On Thu, Jan 21, 2016 at 04:38:36PM +0100, Cornelia Huck wrote:

On Thu, 21 Jan 2016 15:39:26 +0200
"Michael S. Tsirkin" [off-list ref] wrote:

quoted

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.

quoted

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?

On x86 is seems pretty stable.
It's a question of setting VMEXIT_CYCLES and VMENTRY_CYCLES correctly.

What about things like
endianness conversions?

I didn't bother with them yet.

quoted

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.