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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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9
include/block/aio.h
View file

@ -60,8 +60,14 @@ struct AioContext {
*/
int walking_handlers;
/* Used to avoid unnecessary event_notifier_set calls in aio_notify.
* Writes protected by lock or BQL, reads are lockless.
*/
bool dispatching;
/* lock to protect between bh's adders and deleter */
QemuMutex bh_lock;
/* Anchor of the list of Bottom Halves belonging to the context */
struct QEMUBH *first_bh;
@ -83,6 +89,9 @@ struct AioContext {
QEMUTimerListGroup tlg;
};
/* Used internally to synchronize aio_poll against qemu_bh_schedule. */
void aio_set_dispatching(AioContext *ctx, bool dispatching);
/**
* aio_context_new: Allocate a new AioContext.
*