linux-kernel - Re: long sleep_on_page delays writing to slow storage

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Message-ID: <20111114184717.GA7006@redhat.com>
Date:	Mon, 14 Nov 2011 13:47:17 -0500
From:	Dave Jones <davej@...hat.com>
To:	Johannes Weiner <jweiner@...hat.com>
Cc:	Andrea Arcangeli <aarcange@...hat.com>,
	Mel Gorman <mgorman@...e.de>, Jan Kara <jack@...e.cz>,
	Andy Isaacson <adi@...apodia.org>,
	linux-kernel@...r.kernel.org, linux-mm@...r.kernel.org,
	kernel-team@...oraproject.org
Subject: Re: long sleep_on_page delays writing to slow storage

On Thu, Nov 10, 2011 at 10:34:42AM +0100, Johannes Weiner wrote:
 
 > > I wonder if a change like this would be enough?
 > > 
 > >        sync_migration = !(gfp_mask & __GFP_NO_KSWAPD);
 > > 
 > > But even if hidden in a new function, the main downside overall is the
 > > fact we'll pass one more var through the stack of fast paths.
 > > 
 > > Johannes I recall you reported this too and Mel suggested the above
 > > change, did it help in the end?
 > 
 > Yes, it completely fixed the latency problem.
 > 
 > That said, I haven't looked at the impact on the THP success rate, but
 > a regression there is probably less severe than half-minute-stalls in
 > interactive applications.

FWIW, we've had a few reports from Fedora users since we moved to 3.x kernels
about similar problems, so whatever the fix is for this should probably
go to stable too.

I could push an update for Fedora users to test the change above if
that would be helpful ?

	Dave

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