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 <s...@redhat.com>
Cc: Stefan Hajnoczi <stefa...@gmail.com>
Cc: Kevin Wolf <kw...@redhat.com>
Cc: qemu-bl...@nongnu.org
Signed-off-by: Paolo Bonzini <pbonz...@redhat.com>
Message-Id: <1553692145-86728-1-git-send-email-pbonz...@redhat.com>
Signed-off-by: Paolo Bonzini <pbonz...@redhat.com>
---
v1->v2: use qemu_soonest_timeout to handle timeout == -1
util/aio-posix.c | 12 ++++++++----
1 file changed, 8 insertions(+), 4 deletions(-)
diff --git a/util/aio-posix.c b/util/aio-posix.c
index 6fbfa7924f..db11021287 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 = qemu_soonest_timeout(*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.
@@ -585,8 +590,7 @@ static bool run_poll_handlers(AioContext *ctx, int64_t
max_ns, int64_t *timeout)
*/
static bool try_poll_mode(AioContext *ctx, int64_t *timeout)
{
- /* See qemu_soonest_timeout() uint64_t hack */
- int64_t max_ns = MIN((uint64_t)*timeout, (uint64_t)ctx->poll_ns);
+ int64_t max_ns = qemu_soonest_timeout(*timeout, ctx->poll_ns);
if (max_ns && !atomic_read(&ctx->poll_disable_cnt)) {
poll_set_started(ctx, true);
--
2.21.0
