", "29dcfab1")  = 0 <18.339336>
>     rename(".29f26c7c.KNZXEe", "29f26c7c")  = 0 <18.372242>
>     rename(".2a09907b.SXIgev", "2a09907b")  = 0 <18.317119>
>     rename(".2a0c499c.8DiCsM", "2a0c499c")  = 0 <18.380393>
>     rename(".2a64b7e8.FPnsB3", "2a64b7e8")  = 0 <18.372004>
>     rename(".2a6765c9.t7Z0hj", "2a6765c9")  = 0 <18.296044>
>     rename(".2a83d78f.UJVoMu", "2a83d78f")  = 0 <18.380678>
>     rename(".2a94e724.AorYof", "2a94e724")  = 0 <18.360716>
>     rename(".2a9ea651.EWpBHM", "2a9ea651")  = 0 <18.327733>
>     rename(".2a9f1679.xDYq9Q", "2a9f1679")  = 0 <18.312850>
>     rename(".2ab0a134.2GWgmr", "2ab0a134")  = 0 <18.326181>
>     rename(".2aebf110.pGkILq", "2aebf110")  = 0 <0.000188>
>     rename(".2af10031.7Sl5g6", "2af10031")  = 0 <18.342683>
>     rename(".2b095066.MCauJX", "2b095066")  = 0 <18.375003>
>     rename(".2b217bfd.HauJjr", "2b217bfd")  = 0 <18.427703>
>     rename(".2b336a06.w5NN0p", "2b336a06")  = 0 <18.378774>
>     rename(".2b40b422.i2v0E6", "2b40b422")  = 0 <14.727797>
>     rename(".2b568d13.9zmRRX", "2b568d13")  = 0 <0.000056>
>     rename(".2b5ccc66.AFd86P", "2b5ccc66")  = 0 <0.000063>
>     rename(".2b7d0a43.qWyxge", "2b7d0a43")  = 0 <0.000046>
>     rename(".2b7f968a.QAqOCb", "2b7f968a")  = 0 <0.000041>
>     rename(".2ba6dddf.ynNTvi", "2ba6dddf")  = 0 <0.000039>
>     rename(".2bce23ab.tliDkg", "2bce23ab")  = 0 <0.000040>
>     rename(".2c19e144.CvHPV5", "2c19e144")  = 0 <0.000060>
>     rename(".2c7c0651.8x1kQy", "2c7c0651")  = 0 <0.000057>
>     rename(".2ca1a6b7.QwujH4", "2ca1a6b7")  = 0 <0.000396>
>     rename(".2cc71683.7n9EYA", "2cc71683")  = 0 <0.000045>
>     rename(".2cebde90.ZiGcTa", "2cebde90")  = 0 <0.000042>
>     rename(".2d057cb4.5PGOIP", "2d057cb4")  = 0 <0.000042>
>     rename(".2d29b4a7.A8hfwg", "2d29b4a7")  = 0 <0.000043>
> 
> So during the slow phase it took mostly ~18 seconds and as the phase
> ends, the renames are very fast again.
> 
> Tried to change the priority of the process with renice and
> also enabled some different IO schedulers for the block device,
> but this had no effect.
> 
> Could not find anything in the logs or dmesg when this happens.
> 
> Any idea what could be the cause of this slowdown?

Unfortunately I do not.

> What else can I do to better locate in which part of the kernel
> the IO is stuck?

Linux 6.x has been out there for a while, and until now I am not aware 
of similar reports, so it’s probably hard to reproduce. In light of 
that, bisecting the issue is the only recommendation I can give and 
which you can also do yourself without having to wait for others.

> The system has 1.5TiB memory and the filesystem is ext4 on a MD
> raid10 with 10 nvme drives (Intel P4610):
> 
>     cat /proc/mdstat
>     Personalities : [raid10]
>     md0 : active raid10 nvme1n1[2] nvme4n1[4] nvme5n1[5] nvme3n1[3] nvme9n1[9] nvme8n1[8] nvme7n1[7] nvme6n1[6] nvme2n1[1] nvme0n1[0]
>           7813406720 blocks super 1.2 512K chunks 2 near-copies [10/10] [UUUUUUUUUU]
>           bitmap: 28/59 pages [112KB], 65536KB chunk
> 
> Mounted as follows:
> 
>     /dev/md0 on /u2 type ext4 (rw,nodev,noatime,commit=600,stripe=640)
> 
> The following cron entry is used to trim the device:
> 
>     25 */2 * * * root /usr/sbin/fstrim -v /u2 >> /tmp/u2.trim 2>&1
> 
> A check of the raid was also performed with no issues:
> 
>     [Sun May  5 13:52:01 2024] md: data-check of RAID array md0
>     [Sun May  5 14:54:25 2024] md: md0: data-check done.
>     cat /sys/block/md0/md/mismatch_cnt
>     0
> 
> CPU's are four Intel Xeon Platinum 8268 and server is a Dell Poweredge R940.
> 
> Additional information of the kernel config and other information I have
> uploaded to https://download.dwd.de/pub/afd/test/kernel_problem

Kind regards,

Paul

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