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

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Date:	Thu, 4 Nov 2010 12:05:41 -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

> So this sounds like the backup is just thrashing your cache.

I think it's more than that.  Starting an rxvt shouldn't take 8 seconds,
even with a cold cache.  Actually, it does take a while, so you do have
a point.  I just did

  echo 3 > /proc/sys/vm/drop_caches

and then started rxvt.  That takes about 3 seconds (which seems long,
but I don't know wherein that slowness lies), of which maybe 0.25
seconds is loading and running 'date':

$ time rxvt -e date
real	0m2.782s
user	0m0.148s
sys	0m0.032s

The 8-second delay during the rsync must have at least two causes: (1)
the cache is wiped out, and (2) the rxvt binary cannot be paged in
quickly because the disk is doing lots of other I/O.  

Can the system someknow that paging in the rxvt binary and shared
libraries is interactive I/O, because it was started by an interactive
process, and therefore should take priority over the rsync?

> Does rsync have the option to do an fadvise DONTNEED?

I couldn't find one.  It would be good to have a solution that is
independent of the backup app.  (The 'locate' cron job does a similar
thrashing of the interactive response.)

-Sanjoy

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