netdev - Re: [RFC PATCH 1/1] vhost: TX used buffer guest signal accumulation

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Date:	Fri, 29 Oct 2010 08:43:08 -0700
From:	Shirley Ma <mashirle@...ibm.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
Cc:	David Miller <davem@...emloft.net>, netdev@...r.kernel.org,
	kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 1/1] vhost: TX used buffer guest signal accumulation

On Fri, 2010-10-29 at 10:10 +0200, Michael S. Tsirkin wrote:
> Hmm. I don't yet understand. We are still doing copies into the per-vq
> buffer, and the data copied is really small.  Is it about cache line
> bounces?  Could you try figuring it out?

per-vq buffer is much less expensive than 3 put_copy() call. I will
collect the profiling data to show that.

> > > 2. How about flushing out queued stuff before we exit
> > >    the handle_tx loop? That would address most of
> > >    the spec issue. 
> > 
> > The performance is almost as same as the previous patch. I will
> resubmit
> > the modified one, adding vhost_add_used_and_signal_n after handle_tx
> > loop for processing pending queue.
> > 
> > This patch was a part of modified macvtap zero copy which I haven't
> > submitted yet. I found this helped vhost TX in general. This pending
> > queue will be used by DMA done later, so I put it in vq instead of a
> > local variable in handle_tx.
> > 
> > Thanks
> > Shirley
> 
> BTW why do we need another array? Isn't heads field exactly what we
> need
> here?

head field is only for up to 32, the more used buffers add and signal
accumulated the better performance is from test results. That's was one
of the reason I didn't use heads. The other reason was I used these
buffer for pending dma done in mavctap zero copy patch. It could be up
to vq->num in worse case.

Thanks
Shirley

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