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[Qemu-block] [PULL 7/7] iothread: document about why we need explicit ai

From: Stefan Hajnoczi
Subject: [Qemu-block] [PULL 7/7] iothread: document about why we need explicit aio_poll()
Date: Fri, 8 Mar 2019 16:53:44 +0000

From: Peter Xu <address@hidden>

After consulting Paolo I know why we'd better keep the explicit
aio_poll() in iothread_run().  Document it directly into the code so
that future readers will know the answer from day one.

Signed-off-by: Peter Xu <address@hidden>
Reviewed-by: Marc-André Lureau <address@hidden>
Message-id: address@hidden
Message-Id: <address@hidden>
Signed-off-by: Stefan Hajnoczi <address@hidden>
 iothread.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/iothread.c b/iothread.c
index ad64c757ac..7130be58e3 100644
--- a/iothread.c
+++ b/iothread.c
@@ -63,6 +63,15 @@ static void *iothread_run(void *opaque)
     while (iothread->running) {
+        /*
+         * Note: from functional-wise the g_main_loop_run() below can
+         * already cover the aio_poll() events, but we can't run the
+         * main loop unconditionally because explicit aio_poll() here
+         * is faster than g_main_loop_run() when we do not need the
+         * gcontext at all (e.g., pure block layer iothreads).  In
+         * other words, when we want to run the gcontext with the
+         * iothread we need to pay some performance for functionality.
+         */
         aio_poll(iothread->ctx, true);

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