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

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Message-ID: <33307c790809181352h14f2cf26kc73de75b939177b5@mail.gmail.com>
Date:	Thu, 18 Sep 2008 13:52:04 -0700
From:	"Martin Bligh" <mbligh@...gle.com>
To:	"Jeremy Fitzhardinge" <jeremy@...p.org>
Cc:	"Christoph Lameter" <cl@...ux-foundation.org>,
	"Chris Snook" <csnook@...hat.com>,
	"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

>
> 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".
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