Re: [LKP] Re: [ext4] b1b4705d54: filebench.sum_bytes_mb/s -20.2% regression

Xing Zhengjun
2020-04-15T07:55:39Z

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On 3/25/2020 10:31 PM, Jan Kara wrote:
> On Wed 25-03-20 13:50:09, Xing Zhengjun wrote:
>> ping...
>> The issue still exists in v5.6-rc7.
>
> So I have tried again to reproduce this so that I can look into the
> regression. When observing what is actually happening in the system I have
> to say that this workfile (or actually its implementation in filebench) is
> pretty dubious. The problem is that filebench first creates the files by
> writing them through ordinary write(2). Then it immediately starts reading
> the files with direct IO read. So what happens is that by the time direct
> IO read is running, the system is still writing back the create files and
> depending on how read vs writes get scheduled, you get different results.
> Also direct IO read will first flush the range it is going to read from the
> page cache so to some extent this is actually parallel small ranged
> fsync(2) benchmark. Finally differences in how we achieve integrity of
> direct IO reads with dirty page cache are going to impact this benchmark.
>

Sounds reasonable! Thanks for the clarification!

> So overall can now see why this commit makes a difference but the workload
> is IMHO largely irrelevant. What would make sense is to run filebench once,
> then unmount & mount the fs to force files to disk and clear page cache and
> then run it again. Filebench will reuse the files in this case and then
> parallel direct IO readers without page cache are a sensible workload. But
> I didn't see any difference in that (even with rotating disk) on my
> machines.
>
We do a test per your suggestion, run "filebench" once during setup
stage, then do a "sync", after that run "filebench" again, from the
attached test result "compare", "filebench.sum_bytes_mb/s" regression is
disappeared.

> Honza
>>
>> On 3/4/2020 4:15 PM, Xing Zhengjun wrote:
>>> Hi Matthew,
>>>
>>>  We test it in v5.6-rc4, the issue still exist, do you have time to
>>> take a look at this? Thanks.
>>>
>>> On 1/8/2020 10:31 AM, Rong Chen wrote:
>>>>
>>>>
>>>> On 1/8/20 1:28 AM, Jan Kara wrote:
>>>>> On Tue 07-01-20 11:57:08, Theodore Y. Ts'o wrote:
>>>>>> On Tue, Jan 07, 2020 at 02:41:06PM +0100, Jan Kara wrote:
>>>>>>> Hello,
>>>>>>>
>>>>>>> On Tue 24-12-19 08:59:15, kernel test robot wrote:
>>>>>>>> FYI, we noticed a -20.2% regression of
>>>>>>>> filebench.sum_bytes_mb/s due to commit:
>>>>>>>>
>>>>>>>>
>>>>>>>> commit: b1b4705d54abedfd69dcdf42779c521aa1e0fbd3
>>>>>>>> ("ext4: introduce direct I/O read using iomap
>>>>>>>> infrastructure")
>>>>>>>> https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git
>>>>>>>> master
>>>>>>>>
>>>>>>>> in testcase: filebench
>>>>>>>> on test machine: 8 threads Intel(R) Core(TM) i7-4770
>>>>>>>> CPU @ 3.40GHz with 8G memory
>>>>>>>> with following parameters:
>>>>>>>>
>>>>>>>>     disk: 1HDD
>>>>>>>>     fs: ext4
>>>>>>>>     test: fivestreamreaddirect.f
>>>>>>>>     cpufreq_governor: performance
>>>>>>>>     ucode: 0x27
>>>>>>> I was trying to reproduce this but I failed with my test
>>>>>>> VM. I had SATA SSD
>>>>>>> as a backing store though so maybe that's what makes a
>>>>>>> difference. Maybe
>>>>>>> the new code results in somewhat more seeks because the
>>>>>>> five threads which
>>>>>>> compete in submitting sequential IO end up being more interleaved?
>>>>>> A "-20.2% regression" should be read as a "20.2% performance
>>>>>> improvement" is zero-day kernel speak.
>>>>> Are you sure? I can see:
>>>>>
>>>>>       58.30 ±  2%     -20.2%      46.53        filebench.sum_bytes_mb/s
>>>>>
>>>>> which implies to me previously the throughput was 58 MB/s and after the
>>>>> commit it was 46 MB/s?
>>>>>
>>>>> Anyway, in my testing that commit made no difference in that benchmark
>>>>> whasoever (getting around 97 MB/s for each thread before and after the
>>>>> commit).
>>>>>                                 Honza
>>>>
>>>> We're sorry for the misunderstanding, "-20.2%" means the change of
>>>> filebench.sum_bytes_mb/s,
>>>> "regression" means the explanation of this change from LKP.
>>>>
>>>> Best Regards,
>>>> Rong Chen
>>>> _______________________________________________
>>>> LKP mailing list -- lkp@lists.01.org
>>>> To unsubscribe send an email to lkp-leave@lists.01.org
>>>
>>
>> --
>> Zhengjun Xing

--
Zhengjun Xing

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=========================================================================================
tbox_group/testcase/rootfs/kconfig/compiler/debug-setup/disk/fs/test/cpufreq_governor/ucode:
lkp-hsw-d01/filebench/debian-x86_64-20191114.cgz/x86_64-rhel-7.6/gcc-7/test2/1HDD/ext4/fivestreamreaddirect.f/performance/0x27

commit:
b1b4705d54abedfd69dcdf42779c521aa1e0fbd3
09edf4d381957b144440bac18a4769c53063b943
v5.5
v5.7-rc1

b1b4705d54abedfd 09edf4d381957b144440bac18a4 v5.5 v5.7-rc1
---------------- --------------------------- --------------------------- ---------------------------
%stddev %change %stddev %change %stddev %change %stddev
\ | \ | \ | \
59.40 +0.0% 59.40 -0.8% 58.93 -1.0% 58.80 filebench.sum_bytes_mb/s
3570 +0.0% 3570 -0.8% 3541 -1.0% 3533 filebench.sum_operations
59.50 +0.0% 59.50 -0.8% 59.02 -1.0% 58.89 filebench.sum_operations/s
59.33 +0.0% 59.33 +0.0% 59.33 -0.6% 59.00 filebench.sum_reads/s
83.98 -1.5% 82.75 +0.8% 84.62 +1.0% 84.84 filebench.sum_time_ms/op