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Re: [Qemu-devel] [regression] dataplane: throughout -40% by commit 580b6
From: |
Paolo Bonzini |
Subject: |
Re: [Qemu-devel] [regression] dataplane: throughout -40% by commit 580b6b2aa2 |
Date: |
Wed, 02 Jul 2014 18:21:40 +0200 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:24.0) Gecko/20100101 Thunderbird/24.6.0 |
Il 02/07/2014 17:45, Ming Lei ha scritto:
> The attachment debug patch skips aio_notify() if qemu_bh_schedule
> is running from current aio context, but looks there is still 120K
> writes triggered. (without the patch, 400K can be observed in
> same test)
Nice. Another observation is that after aio_dispatch we'll always
re-evaluate everything (bottom halves, file descriptors and timeouts),
so we can skip the aio_notify if we're inside aio_dispatch.
So what about this untested patch:
diff --git a/aio-posix.c b/aio-posix.c
index f921d4f..a23d85d 100644
--- a/aio-posix.c
+++ b/aio-posix.c
@@ -124,6 +124,9 @@ static bool aio_dispatch(AioContext *ctx)
AioHandler *node;
bool progress = false;
+ /* No need to set the event notifier during aio_notify. */
+ ctx->running++;
+
/*
* We have to walk very carefully in case qemu_aio_set_fd_handler is
* called while we're walking.
@@ -169,6 +171,11 @@ static bool aio_dispatch(AioContext *ctx)
/* Run our timers */
progress |= timerlistgroup_run_timers(&ctx->tlg);
+ smp_wmb();
+ ctx->iter_count++;
+ smp_wmb();
+ ctx->running--;
+
return progress;
}
diff --git a/async.c b/async.c
index 5b6fe6b..1f56afa 100644
--- a/async.c
+++ b/async.c
@@ -249,7 +249,19 @@ ThreadPool *aio_get_thread_pool(AioContext *ctx)
void aio_notify(AioContext *ctx)
{
- event_notifier_set(&ctx->notifier);
+ uint32_t iter_count;
+ do {
+ iter_count = ctx->iter_count;
+ /* Read ctx->iter_count before ctx->running. */
+ smb_rmb();
+ if (!ctx->running) {
+ event_notifier_set(&ctx->notifier);
+ return;
+ }
+ /* Read ctx->running before ctx->iter_count. */
+ smb_rmb();
+ /* ctx might have gone to sleep. */
+ } while (iter_count != ctx->iter_count);
}
static void aio_timerlist_notify(void *opaque)
@@ -269,6 +279,7 @@ AioContext *aio_context_new(void)
ctx = (AioContext *) g_source_new(&aio_source_funcs, sizeof(AioContext));
ctx->pollfds = g_array_new(FALSE, FALSE, sizeof(GPollFD));
ctx->thread_pool = NULL;
+ ctx->iter_count = ctx->running = 0;
qemu_mutex_init(&ctx->bh_lock);
rfifolock_init(&ctx->lock, aio_rfifolock_cb, ctx);
event_notifier_init(&ctx->notifier, false);
diff --git a/include/block/aio.h b/include/block/aio.h
index a92511b..9f51c4f 100644
--- a/include/block/aio.h
+++ b/include/block/aio.h
@@ -51,6 +51,9 @@ struct AioContext {
/* Protects all fields from multi-threaded access */
RFifoLock lock;
+ /* Used to avoid aio_notify while dispatching event handlers.
+ * Writes protected by lock or BQL, reads are lockless.
+ */
+ uint32_t iter_count, running;
+
/* The list of registered AIO handlers */
QLIST_HEAD(, AioHandler) aio_handlers;
Please review carefully.
> So is there still other writes not found in the path?
What do perf or gdb say? :)
Paolo
- Re: [Qemu-devel] [regression] dataplane: throughout -40% by commit 580b6b2aa2, (continued)
- Re: [Qemu-devel] [regression] dataplane: throughout -40% by commit 580b6b2aa2, Ming Lei, 2014/07/02
- Re: [Qemu-devel] [regression] dataplane: throughout -40% by commit 580b6b2aa2, Ming Lei, 2014/07/02
- Re: [Qemu-devel] [regression] dataplane: throughout -40% by commit 580b6b2aa2, Paolo Bonzini, 2014/07/02
- Re: [Qemu-devel] [regression] dataplane: throughout -40% by commit 580b6b2aa2, Ming Lei, 2014/07/02
- Re: [Qemu-devel] [regression] dataplane: throughout -40% by commit 580b6b2aa2, Paolo Bonzini, 2014/07/02
- Re: [Qemu-devel] [regression] dataplane: throughout -40% by commit 580b6b2aa2, Ming Lei, 2014/07/02
- Re: [Qemu-devel] [regression] dataplane: throughout -40% by commit 580b6b2aa2,
Paolo Bonzini <=
- Re: [Qemu-devel] [regression] dataplane: throughout -40% by commit 580b6b2aa2, Ming Lei, 2014/07/03
- Re: [Qemu-devel] [regression] dataplane: throughout -40% by commit 580b6b2aa2, Paolo Bonzini, 2014/07/03
- Re: [Qemu-devel] [regression] dataplane: throughout -40% by commit 580b6b2aa2, Ming Lei, 2014/07/03
- Re: [Qemu-devel] [regression] dataplane: throughout -40% by commit 580b6b2aa2, Paolo Bonzini, 2014/07/03
- Re: [Qemu-devel] [regression] dataplane: throughout -40% by commit 580b6b2aa2, Ming Lei, 2014/07/03