Re: [PATCH v2 3/4] virtio: use defer_call() in virtio_irqfd_notify()

[Eric Blake <eblake@xxxxxxxxxx>]

On Thu, Aug 17, 2023 at 11:58:46AM -0400, Stefan Hajnoczi wrote:
> virtio-blk and virtio-scsi invoke virtio_irqfd_notify() to send Used
> Buffer Notifications from an IOThread. This involves an eventfd
> write(2) syscall. Calling this repeatedly when completing multiple I/O
> requests in a row is wasteful.
> 
> Use the defer_call() API to batch together virtio_irqfd_notify() calls
> made during thread pool (aio=threads), Linux AIO (aio=native), and
> io_uring (aio=io_uring) completion processing.
> 
> Behavior is unchanged for emulated devices that do not use
> defer_call_begin()/defer_call_end() since defer_call() immediately
> invokes the callback when called outside a
> defer_call_begin()/defer_call_end() region.
> 
> fio rw=randread bs=4k iodepth=64 numjobs=8 IOPS increases by ~9% with a
> single IOThread and 8 vCPUs. iodepth=1 decreases by ~1% but this could
> be noise. Detailed performance data and configuration specifics are
> available here:
> https://gitlab.com/stefanha/virt-playbooks/-/tree/blk_io_plug-irqfd
> 
> This duplicates the BH that virtio-blk uses for batching. The next
> commit will remove it.
> 
> Signed-off-by: Stefan Hajnoczi <stefanha@xxxxxxxxxx>
> ---
>  block/io_uring.c   |  6 ++++++
>  block/linux-aio.c  |  4 ++++
>  hw/virtio/virtio.c | 11 ++++++++++-
>  util/thread-pool.c |  5 +++++
>  4 files changed, 25 insertions(+), 1 deletion(-)

Reviewed-by: Eric Blake <eblake@xxxxxxxxxx>

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.
Virtualization:  qemu.org | libguestfs.org