Re: [PATCH] util/aio: Defer disabling poll mode as long as possible

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Sun, Jul 10, 2022 at 08:08:49PM +0800, Chao Gao wrote:
> When we measure FIO read performance (cache=writethrough, bs=4k,
> iodepth=64) in VMs, ~80K/s notifications (e.g., EPT_MISCONFIG) are observed
> from guest to qemu.
> 
> It turns out those frequent notificatons are caused by interference from
> worker threads. Worker threads queue bottom halves after completing IO
> requests.  Pending bottom halves may lead to either aio_compute_timeout()
> zeros timeout and pass it to try_poll_mode() or run_poll_handlers() returns
> no progress after noticing pending aio_notify() events. Both cause
> run_poll_handlers() to call poll_set_started(false) to disable poll mode.
> However, for both cases, as timeout is already zeroed, the event loop
> (i.e., aio_poll()) just processes bottom halves and then starts the next
> event loop iteration. So, disabling poll mode has no value but leads to
> unnecessary notifications from guest.
> 
> To minimize unnecessary notifications from guest, defer disabling poll
> mode to when the event loop is about to be blocked.
> 
> With this patch applied, FIO seq-read performance (bs=4k, iodepth=64,
> cache=writethrough) in VMs increases from 330K/s to 413K/s IOPS.
> 
> Suggested-by: Stefan Hajnoczi <stefanha@redhat.com>
> Signed-off-by: Chao Gao <chao.gao@intel.com>
> ---
>  util/aio-posix.c | 21 +++++++++++++++------
>  1 file changed, 15 insertions(+), 6 deletions(-)

I just noticed that I forgot to send a pull request with this for QEMU
7.1. It's my fault that this missed QEMU 7.1, sorry. It will be merged
once the 7.2 merge window opens.

Stefan

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