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

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Message-ID: <20101102131239.GA8680@think>
Date:	Tue, 2 Nov 2010 09:12:39 -0400
From:	Chris Mason <chris.mason@...cle.COM>
To:	Sanjoy Mahajan <sanjoy@...n.edu>
Cc:	Ingo Molnar <mingo@...e.hu>, Pekka Enberg <penberg@...nel.org>,
	Aidar Kultayev <the.aidar@...il.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jens Axboe <axboe@...nel.dk>, Peter.Zijl@....EDU
Subject: Re: 2.6.36 io bring the system to its knees

On Tue, Nov 02, 2010 at 07:47:15AM -0400, Sanjoy Mahajan wrote:
> Chris Mason <chris.mason@...cle.com> wrote:
> 
> > > This has the appearance of some really bad IO or VM latency
> > > problem. Unfixed and present in stable kernel versions going from
> > > years ago all the way to v2.6.36.
> > 
> > Hmmm, the workload you're describing here has two special parts.
> > First it dramatically overloads the disk, and then it has guis doing
> > things waiting for the disk.
> 
> I think I see this same issue every few days when I back up my hard
> drive to a USB hard drive using rsync.  While the backup is running, the
> interactive response is bad.  A reproducible measurement of the badness
> is starting an rxvt with F8 (bound to "rxvt &" in my .twmrc).  Often it
> takes 8 seconds for the window to appear (as it just did about 2 minutes
> ago)!  (Starting a subsequent rxvt is quick.)

So this sounds like the backup is just thrashing your cache.  Latencies
starting an app are less surprising than latencies where a running app
doesn't respond at all.

Does rsync have the option to do an fadvise DONTNEED?

-chris
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