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

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Date:	Thu, 18 Sep 2008 15:22:39 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Martin Bligh <mbligh@...igh.org>
CC:	Christoph Lameter <cl@...ux-foundation.org>,
	Martin Bligh <mbligh@...gle.com>,
	MinChan Kim <minchan.kim@...il.com>,
	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

Martin Bligh wrote:
>>>> 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.
>   

Yes.  My thought was that there should be very little cost to
opportunistically populating the pte for a page which is already
resident anyway.

> 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?)
>   

Right.  The faults themselves are more or less the same as the native
case, but setting a pte requires a hypercall compared to a memory write
in the native case.  But I can set any number of ptes in one hypercall,
so batching would amortize the cost.

    J
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