On Tue, 1 Oct 2019 17:26:09 +0400 Marc-André Lureau <marcandre.lur...@redhat.com> wrote:
> Commit 05e514b1d4d5bd4209e2c8bbc76ff05c85a235f3 introduced an AIO > context optimization to avoid calling event_notifier_test_and_clear() on > ctx->notifier. On Windows, the same notifier is being used to wakeup the > wait on socket events (see commit > d3385eb448e38f828c78f8f68ec5d79c66a58b5d). > > The ctx->notifier event is added to the gpoll sources in > aio_set_event_notifier(), aio_ctx_check() should clear the event > regardless of ctx->notified, since Windows sets the event by itself, > bypassing the aio->notified. This fixes qemu not clearing the event > resulting in a busy loop. > > Paolo suggested to me on irc to call event_notifier_test_and_clear() > after select() >0 from aio-win32.c's aio_prepare. Unfortunately, not all > fds associated with ctx->notifiers are in AIO fd handlers set. > (qemu_set_nonblock() in util/oslib-win32.c calls qemu_fd_register()). > > This is essentially a v2 of a patch that was sent earlier: > https://lists.gnu.org/archive/html/qemu-devel/2017-01/msg00420.html > > that resurfaced when James investigated Spice performance issues on Windows: > https://gitlab.freedesktop.org/spice/spice/issues/36 > > In order to test that patch, I simply tried running test-char on > win32, and it hangs. Applying that patch solves it. QIO idle sources > are not dispatched. I haven't investigated much further, I suspect > source priorities and busy looping still come into play. > > This version keeps the "notified" field, so event_notifier_poll() > should still work as expected. > > Cc: James Le Cuirot <ch...@gentoo.org> > Signed-off-by: Marc-André Lureau <marcandre.lur...@redhat.com> > --- > util/async.c | 6 +++++- > 1 file changed, 5 insertions(+), 1 deletion(-) > > diff --git a/util/async.c b/util/async.c > index 4e4c7af51e..ca83e32c7f 100644 > --- a/util/async.c > +++ b/util/async.c > @@ -354,7 +354,11 @@ void aio_notify(AioContext *ctx) > > void aio_notify_accept(AioContext *ctx) > { > - if (atomic_xchg(&ctx->notified, false)) { > + if (atomic_xchg(&ctx->notified, false) > +#ifdef WIN32 > + || true > +#endif > + ) { > event_notifier_test_and_clear(&ctx->notifier); > } > } I can confirm that this updated patch fixes my performance issue. The idle CPU usage drops from around 35% to around 2%. Moving the mouse now makes the usage go up, not down. :) Many thanks! Regards, -- James Le Cuirot (chewi) Gentoo Linux Developer
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