netdev - Re: [PATCHv7] add mergeable buffers support to vhost

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Message-ID: <OF13BEB29A.FE5BAFD2-ON8825771F.0060D8AA-8825771F.006196FE@us.ibm.com>
Date:	Mon, 10 May 2010 10:46:00 -0700
From:	David Stevens <dlstevens@...ibm.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
Cc:	kvm@...r.kernel.org, kvm-owner@...r.kernel.org,
	netdev@...r.kernel.org, netdev-owner@...r.kernel.org,
	virtualization@...ts.osdl.org
Subject: Re: [PATCHv7] add mergeable buffers support to vhost_net

netdev-owner@...r.kernel.org wrote on 05/10/2010 10:25:57 AM:

> On Mon, May 10, 2010 at 10:09:03AM -0700, David Stevens wrote:
> > Since "datalen" carries the difference and will be negative by that 
amount
> > from the original loop, what about just adding something like:
> > 
> >         }
> >         if (headcount)
> >                 heads[headcount-1].len += datalen;
> > [and really, headcount >0 since datalen > 0, so just:
> > 
> >         heads[headcount-1].len += datalen;
> > 
> >                                                 +-DLS
> 
> This works too, just does more checks and comparisons.
> I am still surprised that you were unable to reproduce the problem.
> 

I'm sure it happened, and probably had a performance
penalty on my systems too, but not as much as yours.
I didn't see any obvious performance difference running
with the patch, though; not sure why. I'll instrument to
see how often it's happening, I think.
        But fixed now, good catch!

                                                        +-DLS

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