netdev - Re: Network performance with small packets

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Message-ID: <20110202154706.GA12738@redhat.com>
Date:	Wed, 2 Feb 2011 17:47:06 +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 07:39:45AM -0800, Shirley Ma wrote:
> On Wed, 2011-02-02 at 12:48 +0200, Michael S. Tsirkin wrote:
> > Yes, I think doing this in the host is much simpler,
> > just send an interrupt after there's a decent amount
> > of space in the queue.
> > 
> > Having said that the simple heuristic that I coded
> > might be a bit too simple.
> 
> >From the debugging out what I have seen so far (a single small message
> TCP_STEAM test), I think the right approach is to patch both guest and
> vhost.

One problem is slowing down the guest helps here.
So there's a chance that just by adding complexity
in guest driver we get a small improvement :(

We can't rely on a patched guest anyway, so
I think it is best to test guest and host changes separately.

And I do agree something needs to be done in guest too,
for example when vqs share an interrupt, we
might invoke a callback when we see vq is not empty
even though it's not requested. Probably should
check interrupts enabled here?

> The problem I have found is a regression for single  small
> message TCP_STEAM test. Old kernel works well for TCP_STREAM, only new
> kernel has problem.

Likely new kernel is faster :)

> For Steven's problem, it's multiple stream TCP_RR issues, the old guest
> doesn't perform well, so does new guest kernel. We tested reducing vhost
> signaling patch before, it didn't help the performance at all.
> 
> Thanks
> Shirley

Yes, it seems unrelated to tx interrupts.

-- 
MST
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