netdev - Re: [PATCH 2/2] vhost-net: add a spin

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Date:	Mon, 20 Feb 2012 22:28:36 -0800
From:	Shirley Ma <mashirle@...ibm.com>
To:	Jason Wang <jasowang@...hat.com>
Cc:	"Michael S. Tsirkin" <mst@...hat.com>,
	Anthony Liguori <aliguori@...ibm.com>, netdev@...r.kernel.org,
	Tom Lendacky <toml@...ibm.com>,
	Cristian Viana <vianac@...ibm.com>
Subject: Re: [PATCH 2/2] vhost-net: add a spin_threshold parameter

On Tue, 2012-02-21 at 13:34 +0800, Jason Wang wrote:
> On 02/21/2012 09:35 AM, Shirley Ma wrote:
> > We tried similar approach before by using a minimum timer for
> handle_tx
> > to stay in the loop to accumulate more packets before enabling the
> guest
> > notification. It did have better TCP_RRs, UDP_RRs results. However,
> we
> > think this is just a debug patch. We really need to understand why
> > handle_tx can't see more packets to process for multiple instances
> > request/response type of workload first. Spinning in this loop is
> not a
> > good solution.
> 
> Spinning help for the latency, but looks like we need some adaptive 
> method to adjust the threshold dynamically such as monitor the
> minimum 
> time gap between two packets. For throughput, if we can improve the 
> batching of small packets we can improve it. I've tired to use event 
> index to delay the tx kick until a specified number of packets were 
> batched in the virtqueue. Test shows improvement of throughput in
> small 
> packets as the number of #exit were reduced greatly ( the
> packets/#exit 
> and cpu utilization were increased), but it damages the performance
> of 
> other. This is only for debug, but it confirms that there's something
> we 
> need to improve the batching.

Our test case was 60 instances 256/256 bytes tcp_rrs or udp_rrs. In
theory there should be multiple packets in the queue by the time vhost
gets notified, but from debugging output, there was only a few or even
one packet in the queue. So the questions here why the time gap between
two packets is that big?

Shirley

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