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

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Date:	Wed, 6 Jun 2012 12:15:44 -0400
From:	Vivek Goyal <vgoyal@...hat.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Fengguang Wu <fengguang.wu@...el.com>,
	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 Wed, Jun 06, 2012 at 08:14:08AM -0400, Vivek Goyal wrote:

[..]
> I think it is happening because sync_file_range() will send all
> the writes as SYNC and it will compete with firefox IO. On the other
> hand, flusher's IO will show up as ASYNC and CFQ  will be penalize it
> heavily and firefox's IO will be prioritized. And this effect should
> just get worse as more processes do sync_file_range().

Ok, this time I tried the same test again but with 4 processes doing
writes in parallel on 4 different files.

And with sync_file_range() things turned ugly. Interactivity was very poor. 

firefox launch test took around 1m:45s with sync_file range() while it
took only about 35seconds with regular flusher threads.

So sending writeback IO synchronously wreaks havoc.

Thanks
Vivek
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