linux-kernel - Re: [PATCH 2/2] lguest: use get_user_pages_fast() instead of get_user

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Date:	Wed, 30 Jul 2008 16:53:09 +1000
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Nick Piggin <nickpiggin@...oo.com.au>
Cc:	lguest <lguest@...abs.org>, linux-kernel@...r.kernel.org,
	"kvm-devel" <kvm-devel@...ts.sourceforge.net>
Subject: Re: [PATCH 2/2] lguest: use get_user_pages_fast() instead of get_user_pages()

On Wednesday 30 July 2008 15:30:10 Nick Piggin wrote:
> On Wednesday 30 July 2008 14:39, Rusty Russell wrote:
> > 	Before: 5.93 seconds
> > 	After: 5.40 seconds
> >
> > (Counts of slow vs fastpath in this usage are 6092 and 2852462
> > respectively.)
> >
> > And more importantly for lguest, the code is simpler.
>
> Cool. That's actually a fairly significant straight-line performance
> improvement, given that the lguest processes are running one-at-a-time
> right? (and not contending on the locks or bouncing cachelines too much)

Yep.  I mean, the performance is abysmal either way (native for this is about 
10x faster), but it would be interesting to see kvm numbers as they have a 
more sophisticated shadow page table setup.  Not as trivial as lguest since 
they seem to use the mm sem to protect other things...

Cheers,
Rusty.
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