Re: EXT4 IOPS degradation in 5.10 compared to 5.4

Linux kernel regression tracking (#update)
2023-01-21T13:08:02Z

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On 19.01.23 16:15, Jan Kara wrote:
> Hello!
>
> On Wed 18-01-23 18:48:10, Bhatnagar, Rishabh wrote:
>> On 1/13/23 2:13 PM, Bhatnagar, Rishabh wrote:
>>>
>>> On 1/12/23 3:38 AM, Jan Kara wrote:
>>>> CAUTION: This email originated from outside of the organization. Do
>>>> not click links or open attachments unless you can confirm the
>>>> sender and know the content is safe.
>>>>
>>>>
>>>>
>>>> Hi!
>>>>
>>>> On Wed 11-01-23 18:06:39, Bhatnagar, Rishabh wrote:
>>>>> We have been seeing a consistent 3% degradation in IOPS score
>>>>> between 5.4
>>>>> and 5.10 stable kernels while running fio tests.
>>>>>
>>>>> I'm running test case on m6g.8xlarge AWS instances using arm64.
>>>>> The test
>>>>> involves:
>>>>>
>>>>> 1. Creating 100GB volume with IO1 500 iops. Attaching it to the
>>>>> instance.
>>>>>
>>>>> 2. Setup and mount fs:
>>>>>
>>>>> mke2fs -m 1 -t ext4 -b 4096 -L /mnt /dev/nvme1n1
>>>>> mount -t ext4 -o noatime,nodiratime,data=ordered /dev/nvme1n1 /mnt
>>>>>
>>>>> 3. Install fio package and run following test:
>>>>> (running 16 threads doing random buffered 16kb writes on a file.
>>>>> ioengine=psync, runtime=60secs)
>>>>>
>>>>> jobs=16
>>>>> blocksize="16k"
>>>>> filesize=1000000
>>>>>
>>>>> if [[ -n $1 ]]; then jobs=$1; fi
>>>>> if [[ -n $2 ]]; then blocksize=$2; fi
>>>>>
>>>>> /usr/bin/fio --name=fio-test --directory=/mnt --rw=randwrite
>>>>> --ioengine=psync --buffered=1 --bs=${blocksize} \
>>>>>          --max-jobs=${jobs} --numjobs=${jobs} --runtime=30 --thread \
>>>>>          --filename=file0 --filesize=${filesize} \
>>>>>          --fsync=1 --group_reporting --create_only=1 > /dev/null
>>>>>
>>>>> sudo echo 1 > /proc/sys/vm/drop_caches
>>>>>
>>>>> set -x
>>>>> echo "Running with jobs=${jobs} filesize=${filesize}
>>>>> blocksize=${blocksize}"
>>>>> /usr/bin/fio --name=fio-test --directory=/mnt --rw=randwrite
>>>>> --ioengine=psync --buffered=1 --bs=${blocksize} \
>>>>>          --max-jobs=${jobs} --numjobs=${jobs} --runtime=60 --thread \
>>>>>          --filename=file0 --filesize=${filesize} \
>>>>>          --fsync=1 --group_reporting --time_based
>>>>>
>>>>> After doing some kernel bisecting between we were able to pinpoint this
>>>>> commit that drops the iops score by 10~15 points (~3%).
>>>>> ext4: avoid unnecessary transaction starts during writeback
>>>>> (6b8ed62008a49751fc71fefd2a4f89202a7c2d4d)
>>>>>
>>>>> We see higher iops/bw/total io after reverting the commit
>>>>> compared to base
>>>>> 5.10 kernel.
>>>>> Although the average clat is higher after reverting the commit
>>>>> the higher bw
>>>>> drives the iops score higher as seen in below fio output.
>>>> I expect the difference is somewhere in waiting for the journal. Can you
>>>> just check whether there's a difference if you use --fdatasync=1
>>>> instead of
>>>> --fsync=1? With this workload that should avoid waiting for the journal
>>>> because the only metadata updates are mtime timestamps in the inode.
