I think these results look a bit more in line with expectations on the
quick sniff test:
QEMU_AIO_POLL_MAX_NS IOPs
unset 26,299
1 25,929
2 25,753
4 27,214
8 27,053
16 26,861
32 24,752
64 25,058
128 24,732
256 25,560
512 24,614
1,024 25,186
2,048 25,829
4,096 25,671
8,192 27,896
16,384 38,086
32,768 35,493
65,536 38,496
131,072 38,296
I did a spot check of CPU utilization when the polling started having
benefits.
Without polling (QEMU_AIO_POLL_MAX_NS=unset) the iothread's CPU usage
looked like this:
user time: 25.94%
system time: 22.11%
With polling and QEMU_AIO_POLL_MAX_NS=16384 the iothread's CPU usage
looked like this:
user time: 78.92%
system time: 20.80%
Karl
On 11/16/2016 11:46 AM, Stefan Hajnoczi wrote:
> 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(-)
>
--
Karl Rister <kris...@redhat.com>
