With aio=thread, adaptive polling makes latency worse rather than
better, because it delays the execution of the ThreadPool's
completion bottom half.
event_notifier_poll() does run while polling, detecting that
a bottom half was scheduled by a worker thread, but because
ctx->notifier is explicitly ignored in run_poll_handlers_once(),
scheduling the BH does not count as making progress and
run_poll_handlers() keeps running. Fix this by recomputing
the deadline after *timeout could have changed.
With this change, ThreadPool still cannot participate in polling
but at least it does not suffer from extra latency.
Reported-by: Sergio Lopez <slp@redhat.com>
Cc: Stefan Hajnoczi <stefanha@gmail.com>
Cc: Kevin Wolf <kwolf@redhat.com>
Cc: qemu-block@nongnu.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
util/aio-posix.c | 9 +++++++--
1 file changed, 7 insertions(+), 2 deletions(-)
diff --git a/util/aio-posix.c b/util/aio-posix.c
index 6fbfa79..b166cda 100644
--- a/util/aio-posix.c
+++ b/util/aio-posix.c
@@ -519,6 +519,10 @@ static bool run_poll_handlers_once(AioContext *ctx, int64_t *timeout)
if (!node->deleted && node->io_poll &&
aio_node_check(ctx, node->is_external) &&
node->io_poll(node->opaque)) {
+ /*
+ * Polling was successful, exit try_poll_mode immediately
+ * to adjust the next polling time.
+ */
*timeout = 0;
if (node->opaque != &ctx->notifier) {
progress = true;
@@ -558,8 +562,9 @@ static bool run_poll_handlers(AioContext *ctx, int64_t max_ns, int64_t *timeout)
do {
progress = run_poll_handlers_once(ctx, timeout);
elapsed_time = qemu_clock_get_ns(QEMU_CLOCK_REALTIME) - start_time;
- } while (!progress && elapsed_time < max_ns
- && !atomic_read(&ctx->poll_disable_cnt));
+ max_ns = MIN(*timeout, max_ns);
+ assert(!(max_ns && progress));
+ } while (elapsed_time < max_ns && !atomic_read(&ctx->poll_disable_cnt));
/* If time has passed with no successful polling, adjust *timeout to
* keep the same ending time.
--
1.8.3.1