>>> There is a difference of 5% with and with the commit if i change this to
>>> fdatasync=1.
>>>>> Fio output (5.10.162):
>>>>> write: io=431280KB, bw=7186.3KB/s, iops=449, runt= 60015msec
>>>>> clat (usec): min=6, max=25942, avg=267.76,stdev=1604.25
>>>>> lat (usec): min=6, max=25943, avg=267.93,stdev=1604.25
>>>>> clat percentiles (usec):
>>>>> | 1.00th=[ 9], 5.00th=[ 10], 10.00th=[ 16], 20.00th=[ 24]
>>>>> | 30.00th=[ 34], 40.00th=[ 45], 50.00th=[ 58], 60.00th=[ 70],
>>>>> | 70.00th=[ 81], 80.00th=[ 94], 90.00th=[ 107], 95.00th=[ 114],
>>>>> | 99.00th=[10048], 99.50th=[14016], 99.90th=[20096], 99.95th=[21888],
>>>>> | 99.99th=[24448]
>>>>> lat (usec) : 10=3.46%, 20=12.54%, 50=26.66%, 100=41.16%, 250=13.64%
>>>>> lat (usec) : 500=0.02%, 750=0.03%, 1000=0.01%
>>>>> lat (msec) : 2=0.23%, 4=0.50%, 10=0.73%, 20=0.91%, 50=0.12%
>>>>> cpu : usr=0.02%, sys=0.42%, ctx=299540, majf=0, minf=0
>>>>> IO depths : 1=100.0%, 2=0.0%, 4=0.0%, 8=0.0%, 16=0.0%, 32=0.0%,
>>>>>> =64=0.0%
>>>>> submit : 0=0.0%, 4=100.0%, 8=0.0%, 16=0.0%, 32=0.0%, 64=0.0%, >=64=0.0%
>>>>> complete : 0=0.0%, 4=100.0%, 8=0.0%, 16=0.0%, 32=0.0%, 64=0.0%,
>>>>>> =64=0.0%
>>>>> issued : total=r=0/w=26955/d=0, short=r=0/w=0/d=0, drop=r=0/w=0/d=0
>>>>> latency : target=0, window=0, percentile=100.00%, depth=1
>>>>> Run status group 0 (all jobs):
>>>>> WRITE: io=431280KB, aggrb=7186KB/s, minb=7186KB/s, maxb=7186KB/s,
>>>>> mint=60015msec, maxt=60015msec
>>>>> Disk stats (read/write):
>>>>> nvme1n1: ios=0/30627, merge=0/2125, ticks=0/410990, in_queue=410990,
>>>>> util=99.94%
>>>>>
>>>>> Fio output (5.10.162 with revert):
>>>>> write: io=441920KB, bw=7363.7KB/s, iops=460, runt= 60014msec
>>>>> clat (usec): min=6, max=35768, avg=289.09, stdev=1736.62
>>>>> lat (usec): min=6, max=35768, avg=289.28,stdev=1736.62
>>>>> clat percentiles (usec):
>>>>> | 1.00th=[ 8], 5.00th=[ 10], 10.00th=[ 16], 20.00th=[ 24],
>>>>> | 30.00th=[ 36], 40.00th=[ 46], 50.00th=[ 59], 60.00th=[ 71],
>>>>> | 70.00th=[ 83], 80.00th=[ 97], 90.00th=[ 110], 95.00th=[ 117],
>>>>> | 99.00th=[10048], 99.50th=[14144], 99.90th=[21632], 99.95th=[25984],
>>>>> | 99.99th=[28288]
>>>>> lat (usec) : 10=4.13%, 20=11.67%, 50=26.59%, 100=39.57%, 250=15.28%
>>>>> lat (usec) : 500=0.03%, 750=0.03%, 1000=0.03%
>>>>> lat (msec) : 2=0.20%, 4=0.64%, 10=0.80%, 20=0.86%, 50=0.18%
>>>>> cpu : usr=0.01%, sys=0.43%, ctx=313909, majf=0, minf=0
>>>>> IO depths : 1=100.0%, 2=0.0%, 4=0.0%, 8=0.0%, 16=0.0%, 32=0.0%,
>>>>>> =64=0.0%
>>>>> submit : 0=0.0%, 4=100.0%, 8=0.0%, 16=0.0%, 32=0.0%, 64=0.0%, >=64=0.0%
>>>>> complete : 0=0.0%, 4=100.0%, 8=0.0%, 16=0.0%, 32=0.0%, 64=0.0%,
>>>>>> =64=0.0%
>>>>> issued : total=r=0/w=27620/d=0, short=r=0/w=0/d=0, drop=r=0/w=0/d=0
