[Qemu-devel] [PATCH v4 00/13] aio: experimental virtio-blk polling mode

[Stefan Hajnoczi]

v4:
 * Added poll time self-tuning algorithm [Christian and Paolo]
 * Try a single iteration of polling to avoid non-blocking 
ppoll(2)/epoll_wait(2) [Paolo]
 * Reordered patches to make performance analysis easier - see below

v3:
 * Avoid ppoll(2)/epoll_wait(2) if polling succeeded [Paolo]
 * Disable guest->host virtqueue notification during polling [Christian]
 * Rebased on top of my virtio-blk/scsi virtqueue notification disable patches

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 -object iothread,poll-max-ns=NUM parameter sets the number of nanoseconds
to poll before entering the usual blocking poll(2) syscall.  Try setting this
parameter to the time from old request completion to new virtqueue kick.  By
default no polling is done so you must set this parameter to get busy polling.

Patch series overview:

Here are convenient points in the patch series at which to do performance
benchmarking to see how various pieces interact.

1. Basic -iothread poll-max-ns=NUM parameter without self-tuning:

aio: add flag to skip fds to aio_dispatch()
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
iothread: add polling parameters

2. My "[PATCH 0/3] virtio: disable notifications in blk and scsi" series is
   needed as a base.  This is unrelated to polling but we'll also disable
   notifications during polling later:

virtio-blk: suppress virtqueue kick during processing
virtio-scsi: suppress virtqueue kick during processing

3. Disable virtqueue notifications (reduce vmexits) during polling:

virtio: turn vq->notification into a nested counter
aio: add .io_poll_begin/end() callbacks
virtio: disable virtqueue notifications during polling

4. Self-tuning, adjusts polling time depending on wait times:

aio: self-tune polling time
iothread: add poll-grow and poll-shrink parameters

Stefan Hajnoczi (13):
  aio: add flag to skip fds to aio_dispatch()
  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
  iothread: add polling parameters
  virtio-blk: suppress virtqueue kick during processing
  virtio-scsi: suppress virtqueue kick during processing
  virtio: turn vq->notification into a nested counter
  aio: add .io_poll_begin/end() callbacks
  virtio: disable virtqueue notifications during polling
  aio: self-tune polling time
  iothread: add poll-grow and poll-shrink parameters

 include/block/aio.h         |  53 +++++++-
 include/sysemu/iothread.h   |   5 +
 aio-posix.c                 | 308 +++++++++++++++++++++++++++++++++++++++-----
 aio-win32.c                 |  32 ++++-
 async.c                     |  21 ++-
 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/block/virtio-blk.c       |  18 ++-
 hw/scsi/virtio-scsi.c       |  36 +++---
 hw/virtio/virtio.c          |  54 ++++++--
 iohandler.c                 |   2 +-
 iothread.c                  |  84 ++++++++++++
 nbd/server.c                |   9 +-
 stubs/set-fd-handler.c      |   1 +
 tests/test-aio.c            |   4 +-
 util/event_notifier-posix.c |   2 +-
 trace-events                |   6 +
 23 files changed, 604 insertions(+), 110 deletions(-)

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2.9.3