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

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Message-ID: <20120607190613.GC18538@redhat.com>
Date:	Thu, 7 Jun 2012 15:06:13 -0400
From:	Vivek Goyal <vgoyal@...hat.com>
To:	Jan Kara <jack@...e.cz>
Cc:	Fengguang Wu <fengguang.wu@...el.com>,
	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 Thu, Jun 07, 2012 at 11:45:04AM +0200, Jan Kara wrote:
[..]
> > Instead of above, I modified sync_file_range() to call
> > __filemap_fdatawrite_range(WB_SYNC_NONE) and I do see now ASYNC writes
> > showing up at elevator.
> > 
> > With 4 processes doing sync_file_range() now, firefox start time test
> > clocks around 18-19 seconds which is better than 30-35 seconds of 4
> > processes doing buffered writes. And system looks pretty good from
> > interactivity point of view.
>   So do you have any idea why is that? Do we drive shallower queues? Also
> how does speed of the writers compare to the speed with normal buffered
> writes + fsync (you'd need fsync for sync_file_range writers as well to
> make comparison fair)?

Ok, I did more tests and few odd things I noticed.

- Results are varying a lot. Sometimes with write+flush workload also firefox
  launched fast. So now it is hard to conclude things.

- For some reason I had nr_requests as 16K on my root drive. I have no
  idea who is setting it. Once I set it to 128, then firefox with
  write+flush workload performs much better and launch time are similar
  to sync_file_range.

- I tried to open new windows in firefox and browse web, load new
  websites. I would say sync_file_range() feels little better but
  I don't have any logical explanation and can't conclude anything yet
  by looking at traces. I am continuing to stare though.

So in summary, at this point of time I really can't conclude that
using sync_file_range() with ASYNC request is providing better latencies
in my setup.

I will keept at it though and if I notice something new, will write back.

Thanks
Vivek
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