netdev - Re: Network performance with small packets

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Date:	Wed, 2 Feb 2011 22:17:31 +0200
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Shirley Ma <mashirle@...ibm.com>
Cc:	Krishna Kumar2 <krkumar2@...ibm.com>,
	David Miller <davem@...emloft.net>, kvm@...r.kernel.org,
	mashirle@...ux.vnet.ibm.com, netdev@...r.kernel.org,
	netdev-owner@...r.kernel.org, Sridhar Samudrala <sri@...ibm.com>,
	Steve Dobbelstein <steved@...ibm.com>
Subject: Re: Network performance with small packets

On Wed, Feb 02, 2011 at 11:29:35AM -0800, Shirley Ma wrote:
> On Wed, 2011-02-02 at 20:27 +0200, Michael S. Tsirkin wrote:
> > On Wed, Feb 02, 2011 at 10:11:51AM -0800, Shirley Ma wrote:
> > > On Wed, 2011-02-02 at 19:32 +0200, Michael S. Tsirkin wrote:
> > > > OK, but this should have no effect with a vhost patch
> > > > which should ensure that we don't get an interrupt
> > > > until the queue is at least half empty.
> > > > Right?
> > > 
> > > There should be some coordination between guest and vhost.
> > 
> > What kind of coordination? With a patched vhost, and a full ring.
> > you should get an interrupt per 100 packets.
> > Is this what you see? And if yes, isn't the guest patch
> > doing nothing then?
> 
> vhost_signal won't be able send any TX interrupts to guest when guest TX
> interrupt is disabled. Guest TX interrupt is only enabled when running
> out of descriptors.

Well, this is also the only case where the queue is stopped, no?

> > > We shouldn't
> > > count the TX packets when netif queue is enabled since next guest TX
> > > xmit will free any used buffers in vhost. We need to be careful here
> > in
> > > case we miss the interrupts when netif queue has stopped.
> > > 
> > > However we can't change old guest so we can test the patches
> > separately
> > > for guest only, vhost only, and the combination.
> > > 
> > > > > > 
> > > > > > Yes, it seems unrelated to tx interrupts. 
> > > > > 
> > > > > The issue is more likely related to latency.
> > > > 
> > > > Could be. Why do you think so?
> > > 
> > > Since I played with latency hack, I can see performance difference
> > for
> > > different latency.
> > 
> > Which hack was that? 
> 
> I tried to accumulate multiple guest to host notifications for TX xmits,
> it did help multiple streams TCP_RR results;

I don't see a point to delay used idx update, do you?
So delaying just signal seems better, right?

> I also forced vhost
> handle_tx to handle more packets; both hack seemed help.
> 
> Thanks
> Shirley

Haven't noticed that part, how does your patch make it
handle more packets?

-- 
MST
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