Re: [PATCH 0/3] virtio: don't poll masked vectors with irqfd

Asias He
2012-12-19T08:50:58Z

On 12/18/2012 08:39 PM, Michael S. Tsirkin wrote:
> At the moment when vector is masked virtio will poll it
> in userspace, even if it is handled by irqfd.
> This is done in order to update pending bits, but
> it's not really required until someone reads the pending bits.
> On the other hand this read results in extra io thread wakeups.
>
> As we only implement the pending bits as a compatibility
> feature (read - real drivers don't use it), we can defer
> the irqfd poll until the read actually happens.
>
> This does not seem to affect vhost-net speed
> in simple benchmarks but could help block: both
> vhost-blk and dataplane when using irqfd,
> and I also think this is cleaner than enabling/disabling
> notifiers all the time.
>
> This will also be the basis for future optimizations.
>
> Michael S. Tsirkin (3):
> msi: add API to get notified about pending bit poll
> msix: expose access to masked/pending state
> virtio-pci: don't poll masked vectors
>
> hw/pci/msix.c | 19 +++++++++++++++----
> hw/pci/msix.h | 6 +++++-
> hw/pci/pci.h | 4 ++++
> hw/vfio_pci.c | 2 +-
> hw/virtio-pci.c | 53 +++++++++++++++++++++++++++++++++++++++++------------
> 5 files changed, 66 insertions(+), 18 deletions(-)

The performance boost is significant here. It is close to the
result of dropping msix_fire_vector_notifier() hack.

--
Asias