netdev - performance regression in virtio-net in 2.6.32-rc4

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Date:	Mon, 26 Oct 2009 20:48:35 +0200
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Rusty Russell <rusty@...tcorp.com.au>
Cc:	virtualization@...ts.linux-foundation.org, kvm@...r.kernel.org,
	netdev@...r.kernel.org
Subject: performance regression in virtio-net in 2.6.32-rc4

Hi!
I noticed a performance regression in virtio net: going from
2.6.31 to 2.6.32-rc4 I see this, for guest to host communication:

[mst@...k ~]$ ssh robin sh streamtest1
TCP STREAM TEST from 0.0.0.0 (0.0.0.0) port 0 AF_INET to 11.0.0.3
(11.0.0.3) port 0 AF_INET : demo
Recv   Send    Send
Socket Socket  Message  Elapsed
Size   Size    Size     Time     Throughput
bytes  bytes   bytes    secs.    10^6bits/sec

 87380  16384  16384    10.20    7806.48

[mst@...k ~]$ ssh robin sh streamtest1
TCP STREAM TEST from 0.0.0.0 (0.0.0.0) port 0 AF_INET to 11.0.0.3
(11.0.0.3) port 0 AF_INET : demo
Recv   Send    Send
Socket Socket  Message  Elapsed
Size   Size    Size     Time     Throughput
bytes  bytes   bytes    secs.    10^6bits/sec

 87380  16384  16384    10.00    6814.60

Note: I had to revert 48925e372f04f5e35fec6269127c62b2c71ab794,
and I applied a patch
	virtio-pci: fix per-vq MSI-X request logic
which fixes a bug introduced by f68d24082e22ccee3077d11aeb6dc5354f0ca7f1.

Any tips on debugging this?

-- 
MST
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