Linux Virtualization - Jan 2016 - virtio ring layout changes for optimal single-stream performance

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.

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.

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.

TODO:
- documentation+discussion of interaction with CPU caching
- thorough benchmarking of different configurations/hosts
- experiment with event index replacements
- better emulate vmexit/vmentry cost overhead
- virtio spec proposal

Thanks!
-- 
MST

On Thu, 21 Jan 2016 15:39:26 +0200
"Michael S. Tsirkin" <mst at redhat.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.

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" <mst at redhat.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?
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.
> > 
> > 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.