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aio-posix: completely stop polling when disabled

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One iteration of polling is always performed even when polling is
disabled.  This is done because:
1. Userspace polling is cheaper than making a syscall.  We might get
   lucky.
2. We must poll once more after polling has stopped in case an event
   occurred while stopping polling.

However, there are downsides:
1. Polling becomes a bottleneck when the number of event sources is very
   high.  It's more efficient to monitor fds in that case.
2. A high-frequency polling event source can starve non-polling event
   sources because ppoll(2)/epoll(7) is never invoked.

This patch removes the forced polling iteration so that poll_ns=0 really
means no polling.

IOPS increases from 10k to 60k when the guest has 100
virtio-blk-pci,num-queues=32 devices and 1 virtio-blk-pci,num-queues=1
device because the large number of event sources being polled slows down
the event loop.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Link: https://lore.kernel.org/r/20200305170806.1313245-2-stefanha@redhat.com
Message-Id: <20200305170806.1313245-2-stefanha@redhat.com>
This commit is contained in:
Stefan Hajnoczi 2020-03-05 17:08:00 +00:00
parent c39cbedb54
commit e4346192f1
1 changed files with 15 additions and 7 deletions
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22
util/aio-posix.c
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@ -361,12 +361,13 @@ void aio_set_event_notifier_poll(AioContext *ctx,
(IOHandler *)io_poll_end);
}
static void poll_set_started(AioContext *ctx, bool started)
static bool poll_set_started(AioContext *ctx, bool started)
{
AioHandler *node;
bool progress = false;
if (started == ctx->poll_started) {
return;
return false;
}
ctx->poll_started = started;
@ -388,8 +389,15 @@ static void poll_set_started(AioContext *ctx, bool started)
if (fn) {
fn(node->opaque);
}
/* Poll one last time in case ->io_poll_end() raced with the event */
if (!started) {
progress = node->io_poll(node->opaque) || progress;
}
}
qemu_lockcnt_dec(&ctx->list_lock);
return progress;
}
@ -670,12 +678,12 @@ static bool try_poll_mode(AioContext *ctx, int64_t *timeout)
}
}
poll_set_started(ctx, false);
if (poll_set_started(ctx, false)) {
*timeout = 0;
return true;
}
/* Even if we don't run busy polling, try polling once in case it can make
* progress and the caller will be able to avoid ppoll(2)/epoll_wait(2).
*/
return run_poll_handlers_once(ctx, timeout);
return false;
}
bool aio_poll(AioContext *ctx, bool blocking)