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

[Huang Ying <ying.huang@intel.com>]

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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>

Best Regards,
Huang, Ying

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