netdev - Re: Network performance with small packets

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Date:	Wed, 2 Feb 2011 12:04:37 +0530
From:	Krishna Kumar2 <krkumar2@...ibm.com>
To:	Shirley Ma <mashirle@...ibm.com>
Cc:	David Miller <davem@...emloft.net>, kvm@...r.kernel.org,
	mashirle@...ux.vnet.ibm.com, "Michael S. Tsirkin" <mst@...hat.com>,
	netdev@...r.kernel.org, netdev-owner@...r.kernel.org,
	Sridhar Samudrala <sri@...ibm.com>,
	Steve Dobbelstein <steved@...ibm.com>
Subject: Re: Network performance with small packets

> On Tue, 2011-02-01 at 22:05 -0800, Shirley Ma wrote:
> >
> > The way I am changing is only when netif queue has stopped, then we
> > start to count num_free descriptors to send the signal to wake netif
> > queue.
>
> I forgot to mention, the code change I am making is in guest kernel, in
> xmit call back only wake up the queue when it's stopped && num_free >=
> 1/2 *vq->num, I add a new API in virtio_ring.

FYI :)

I have tried this before. There are a couple of issues:

1. the free count will not reduce until you run free_old_xmit_skbs,
   which will not run anymore since the tx queue is stopped.
2. You cannot call free_old_xmit_skbs directly as it races with a
   queue that was just awakened (current cb was due to the delay
   in disabling cb's).

You have to call free_old_xmit_skbs() under netif_queue_stopped()
check to avoid the race.

I got a small improvement in my testing upto some number of threads
(32 or 48?), but beyond that I was getting a regression.

Thanks,

- KK

> However vhost signaling reduction is needed as well. The patch I
> submitted a while ago showed both CPUs and BW improvement.

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