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

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Message-ID: <8f3aa8d60809181518r697b5d7cp9e154fe73f804cc7@mail.gmail.com>
Date:	Thu, 18 Sep 2008 15:18:05 -0700
From:	"Martin Bligh" <mbligh@...igh.org>
To:	"Christoph Lameter" <cl@...ux-foundation.org>
Cc:	"Martin Bligh" <mbligh@...gle.com>,
	"MinChan Kim" <minchan.kim@...il.com>,
	"Chris Snook" <csnook@...hat.com>,
	"Jeremy Fitzhardinge" <jeremy@...p.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

>>> My patches were only for anonymous pages not for file backed because readahead
>>> is available for file backed mappings.
>>
>> Do we populate the PTEs though? I didn't think that was batched, but I
>> might well be wrong.
>
> We do not populate the PTEs and AFAICT PTE population was assumed not to be
> performance critical since the backing media is comparatively slow.

I think the times when this matters are things like glibc, which are
heavily shared -
we were only 'prefaulting' when the pagecache was already there. So it's a case
for a "readahead like algorithm", not necessarily a direct hook.

Anonymous pages seem much riskier, as presumably there's a no backing page
except in the fork case.

I presume the reason Jeremy is interested is because his pagefaults are more
expensive than most (under virtualization), so he may well find a
different tradeoff
than I did (try running kernbench?)
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