Re: [Qemu-devel] [RFC 0/3] aio: experimental virtio-blk polling mode

[Stefan Hajnoczi]

On Mon, Nov 14, 2016 at 03:59:32PM +0100, Christian Borntraeger wrote:
> On 11/09/2016 06:13 PM, Stefan Hajnoczi wrote:
> > Recent performance investigation work done by Karl Rister shows that the
> > guest->host notification takes around 20 us.  This is more than the 
> > "overhead"
> > of QEMU itself (e.g. block layer).
> > 
> > One way to avoid the costly exit is to use polling instead of notification.
> > The main drawback of polling is that it consumes CPU resources.  In order to
> > benefit performance the host must have extra CPU cycles available on 
> > physical
> > CPUs that aren't used by the guest.
> > 
> > This is an experimental AioContext polling implementation.  It adds a 
> > polling
> > callback into the event loop.  Polling functions are implemented for 
> > virtio-blk
> > virtqueue guest->host kick and Linux AIO completion.
> > 
> > The QEMU_AIO_POLL_MAX_NS environment variable sets the number of 
> > nanoseconds to
> > poll before entering the usual blocking poll(2) syscall.  Try setting this
> > variable to the time from old request completion to new virtqueue kick.
> > 
> > By default no polling is done.  The QEMU_AIO_POLL_MAX_NS must be set to get 
> > any
> > polling!
> > 
> > Karl: I hope you can try this patch series with several QEMU_AIO_POLL_MAX_NS
> > values.  If you don't find a good value we should double-check the tracing 
> > data
> > to see if this experimental code can be improved.
> > 
> > Stefan Hajnoczi (3):
> >   aio-posix: add aio_set_poll_handler()
> >   virtio: poll virtqueues for new buffers
> >   linux-aio: poll ring for completions
> > 
> >  aio-posix.c         | 133 
> > ++++++++++++++++++++++++++++++++++++++++++++++++++++
> >  block/linux-aio.c   |  17 +++++++
> >  hw/virtio/virtio.c  |  19 ++++++++
> >  include/block/aio.h |  16 +++++++
> >  4 files changed, 185 insertions(+)
> > 
> 
> Another observation: With more iothreads than host CPUs the performance drops 
> significantly.

This makes sense although we can eliminate it in common cases by only
polling when we actually need to monitor events.

The current series wastes CPU on Linux AIO polling when no requests are
pending ;).

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