Re: [PATCH net V2] vhost: net: switch to use data copy if pending DMAs exceed the limit

[David Miller]

From: Jason Wang <jasowang@xxxxxxxxxx>
Date: Fri,  7 Mar 2014 13:28:27 +0800

> This is because the delay added by htb may lead the delay the finish
> of DMAs and cause the pending DMAs for tap0 exceeds the limit
> (VHOST_MAX_PEND). In this case vhost stop handling tx request until
> htb send some packets. The problem here is all of the packets
> transmission were blocked even if it does not go to VM2.

Isn't this essentially head of line blocking?

> We can solve this issue by relaxing it a little bit: switching to use
> data copy instead of stopping tx when the number of pending DMAs
> exceed half of the vq size. This is safe because:
> 
> - The number of pending DMAs were still limited (half of the vq size)
> - The out of order completion during mode switch can make sure that
>   most of the tx buffers were freed in time in guest.
> 
> So even if about 50% packets were delayed in zero-copy case, vhost
> could continue to do the transmission through data copy in this case.
> 
> Test result:
> 
> Before this patch:
> VM1 to VM2 throughput is 9.3Mbit/s
> VM1 to External throughput is 40Mbit/s
> CPU utilization is 7%
> 
> After this patch:
> VM1 to VM2 throughput is 9.3Mbit/s
> Vm1 to External throughput is 93Mbit/s
> CPU utilization is 16%
> 
> Completed performance test on 40gbe shows no obvious changes in both
> throughput and cpu utilization with this patch.
> 
> The patch only solve this issue when unlimited sndbuf. We still need a
> solution for limited sndbuf.
> 
> Cc: Michael S. Tsirkin <mst@xxxxxxxxxx>
> Cc: Qin Chuanyu <qinchuanyu@xxxxxxxxxx>
> Signed-off-by: Jason Wang <jasowang@xxxxxxxxxx>

I'd like some vhost experts reviewing this before I apply it.
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