netdev - Re: [RFC PATCH 00/17] virtual-bus

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Message-Id: <200904011638.45135.rusty@rustcorp.com.au>
Date:	Wed, 1 Apr 2009 16:38:44 +1030
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Gregory Haskins <ghaskins@...ell.com>
Cc:	linux-kernel@...r.kernel.org, agraf@...e.de, pmullaney@...ell.com,
	pmorreale@...ell.com, anthony@...emonkey.ws,
	netdev@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [RFC PATCH 00/17] virtual-bus

On Wednesday 01 April 2009 05:12:47 Gregory Haskins wrote:
> Bare metal: tput = 4078Mb/s, round-trip = 25593pps (39us rtt)
> Virtio-net: tput = 4003Mb/s, round-trip = 320pps (3125us rtt)
> Venet: tput = 4050Mb/s, round-trip = 15255 (65us rtt)

That rtt time is awful.  I know the notification suppression heuristic
in qemu sucks.

I could dig through the code, but I'll ask directly: what heuristic do
you use for notification prevention in your venet_tap driver?

As you point out, 350-450 is possible, which is still bad, and it's at least
partially caused by the exit to userspace and two system calls.  If virtio_net
had a backend in the kernel, we'd be able to compare numbers properly.

> Bare metal: tput = 9717Mb/s, round-trip = 30396pps (33us rtt)
> Virtio-net: tput = 4578Mb/s, round-trip = 249pps (4016us rtt)
> Venet: tput = 5802Mb/s, round-trip = 15127 (66us rtt)
> 
> Note that even the throughput was slightly better in this test for venet, though
> neither venet nor virtio-net could achieve line-rate.  I suspect some tuning may
> allow these numbers to improve, TBD.

At some point, the copying will hurt you.  This is fairly easy to avoid on
xmit tho.

Cheers,
Rusty.
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