netdev - Re: Network performance with small packets

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Message-ID: <20110203162042.GA10028@redhat.com>
Date:	Thu, 3 Feb 2011 18:20:42 +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 Thu, Feb 03, 2011 at 07:58:00AM -0800, Shirley Ma wrote:
> On Thu, 2011-02-03 at 08:13 +0200, Michael S. Tsirkin wrote:
> > > Initial TCP_STREAM performance results I got for guest to local
> > host 
> > > 4.2Gb/s for 1K message size, (vs. 2.5Gb/s)
> > > 6.2Gb/s for 2K message size, and (vs. 3.8Gb/s)
> > > 9.8Gb/s for 4K message size. (vs.5.xGb/s)
> > 
> > What is the average packet size, # bytes per ack, and the # of
> > interrupts
> > per packet? It could be that just slowing down trahsmission
> > makes GSO work better. 
> 
> There is no TX interrupts with dropping packet.
> 
> GSO/TSO is the key for small message performance, w/o GSO/TSO, the
> performance is limited to about 2Gb/s no matter how big the message size
> it is. I think any work we try here will increase large packet size
> rate. BTW for dropping packet, TCP increased fast retrans, not slow
> start. 
> 
> I will collect tcpdump, netstart before and after data to compare packet
> size/rate w/o w/i the patch.
> 
> Thanks
> Shirley

Just a thought: does it help to make tx queue len of the
virtio device smaller?
E.g. match the vq size?

-- 
MST
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