linux-kernel - Re: kswapd high CPU usage with no swap

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Message-ID: <20070925213156.68fceea2@bree.surriel.com>
Date:	Tue, 25 Sep 2007 21:31:56 -0400
From:	Rik van Riel <riel@...hat.com>
To:	Jan Kundrát <jkt@...too.org>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: kswapd high CPU usage with no swap

On Tue, 25 Sep 2007 12:13:41 +0200
Jan Kundrát <jkt@...too.org> wrote:

> Rik van Riel wrote:
> > How much memory did you have in "cached" when you looked
> > with top (and no swap enabled) ?
> 
> Hi Rik,
> it was pretty low number (several thousands, or maybe tens of
> thousands).
> 
> In the meanwhile, I've come across your patch [1] ("prevent kswapd
> from freeing excessive amounts of lowmem") and applied it locally.

Could you try out the attached patch, too?

Kswapd and try_to_free_pages() have a built-in pause, where
it waits for IO to complete.  However, the current code also
calls blk_congestion_wait() when there is no IO in flight!

This patch should only make the pageout code wait for IO when
there actually is a significant amount of pageout IO in flight.

Signed-off-by: Rik van Riel <riel@...hat.com>

View attachment "linux-2.6-kswapd-iowait.patch" of type "text/x-patch" (2215 bytes)