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 <pbonzini@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

diff --git a/tools/virtio/ringtest/main.c b/tools/virtio/ringtest/main.c
index b00ecd619969ac36afe45c0929dce48f0d675cd1..f31353fac5415d8b9f5614e6f46f71a8f062f09b 100644 (file)
--- 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();