netdev - Re: [v2 RFC PATCH 0/4] Implement multiqueue virtio-net

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Message-ID: <20101012170907.GA30613@redhat.com>
Date:	Tue, 12 Oct 2010 19:09:07 +0200
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Krishna Kumar2 <krkumar2@...ibm.com>
Cc:	anthony@...emonkey.ws, arnd@...db.de, avi@...hat.com,
	davem@...emloft.net, kvm@...r.kernel.org, netdev@...r.kernel.org,
	rusty@...tcorp.com.au
Subject: Re: [v2 RFC PATCH 0/4] Implement multiqueue virtio-net

On Mon, Oct 11, 2010 at 12:51:27PM +0530, Krishna Kumar2 wrote:
> "Michael S. Tsirkin" <mst@...hat.com> wrote on 10/06/2010 07:04:31 PM:
> 
> > On Fri, Sep 17, 2010 at 03:33:07PM +0530, Krishna Kumar wrote:
> > > For 1 TCP netperf, I ran 7 iterations and summed it. Explanation
> > > for degradation for 1 stream case:
> >
> > I thought about possible RX/TX contention reasons, and I realized that
> > we get/put the mm counter all the time.  So I write the following: I
> > haven't seen any performance gain from this in a single queue case, but
> > maybe this will help multiqueue?
> 
> Sorry for the delay, I was sick last couple of days. The results
> with your patch are (%'s over original code):
> 
> Code               BW%       CPU%       RemoteCPU
> MQ     (#txq=16)   31.4%     38.42%     6.41%
> MQ+MST (#txq=16)   28.3%     18.9%      -10.77%
> 
> The patch helps CPU utilization but didn't help single stream
> drop.
> 
> Thanks,

What other shared TX/RX locks are there?  In your setup, is the same
macvtap socket structure used for RX and TX?  If yes this will create
cacheline bounces as sk_wmem_alloc/sk_rmem_alloc share a cache line,
there might also be contention on the lock in sk_sleep waitqueue.
Anything else?

-- 
MST
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