linux-kernel - Re: write-behind on streaming writes

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Date:	Wed, 6 Jun 2012 22:08:59 +0800
From:	Fengguang Wu <fengguang.wu@...el.com>
To:	Vivek Goyal <vgoyal@...hat.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>,
	"Myklebust, Trond" <Trond.Myklebust@...app.com>,
	linux-fsdevel@...r.kernel.org,
	Linux Memory Management List <linux-mm@...ck.org>,
	Jens Axboe <axboe@...nel.dk>
Subject: Re: write-behind on streaming writes

On Tue, Jun 05, 2012 at 10:57:30PM -0400, Vivek Goyal wrote:
> On Tue, Jun 05, 2012 at 04:10:45PM -0400, Vivek Goyal wrote:
> > On Tue, Jun 05, 2012 at 02:48:53PM -0400, Vivek Goyal wrote:
> > 
> > [..]
> > > So sync_file_range() test keeps less in flight requests on on average
> > > hence better latencies. It might not produce throughput drop on SATA
> > > disks but might have some effect on storage array luns. Will give it
> > > a try.
> > 
> > Well, I ran dd and syn_file_range test on a storage array Lun. Wrote a
> > file of size 4G on ext4. Got about 300MB/s write speed. In fact when I
> > measured time using "time", sync_file_range test finished little faster.
> > 
> > Then I started looking at blktrace output. sync_file_range() test
> > initially (for about 8 seconds), drives shallow queue depth (about 16),
> > but after 8 seconds somehow flusher gets involved and starts submitting
> > lots of requests and we start driving much higher queue depth (upto more than
> > 100). Not sure why flusher should get involved. Is everything working as
> > expected. I thought that as we wait for last 8MB IO to finish before we
> > start new one, we should have at max 16MB of IO in flight. Fengguang?
> 
> Ok, found it. I am using "int index" which in turn caused signed integer
> extension of (i*BUFSIZE). Once "i" crosses 255, integer overflow happens
> and 64bit offset is sign extended and offsets are screwed. So after 2G
> file size, sync_file_range() effectively stops working leaving dirty
> pages which are cleaned up by flusher. So that explains why flusher
> was kicking during my tests. Change "int" to "unsigned int" and problem
> if fixed.

Good catch! Besides that, I do see a small chance for the flusher
thread to kick in: at the time when the inode dirty expires after 30s.
Just a kind reminder, because I don't see how it can impact this
workload in some noticeable way.

Thanks,
Fengguang
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