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

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Date:	Tue, 21 Feb 2012 13:34:20 +0800
From:	Jason Wang <jasowang@...hat.com>
To:	Shirley Ma <mashirle@...ibm.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 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.

>
> Thanks
> Shirley
>
> --
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