linux-kernel - Re: Populating multiple ptes at fault time

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Message-ID: <48D2BFB8.6010503@redhat.com>
Date:	Thu, 18 Sep 2008 16:53:12 -0400
From:	Chris Snook <csnook@...hat.com>
To:	Martin Bligh <mbligh@...gle.com>
CC:	Jeremy Fitzhardinge <jeremy@...p.org>,
	Christoph Lameter <cl@...ux-foundation.org>,
	Nick Piggin <nickpiggin@...oo.com.au>,
	Hugh Dickens <hugh@...itas.com>,
	Linux Memory Management List <linux-mm@...ck.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Avi Kivity <avi@...ranet.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Rik van Riel <riel@...hat.com>
Subject: Re: Populating multiple ptes at fault time

Martin Bligh wrote:
>> Thanks, that was exactly what I was hoping to see.  I didn't see any
>> definitive statements against the patch set, other than a concern that
>> it could make things worse.  Was the upshot that no consensus was
>> reached about how to detect when its beneficial to preallocate anonymous
>> pages?
>>
>> Martin, in that thread you mentioned that you had tried pre-populating
>> file-backed mappings as well, but "Mmmm ... we tried doing this before
>> for filebacked pages by sniffing the
>> pagecache, but it crippled forky workloads (like kernel compile) with the
>> extra cost in zap_pte_range, etc. ".
>>
>> Could you describe, or have a pointer to, what you tried and how it
>> turned out?
> 
> Don't have the patches still, but it was fairly simple - just faulted in
> the next 3 pages whenever we took a fault, if the pages were already
> in pagecache. I would have thought that was pretty lightweight and
> non-invasive, but turns out it slowed things down.
> 
>> Did you end up populating so many (unused) ptes that
>> zap_pte_range needed to do lots more work?
> 
> Yup, basically you're assuming good locality of reference, but it turns
> out that (as davej would say) "userspace sucks".

Well, *most* userspace sucks.  It might still be worthwhile to do this when 
userspace is using madvise().

-- Chris
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