ringtest: poll for new buffers once before updating event index

Updating the event index has a memory barrier and causes more work on the other side to actually signal the event. It is unnecessary if a new buffer has already appeared on the ring, so poll once before doing the update. The effect of this on the 0.9 ring implementation is pretty much invisible, but on the new-style ring it provides a consistent 3% performance improvement. Signed-off-by: Paolo Bonzini <[email protected]> Signed-off-by: Michael S. Tsirkin <[email protected]>
opendata26 · Oct 30, 2016 · 948a8ac · 948a8ac
1 parent d3c3589
commit 948a8ac
Showing 1 changed file with 2 additions and 2 deletions.
diff --git a/tools/virtio/ringtest/main.c b/tools/virtio/ringtest/main.c
@@ -147,7 +147,7 @@ static void __attribute__((__flatten__)) run_guest(void)
 		assert(completed <= bufs);
 		assert(started <= bufs);
 		if (do_sleep) {
-			if (enable_call())
+			if (used_empty() && enable_call())
 				wait_for_call();
 		} else {
 			poll_used();
@@ -172,7 +172,7 @@ static void __attribute__((__flatten__)) run_host(void)
 
 	for (;;) {
 		if (do_sleep) {
-			if (enable_kick())
+			if (avail_empty() && enable_kick())
 				wait_for_kick();
 		} else {
 			poll_avail();