linux-kernel - Re: 2.6.36 io bring the system to its knees

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Message-ID: <20101028135445.GB32157@elte.hu>
Date:	Thu, 28 Oct 2010 15:54:45 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Christoph Hellwig <hch@...radead.org>
Cc:	Pekka Enberg <penberg@...nel.org>,
	Aidar Kultayev <the.aidar@...il.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	npiggin@...nel.dk, Dave Chinner <david@...morbit.com>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: 2.6.36 io bring the system to its knees


* Christoph Hellwig <hch@...radead.org> wrote:

> On Thu, Oct 28, 2010 at 02:48:20PM +0300, Pekka Enberg wrote:
> > On Thu, Oct 28, 2010 at 2:33 PM, Aidar Kultayev <the.aidar@...il.com> wrote:
> > >
> > > if it wasn't picasa, it would have been something else. I mean if I kill 
> > > picasa ( later on it was done indexing new pics anyway ), it would have been 
> > > for virtualbox to thrash the io. So, nope, getting rid of picasa doesn't help 
> > > either. In general the systems responsiveness or sluggishness is dominated by 
> > > those io operations going on - the DD & CP & probably VBOX issuing whole bunch 
> > > of its load for IO.
> > 
> > Do you still see high latencies in vfs_lseek() and vfs_fsync()? I'm not a VFS 
> > expert but looking at your latencytop output, it seems that fsync grabs 
> > ->i_mutex which blocks vfs_llseek(), for example. I'm not sure why that causes 
> > high latencies though it's a mutex we're holding.
> 
> It does.  But what workload does a lot of llseeks while fsyncing the same file?  
> I'd bet some application is doing really stupid things here.

Seeking in a file and fsync-ing it does not seem like an inherently bad or even 
stupid thing to do - why do you claim that it is stupid?

If mixed seek()+fsync() is the reason for these latencies (which is just an 
assumption right now) then it needs to be fixed in the kernel, not in apps.

Thanks,

	Ingo
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