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aio-posix: keep aio_notify_me disabled during polling

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Polling only monitors the ctx->notified field and does not need the
ctx->notifier EventNotifier to be signalled. Keep ctx->aio_notify_me
disabled while polling to avoid unnecessary EventNotifier syscalls.

This optimization improves virtio-blk 4KB random read performance by
18%. The following results are with an IOThread and the null-co block
driver:

Test         IOPS   Error
Before  244518.62 ± 1.20%
After   290706.11 ± 0.44%

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-id: 20200806131802.569478-4-stefanha@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
This commit is contained in:
Stefan Hajnoczi 2020-08-06 14:18:02 +01:00
parent 601829f88e
commit 44277bf914
1 changed files with 29 additions and 26 deletions
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55
util/aio-posix.c
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@ -464,9 +464,6 @@ static bool remove_idle_poll_handlers(AioContext *ctx, int64_t now)
*
* Polls for a given time.
*
* Note that ctx->notify_me must be non-zero so this function can detect
* aio_notify().
*
* Note that the caller must have incremented ctx->list_lock.
*
* Returns: true if progress was made, false otherwise
@ -476,7 +473,6 @@ static bool run_poll_handlers(AioContext *ctx, int64_t max_ns, int64_t *timeout)
bool progress;
int64_t start_time, elapsed_time;
assert(ctx->notify_me);
assert(qemu_lockcnt_count(&ctx->list_lock) > 0);
trace_run_poll_handlers_begin(ctx, max_ns, *timeout);
@ -520,8 +516,6 @@ static bool run_poll_handlers(AioContext *ctx, int64_t max_ns, int64_t *timeout)
* @timeout: timeout for blocking wait, computed by the caller and updated if
* polling succeeds.
*
* ctx->notify_me must be non-zero so this function can detect aio_notify().
*
* Note that the caller must have incremented ctx->list_lock.
*
* Returns: true if progress was made, false otherwise
@ -556,6 +550,7 @@ bool aio_poll(AioContext *ctx, bool blocking)
AioHandlerList ready_list = QLIST_HEAD_INITIALIZER(ready_list);
int ret = 0;
bool progress;
bool use_notify_me;
int64_t timeout;
int64_t start = 0;
@ -566,23 +561,6 @@ bool aio_poll(AioContext *ctx, bool blocking)
*/
assert(in_aio_context_home_thread(ctx));
/* aio_notify can avoid the expensive event_notifier_set if
* everything (file descriptors, bottom halves, timers) will
* be re-evaluated before the next blocking poll(). This is
* already true when aio_poll is called with blocking == false;
* if blocking == true, it is only true after poll() returns,
* so disable the optimization now.
*/
if (blocking) {
atomic_set(&ctx->notify_me, atomic_read(&ctx->notify_me) + 2);
/*
* Write ctx->notify_me before computing the timeout
* (reading bottom half flags, etc.). Pairs with
* smp_mb in aio_notify().
*/
smp_mb();
}
qemu_lockcnt_inc(&ctx->list_lock);
if (ctx->poll_max_ns) {
@ -593,6 +571,29 @@ bool aio_poll(AioContext *ctx, bool blocking)
progress = try_poll_mode(ctx, &timeout);
assert(!(timeout && progress));
/*
* aio_notify can avoid the expensive event_notifier_set if
* everything (file descriptors, bottom halves, timers) will
* be re-evaluated before the next blocking poll(). This is
* already true when aio_poll is called with blocking == false;
* if blocking == true, it is only true after poll() returns,
* so disable the optimization now.
*/
use_notify_me = timeout != 0;
if (use_notify_me) {
atomic_set(&ctx->notify_me, atomic_read(&ctx->notify_me) + 2);
/*
* Write ctx->notify_me before reading ctx->notified. Pairs with
* smp_mb in aio_notify().
*/
smp_mb();
/* Don't block if aio_notify() was called */
if (atomic_read(&ctx->notified)) {
timeout = 0;
}
}
/* If polling is allowed, non-blocking aio_poll does not need the
* system call---a single round of run_poll_handlers_once suffices.
*/
@ -600,12 +601,14 @@ bool aio_poll(AioContext *ctx, bool blocking)
ret = ctx->fdmon_ops->wait(ctx, &ready_list, timeout);
}
if (blocking) {
if (use_notify_me) {
/* Finish the poll before clearing the flag. */
atomic_store_release(&ctx->notify_me, atomic_read(&ctx->notify_me) - 2);
aio_notify_accept(ctx);
atomic_store_release(&ctx->notify_me,
atomic_read(&ctx->notify_me) - 2);
}
aio_notify_accept(ctx);
/* Adjust polling time */
if (ctx->poll_max_ns) {
int64_t block_ns = qemu_clock_get_ns(QEMU_CLOCK_REALTIME) - start;