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

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Message-Id: <20140307.163948.633184079575086092.davem@davemloft.net>
Date:	Fri, 07 Mar 2014 16:39:48 -0500 (EST)
From:	David Miller <davem@...emloft.net>
To:	jasowang@...hat.com
Cc:	mst@...hat.com, kvm@...r.kernel.org,
	virtio-dev@...ts.oasis-open.org,
	virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org, qinchuanyu@...wei.com
Subject: Re: [PATCH net V2] vhost: net: switch to use data copy if pending
 DMAs exceed the limit

From: Jason Wang <jasowang@...hat.com>
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@...hat.com>
> Cc: Qin Chuanyu <qinchuanyu@...wei.com>
> Signed-off-by: Jason Wang <jasowang@...hat.com>

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