netdev - Re: Network performance with small packets

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Message-ID: <20110201214114.GA31105@redhat.com>
Date:	Tue, 1 Feb 2011 23:41:14 +0200
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Shirley Ma <mashirle@...ibm.com>
Cc:	David Miller <davem@...emloft.net>, steved@...ibm.com,
	netdev@...r.kernel.org, kvm@...r.kernel.org, rusty@...tcorp.com.au
Subject: Re: Network performance with small packets

On Tue, Feb 01, 2011 at 01:28:45PM -0800, Shirley Ma wrote:
> On Tue, 2011-02-01 at 23:21 +0200, Michael S. Tsirkin wrote:
> > Confused. We compare capacity to skb frags, no?
> > That's sg I think ...
> 
> Current guest kernel use indirect buffers, num_free returns how many
> available descriptors not skb frags. So it's wrong here.
> 
> Shirley

I see. Good point. In other words when we complete the buffer
it was indirect, but when we add a new one we
can not allocate indirect so we consume.
And then we start the queue and add will fail.
I guess we need some kind of API to figure out
whether the buf we complete was indirect?

Another failure mode is when skb_xmit_done
wakes the queue: it might be too early, there
might not be space for the next packet in the vq yet.

A solution might be to keep some kind of pool
around for indirect, we wanted to do it for block anyway ...

-- 
MST
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