linux-kernel - Re: virtio ring layout changes for optimal single-stream performance

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Date:	Thu, 21 Jan 2016 16:38:36 +0100
From:	Cornelia Huck <cornelia.huck@...ibm.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
Cc:	virtio@...ts.oasis-open.org, virtio-dev@...ts.oasis-open.org,
	kvm@...r.kernel.org, dev@...k.org, linux-kernel@...r.kernel.org,
	qemu-devel@...gnu.org, virtualization@...ts.linux-foundation.org,
	"Xie, Huawei" <huawei.xie@...el.com>
Subject: Re: virtio ring layout changes for optimal single-stream
 performance

On Thu, 21 Jan 2016 15:39:26 +0200
"Michael S. Tsirkin" <mst@...hat.com> wrote:

> Hi all!
> I have been experimenting with alternative virtio ring layouts,
> in order to speed up single stream performance.
> 
> I have just posted a benchmark I wrote for the purpose, and a (partial)
> alternative layout implementation.  This achieves 20-40% reduction in
> virtio overhead in the (default) polling mode.
> 
> http://article.gmane.org/gmane.linux.kernel.virtualization/26889
> 
> The layout is trying to be as simple as possible, to reduce
> the number of cache lines bouncing between CPUs.

Some kind of diagram or textual description would really help to review
this.

> 
> For benchmarking, the idea is to emulate virtio in user-space,
> artificially adding overhead for e.g. signalling to match what happens
> in case of a VM.

Hm... is this overhead comparable enough between different platform so
that you can get a halfway realistic scenario? What about things like
endianness conversions?

> 
> I'd be very curious to get feedback on this, in particular, some people
> discussed using vectored operations to format virtio ring - would it
> conflict with this work?
> 
> You are all welcome to post enhancements or more layout alternatives as
> patches.

Let me see if I can find time to experiment a bit.