netdev - Re: [PATCH 2/2] virtio_net: remove send completion interrupts and avoid TX queue overrun through packet drop

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Date:	Fri, 18 Mar 2011 18:41:55 -0700
From:	Shirley Ma <mashirle@...ibm.com>
To:	Herbert Xu <herbert@...dor.apana.org.au>
Cc:	mst@...hat.com, rusty@...tcorp.com.au, davem@...emloft.net,
	kvm@...r.kernel.org, netdev@...r.kernel.org
Subject: Re: [PATCH 2/2] virtio_net: remove send completion interrupts and
 avoid TX queue overrun through packet drop

On Fri, 2011-03-18 at 08:33 -0500, Herbert Xu wrote:
> Shirley Ma <mashirle@...ibm.com> wrote:
> >
> > +       /* Drop packet instead of stop queue for better performance
> */
> 
> I would like to see some justification as to why this is the right
> way to go and not just papering over the real problem. 

Fair. KVM guest virtio_net TX queue stop/restart is pretty expensive,
which involves:

1. Guest enable callback: one memory barrier, interrupt flag set

2. Host signals guest: one memory barrier, and a TX interrupt from host
to KVM guest through evenfd_signal.

Most of the workload so far we barely see TX over run, except for small
messages TCP_STREAM. 

For small message size TCP_STREAM workload, no matter how big the TX
queue size is, it always causes overrun. I even re-enable the TX queue
when it's empty, it still hits TX overrun again and again.

Somehow KVM guest and host is not in pace on processing small packets. I
tried to pin each thread to different CPU, it didn't help. So it didn't
seem to be scheduling related.

>>From the performance results, we can see dramatically performance gain
with this patch.

I would like to dig out the real reason why host can't in pace with
guest, but haven't figured it out in month, that's the reason I held
this patch for a while. However if anyone can give me any ideas on how
to debug the real problem, I am willing to try it out. 

Thanks
Shirley

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