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AioContext: speed up aio_notify

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In many cases, the call to event_notifier_set in aio_notify is unnecessary.
In particular, if we are executing aio_dispatch, or if aio_poll is not
blocking, we know that we will soon get to the next loop iteration (if
necessary); the thread that hosts the AioContext's event loop does not
need any nudging.

The patch includes a Promela formal model that shows that this really
works and does not need any further complication such as generation
counts.  It needs a memory barrier though.

The generation counts are not needed because any change to
ctx->dispatching after the memory barrier is okay for aio_notify.
If it changes from zero to one, it is the right thing to skip
event_notifier_set.  If it changes from one to zero, the
event_notifier_set is unnecessary but harmless.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
This commit is contained in:
Paolo Bonzini 2014-07-07 15:18:04 +02:00 • committed by Kevin Wolf
parent ef508f427b
commit 0ceb849bd3
4 changed files with 164 additions and 2 deletions
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19
async.c
View file

@ -26,6 +26,7 @@
#include "block/aio.h"
#include "block/thread-pool.h"
#include "qemu/main-loop.h"
#include "qemu/atomic.h"
/***********************************************************/
/* bottom halves (can be seen as timers which expire ASAP) */
@ -247,9 +248,25 @@ ThreadPool *aio_get_thread_pool(AioContext *ctx)
return ctx->thread_pool;
}
void aio_set_dispatching(AioContext *ctx, bool dispatching)
{
ctx->dispatching = dispatching;
if (!dispatching) {
/* Write ctx->dispatching before reading e.g. bh->scheduled.
* Optimization: this is only needed when we're entering the "unsafe"
* phase where other threads must call event_notifier_set.
*/
smp_mb();
}
}
void aio_notify(AioContext *ctx)
{
event_notifier_set(&ctx->notifier);
/* Write e.g. bh->scheduled before reading ctx->dispatching. */
smp_mb();
if (!ctx->dispatching) {
event_notifier_set(&ctx->notifier);
}
}
static void aio_timerlist_notify(void *opaque)