Re: [Btrfs] 3a8b36f3780: -62.6% fileio.requests_per_sec

[Filipe Manana <fdmanana@suse.com>]

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On 04/01/2015 05:59 AM, Huang Ying wrote:
> On Tue, 2015-03-31 at 15:59 +0100, Filipe Manana wrote:
> 
> [snip]
> 
>> From: Filipe Manana <fdmanana@suse.com>
>> Date: Tue, 31 Mar 2015 14:16:52 +0100
>> Subject: [PATCH] Btrfs: avoid syncing log in the fast fsync path when not
>>  necessary
>>
>> Commit 3a8b36f37806 ("Btrfs: fix data loss in the fast fsync path") added
>> a performance regression for that causes an unnecessary sync of the log
>> trees (fs/subvol and root log trees) when 2 consecutive fsyncs are done
>> against a file, without no writes or any metadata updates to the inode in
>> between them and if a transaction is committed before the second fsync is
>> called.
>>
>> Huang Ying reported this to lkml after a test sysbench test that measured
>> a -62% decrease of file io requests for that tests' workload.
>>
>> The test is:
>>
>>   echo performance > /sys/devices/system/cpu/cpu0/cpufreq/scaling_governor
>>   echo performance > /sys/devices/system/cpu/cpu1/cpufreq/scaling_governor
>>   echo performance > /sys/devices/system/cpu/cpu2/cpufreq/scaling_governor
>>   echo performance > /sys/devices/system/cpu/cpu3/cpufreq/scaling_governor
>>   mkfs -t btrfs /dev/sda2
>>   mount -t btrfs /dev/sda2 /fs/sda2
>>   cd /fs/sda2
>>   for ((i = 0; i < 1024; i++)); do fallocate -l 67108864 testfile.$i; done
>>   sysbench --test=fileio --max-requests=0 --num-threads=4 --max-time=600 \
>>     --file-test-mode=rndwr --file-total-size=68719476736 --file-io-mode=sync \
>>     --file-num=1024 run
>>
>> A test on kvm guest, running a debug kernel gave me the following results:
>>
>> Without 3a8b36f378060d:             16.01 reqs/sec
>> With 3a8b36f378060d:                 3.39 reqs/sec
>> With 3a8b36f378060d and this patch: 16.04 reqs/sec
>>
>> Reported-by: Huang Ying <ying.huang@intel.com>
> 
> I have tested your patch, the regression restored in our test.  Thanks!
> 
> Tested-by: Huang, Ying <ying.huang@intel.com>

Thank you very much for testing it and the report.
I'll now send the exact same patch, with your Tested-by tag, to the
btrfs mailing list, so that Chris can pick it from patchwork and can
more easily be noticed by the btrfs community (for review, test, etc).

regards,
Filipe Manana

> 
> Best Regards,
> Huang, Ying
> 
> [snip]
> 
> 
>