Date: Thu, 30 Jun 2011 16:44:32 -0400
From: Vivek Goyal <vgoyal@redhat.com>
To: Dave Chinner <david@fromorbit.com>
Cc: linux-kernel@vger.kernel.org, jaxboe@fusionio.com,
        linux-fsdevel@vger.kernel.org, andrea@betterlinux.com,
        linux-ext4@vger.kernel.org
Subject: Re: fsync serialization on ext4 with blkio throttling (Was: Re:
 [PATCH 0/8][V2] blk-throttle: Throttle buffered WRITEs in
 balance_dirty_pages())
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On Thu, Jun 30, 2011 at 04:04:59PM -0400, Vivek Goyal wrote:

[..]
> Dave,
> 
> Just another example where serialization is taking place with ext4.
> 
> I created a group with 1MB/s write limit and ran tedso's fsync tester
> program with little modification. I used write() system call instead
> of pwrite() so that file size grows. This program basically writes
> 1MB of data and then fsync's it and then measures the fsync time.
> 
> I ran two instances of prgram in two groups on two separate files. One
> instances is throttled to 1MB/s and other is in root group unthrottled.
> 
> Unthrottled program gets serialized behind throttled one. Following
> are fsync times.
> 
> Throttled instance	Unthrottled Instance
> ------------------ 	--------------------
> fsync time: 1.0051	fsync time: 1.0067
> fsync time: 1.0049	fsync time: 1.0075
> fsync time: 1.0048	fsync time: 1.0063
> fsync time: 1.0073	fsync time: 1.0062
> fsync time: 1.0070	fsync time: 1.0078
> fsync time: 1.0032	fsync time: 1.0049
> fsync time: 0.0154	fsync time: 1.0068
> fsync time: 0.0137	fsync time: 1.0048
> 
> Without any throttling both the instances do fine
> -------------------------------------------------
> Throttled instance	Unthrottled Instance
> ------------------ 	--------------------
> fsync time: 0.0139	fsync time: 0.0162
> fsync time: 0.0132	fsync time: 0.0156
> fsync time: 0.0149	fsync time: 0.0169
> fsync time: 0.0165	fsync time: 0.0152
> fsync time: 0.0188	fsync time: 0.0135
> fsync time: 0.0137	fsync time: 0.0142
> fsync time: 0.0148	fsync time: 0.0149
> fsync time: 0.0168	fsync time: 0.0163
> fsync time: 0.0153	fsync time: 0.0143
> 
> So when we are inreasing the size of file and fsyncing it, other
> unthrottled instances of similar activities will get throttled
> behind it.
> 
> IMHO, this is a problem and should be fixed. If filesystem can fix it great.
> But if not, then we should consider the option of throttling buffered writes 
> in balance_dirty_pages().

XFS seems to be doing well for this particular test. Unthrottled
fsyncer does not get serialized behind throttled one.

Throttled instance	Unthrottled Instance
------------------ 	--------------------
fsync time: 1.0511	fsync time: 0.0204
fsync time: 1.0486	fsync time: 0.0260
fsync time: 1.0445	fsync time: 0.0260
fsync time: 1.0485	fsync time: 0.0260
fsync time: 1.0446	fsync time: 0.0260
fsync time: 1.2157	fsync time: 0.0260
fsync time: 1.0446	fsync time: 0.0300
fsync time: 1.0484	fsync time: 0.0340
fsync time: 1.0446	fsync time: 0.0221
fsync time: 1.0486	fsync time: 0.0340
fsync time: 1.0406	fsync time: 0.0340

Thanks
Vivek
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