linux-kernel - Re: [PATCH] mm: Do not stall in synchronous compaction for THP allocations

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Date:	Thu, 10 Nov 2011 10:51:00 +0000
From:	Alan Cox <alan@...rguk.ukuu.org.uk>
To:	Mel Gorman <mgorman@...e.de>
Cc:	Andrew Morton <akpm@...ux-foundation.org>, Jan Kara <jack@...e.cz>,
	Andy Isaacson <adi@...apodia.org>,
	Johannes Weiner <jweiner@...hat.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH] mm: Do not stall in synchronous compaction for THP
 allocations

On Thu, 10 Nov 2011 10:06:16 +0000
Mel Gorman <mgorman@...e.de> wrote:

> Occasionally during large file copies to slow storage, there are still
> reports of user-visible stalls when THP is enabled. Reports on this
> have been intermittent and not reliable to reproduce locally but;

If you want to cause a massive stall take a cheap 32GB USB flash drive
plug it into an 8GB box and rsync a lot of small files to it. 400,000
emails in maildir format does the trick and can easily be simulated. The
drive drops to about 1-2 IOPS with all the small mucking around and the
backlog becomes massive.

> Internally in SUSE, I received a bug report related to stalls in firefox
> 	when using Java and Flash heavily while copying from NFS
> 	to VFAT on USB. It has not been confirmed to be the same problem
> 	but if it looks like a duck and quacks like a duck.....

With the 32GB USB flash rsync I see firefox block for up to 45 minutes
although operating entirely on an unrelated filesystem. I suspect it may
be a problem that is visible because an fsync is getting jammed up in
the mess.

Alan
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