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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>
This commit is contained in:
Paolo Bonzini 2016-10-06 11:39:12 +02:00 committed by Michael S. Tsirkin
parent d3c3589b8b
commit 948a8ac296
1 changed files with 2 additions and 2 deletions
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4
tools/virtio/ringtest/main.c
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@ -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();