linux-kernel - Re: [PATCH] mmu notifiers #v8

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Date:	Mon, 3 Mar 2008 12:23:08 -0600
From:	Jack Steiner <steiner@....com>
To:	Avi Kivity <avi@...ranet.com>
Cc:	Nick Piggin <npiggin@...e.de>,
	Andrea Arcangeli <andrea@...ranet.com>,
	akpm@...ux-foundation.org, Robin Holt <holt@....com>,
	Izik Eidus <izike@...ranet.com>,
	kvm-devel@...ts.sourceforge.net,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	general@...ts.openfabrics.org,
	Steve Wise <swise@...ngridcomputing.com>,
	Roland Dreier <rdreier@...co.com>,
	Kanoj Sarcar <kanojsarcar@...oo.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	daniel.blueman@...drics.com, Christoph Lameter <clameter@....com>
Subject: Re: [PATCH] mmu notifiers #v8

On Mon, Mar 03, 2008 at 08:09:49PM +0200, Avi Kivity wrote:
> Jack Steiner wrote:
> >The range invalidates have a performance advantage for the GRU. TLB 
> >invalidates
> >on the GRU are relatively slow (usec) and interfere somewhat with the 
> >performance
> >of other active GRU instructions. Invalidating a large chunk of addresses 
> >with
> >a single GRU TLBINVAL operation is must faster than issuing a stream of 
> >single
> >page TLBINVALs.
> >
> >I expect this performance advantage will also apply to other users of 
> >mmuops.
> >  
> 
> In theory this would apply to kvm as well (coalesce tlb flush IPIs, 
> lookup shadow page table once), but is it really a fast path?  What 
> triggers range operations for your use cases?
 

Although not frequent, an unmap of a multiple TB object could be quite painful
if each page was invalidated individually instead of 1 invalidate for the entire range.
This is even worse if the application is threaded and the object has been reference by
many GRUs (there are 16 GRU ports per node - each potentially has to be invalidated).

Forks (again, not frequent) would be another case.



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