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

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Message-ID: <E1PDFKe-0005sq-2D@approx.mit.edu>
Date:	Tue, 2 Nov 2010 07:47:15 -0400
From:	Sanjoy Mahajan <sanjoy@...n.edu>
To:	Chris Mason <chris.mason@...cle.com>
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

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.)

The command for running the backup:

  rsync -av --delete /etc /home /media/usbdrive/bak > /tmp/homebackup.log

The hardware is a T60 w/ Intel graphics and wireless, 1.5GB RAM, 5400rpm
160GB harddrive w/ ext3 filesystems, and it's running vanilla 2.6.36.
There's not much memory pressure.  The swap is mostly empty, and there's
usually a Firefox eating 500MB of RAM.  Even Emacs at 50MB is in the
noise compared to the Firefox.

Here's the 'free' output:

             total       used       free     shared    buffers     cached
Mem:       1545292    1500288      45004          0      92848     713988
-/+ buffers/cache:     693452     851840
Swap:      2000088      22680    1977408

What tests or probes are worth running when the problem reappears in
order to find the root cause?

-Sanjoy

`Until lions have their historians, tales of the hunt shall always
 glorify the hunters.'  --African Proverb
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