>>>>> latency : target=0, window=0, percentile=100.00%, depth=1
>>>>> Run status group 0 (all jobs):
>>>>> WRITE: io=441920KB, aggrb=7363KB/s, minb=7363KB/s, maxb=7363KB/s,
>>>>> mint=60014msec, maxt=60014msec
>>>>> Disk stats (read/write):
>>>>> nvme1n1: ios=0/31549, merge=0/2348, ticks=0/409221, in_queue=409221,
>>>>> util=99.88%
>>>>>
>>>>>
>>>>> Also i looked ext4_writepages latency which increases when the
>>>>> commit is
>>>>> reverted. (This makes sense since the commit avoids unnecessary
>>>>> transactions).
>>>>>
>>>>> ./funclatency ext4_writepages -->(5.10.162)
>>>>> avg = 7734912 nsecs, total: 134131121171 nsecs, count: 17341
>>>>>
>>>>> ./funclatency ext4_writepages -->(5.10.162 with revert)
>>>>> avg = 9036068 nsecs, total: 168956404886 nsecs, count: 18698
>>>>>
>>>>>
>>>>> Looking at the journal transaction data I can see that the average
>>>>> transaction commit time decreases after reverting the commit.
>>>>> This probably helps in the IOPS score.
>>>> So what the workload is doing is:
>>>> write()
>>>>    inode lock
>>>>    dirty 16k of page cache
>>>>    dirty inode i_mtime
>>>>    inode unlock
>>>> fsync()
>>>>    walk all inode pages, write dirty ones
>>>>    wait for all pages under writeback in the inode to complete IO
>>>>    force transaction commit and wait for it
>>>>
>>>> Now this has the best throughput (and the worst latency) if all 16
>>>> processes work in lockstep - i.e., like:
>>>>
>>>>    task1         task2           task3 ...
>>>>    write()
>>>>                  write()
>>>>                                  write()
>>>>    fsync()
>>>>                  fsync()
>>>>                                  fsync()
>>>>
>>>> because in that case we writeout all dirty pages from 16 processes
>>>> in one
>>>> sweep together and also we accumulate 16 mtime updates in single
>>>> transaction commit.
>>>>
>>>> Now I suspect the commit you've identified leads to less synchronization
>>>> between the processes and thus in less batching happening. In particular
>>>> before the commit we've called mpage_prepare_extent_to_map() twice
>>>> and the
>>>> second invocation starts at where the first invocation saw last
>>>> dirty page.
>>>> So it potentially additionally writes newly dirtied pages beyond
>>>> that place
>>>> and that effectively synchronizes processes more.
>>>>
>>>> To confirm the theory, it might be interesting to gather a histogram
>>>> of a
>>>> number of pages written back by ext4_writepages() call with /
>>>> without the
>>>> commit.
>>>>
>>>> Honza
>>>> --
>>>> Jan Kara <jack@suse.com>
>>>> SUSE Labs, CR
>>>
>>> Hi Jan
>>>
>>> I collected some data w.r.t to number of pages being written by
>>> ext4_writepages. What you pointed out seems to be correct.
