linux-kernel - Re: [PATCH -mm] throttle direct reclaim when too many pages are isolated already

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Message-Id: <20090715205109.5f86e416.akpm@linux-foundation.org>
Date:	Wed, 15 Jul 2009 20:51:09 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Rik van Riel <riel@...hat.com>
Cc:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	LKML <linux-kernel@...r.kernel.org>,
	linux-mm <linux-mm@...ck.org>,
	Wu Fengguang <fengguang.wu@...el.com>
Subject: Re: [PATCH -mm] throttle direct reclaim when too many pages are
 isolated already

On Wed, 15 Jul 2009 23:42:28 -0400 Rik van Riel <riel@...hat.com> wrote:

> Andrew Morton wrote:
> > On Wed, 15 Jul 2009 23:28:14 -0400 Rik van Riel <riel@...hat.com> wrote:
> 
> >> If we are stuck at this point in the page reclaim code,
> >> it is because too many other tasks are reclaiming pages.
> >>
> >> That makes it fairly safe to just return SWAP_CLUSTER_MAX
> >> here and hope that __alloc_pages() can get a page.
> >>
> >> After all, if __alloc_pages() thinks it made progress,
> >> but still cannot make the allocation, it will call the
> >> pageout code again.
> > 
> > Which will immediately return because the caller still has
> > fatal_signal_pending()?
> 
> Other processes are in the middle of freeing pages at
> this point, so we should succeed in __alloc_pages()
> fairly quickly (and then die and free all our memory).

What if it's a uniprocessor machine and all those processes are
scheduled out?  We sit there chewing 100% CPU and not doing anything
afaict.

Even if it _is_ SMP, we could still chew decent-sized blips of CPU time
rattling around waiting for something to happen.
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