[Qemu-devel] [PATCH v2 0/4] aio: experimental virtio-blk polling mode

[Stefan Hajnoczi <stefanha@redhat.com>]

v2:
 * Uninitialized node->deleted gone [Fam]
 * Removed 1024 polling loop iteration qemu_clock_get_ns() optimization which
   created a weird step pattern [Fam]
 * Unified with AioHandler, dropped AioPollHandler struct [Paolo]
   (actually I think Paolo had more in mind but this is the first step)
 * Only poll when all event loop resources support it [Paolo]
 * Added run_poll_handlers_begin/end trace events for perf analysis
 * Sorry, Christian, no virtqueue kick suppression yet

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!

Stefan Hajnoczi (4):
  aio: add AioPollFn and io_poll() interface
  aio: add polling mode to AioContext
  virtio: poll virtqueues for new buffers
  linux-aio: poll ring for completions

 aio-posix.c                 | 115 ++++++++++++++++++++++++++++++++++++++++++--
 async.c                     |  14 +++++-
 block/curl.c                |   8 +--
 block/iscsi.c               |   3 +-
 block/linux-aio.c           |  19 +++++++-
 block/nbd-client.c          |   8 +--
 block/nfs.c                 |   7 +--
 block/sheepdog.c            |  26 +++++-----
 block/ssh.c                 |   4 +-
 block/win32-aio.c           |   4 +-
 hw/virtio/virtio.c          |  18 ++++++-
 include/block/aio.h         |   8 ++-
 iohandler.c                 |   2 +-
 nbd/server.c                |   9 ++--
 stubs/set-fd-handler.c      |   1 +
 tests/test-aio.c            |   4 +-
 trace-events                |   4 ++
 util/event_notifier-posix.c |   2 +-
 18 files changed, 207 insertions(+), 49 deletions(-)

-- 
2.7.4