linux-kernel - Re: [PATCH 2/3] Fix fsync livelock

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Message-ID: <20081005173019.0a358b09@infradead.org>
Date:	Sun, 5 Oct 2008 17:30:19 -0700
From:	Arjan van de Ven <arjan@...radead.org>
To:	Mikulas Patocka <mpatocka@...hat.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, agk@...hat.com, mbroz@...hat.com,
	chris@...chsys.com
Subject: Re: [PATCH 2/3] Fix fsync livelock

On Sun, 5 Oct 2008 20:01:46 -0400 (EDT)
Mikulas Patocka <mpatocka@...hat.com> wrote:

> I assume that if very few people complained about the livelock till
> now, very few people will see degraded write performance. My patch
> blocks the writes only if the livelock happens, so if the livelock
> doesn't happen in unpatched kernel for most people, the patch won't
> make it worse.

I object to calling this a livelock. It's not. 
And yes, fsync is slow and lots of people are seeing that.
It's not helped by how ext3 is implemented (where fsync is effectively
equivalent of a sync for many cases).
But again, moving the latency to "innocent" parties is not acceptable.

> 
> > If the fsync() implementation isn't smart enough, sure, lets improve
> > it. But not by shifting latency around... lets make it more
> > efficient at submitting IO.
> > If we need to invent something like "chained IO" where if you wait
> > on the last of the chain, you wait on the entirely chain, so be it.
> 
> This looks madly complicated. And ineffective, because if some page
> was submitted before fsync() was invoked, and is under writeback
> while fsync() is called, fsync() still has to wait on it.

so?
just make a chain per inode always...


-- 
Arjan van de Ven 	Intel Open Source Technology Centre
For development, discussion and tips for power savings, 
visit http://www.lesswatts.org
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