>>> Without the commit I see more batching (more writeback count from 4-20
>>> pages)happening compared to with the commit.
>>>
>>> Without the commit (reverted):
>>>
>>> [0-1]   —>  4246
>>> [2-3]   —>  312
>>> [4-5]   —>  20836
>>> [6-7]   —>  205
>>> [8-9]   —>  895
>>> [10-11] —>  56
>>> [12-13] —>  422
>>> [14-15] —>  62
>>> [16-17] —>  234
>>> [18-19] —>  66
>>> [20-21] —>  77
>>> [22-23] —>  9
>>> [24-25] —>  26
>>> [26-27] —>  1
>>> [28-29] —>  13
>>>
>>> Average page count : 3.9194
>>>
>>>
>>> With the commit:
>>>
>>> [0-1]   —> 1635
>>> [2-3]   —> 123
>>> [4-5]   —> 24302
>>> [6-7]   —> 38
>>> [8-9]   —> 604
>>> [10-11] —> 19
>>> [12-13] —> 123
>>> [14-15] —> 12
>>> [16-17] —> 24
>>> [18-19] —> 3
>>> [20-21] —> 8
>>> [22-23] —> 1
>>> [24-25] —> 3
>>> [26-27] —> 0
>>> [28-29] —> 1
>>>
>>> Average page count : 3.9184
>>>
>>> Also looking at journal data I see that without the commit we have more
>>> handles per journal transaction:
>>>
>>> Without the commit:
>>> cat /proc/fs/jbd2/nvme1n1-8/info
>>> 2092 transactions (2091 requested), each up to 8192 blocks
>>> average:
>>> 0ms waiting for transaction
>>> 0ms request delay
>>> 20ms running transaction
>>> 0ms transaction was being locked
>>> 0ms flushing data (in ordered mode)
>>> 20ms logging transaction
>>> 15981us average transaction commit time
>>> 67 handles per transaction
>>> 1 blocks per transaction
>>> 2 logged blocks per transaction
>>>
>>> With the commit:
>>> cat /proc/fs/jbd2/nvme1n1-8/info
>>> 2143 transactions (2143 requested), each up to 8192 blocks
>>> average:
>>> 0ms waiting for transaction
>>> 0ms request delay
>>> 0ms running transaction
>>> 0ms transaction was being locked
>>> 0ms flushing data (in ordered mode)
>>> 20ms logging transaction
>>> 20731us average transaction commit time
>>> 51 handles per transaction
>>> 1 blocks per transaction
>>> 3 logged blocks per transaction
>>
>> Gentle reminder.
>>
>> Any thoughts on this data? Is there a usecase where this commit brings
>> benefits that are worth considering if we decide to revert this?
>
> Yes. This change was actually added because it led to significant
> improvement for workloads where one process was just overwriting allocated
> file (so it didn't need to start a transaction in ext4_writepages()) while
> other processes were creating load on the journal (so starting a
> transaction was actually rather expensive). So I don't think reverting this
> commit to re-introduce more synchronization on the journal into
> ext4_writepages() is actually a good tradeoff.
>
> I'm trying to think if we could somehow improve the performance of your
> workload but it's always going to be a latency vs throughput tradeoff so
> different people will want different behavior. Maybe some fsync batching
> but that is more of an research project than an easy fix.

Thx for this. In that case I'll move this issue to the list of
regression on the backburner -- and likely will remove it in a few
months while reminding everyone a final time about it. Sure, in an ideal
world this would be fixed, but tradeoffs like "latency vs throughput"
are always tricky -- and if this is only noticed now it's likely not
that common.

#regzbot title: ext4: 3% degradation in IOPS score, likely due to a
change that influences latency vs throughput
#regzbot backburner: latency vs throughput issue -- fixing it would be
more of an research project than an easy fix
#regzbot ignore-activity

Ciao, Thorsten (wearing his 'the Linux kernel's regression tracker' hat)
--
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