Reduce the number of flushes while using glQueries.

gpu/command_buffer/client/query_tracker.cc

Index: gpu/command_buffer/client/query_tracker.cc
diff --git a/gpu/command_buffer/client/query_tracker.cc b/gpu/command_buffer/client/query_tracker.cc
index 5ac89e07e0f5ca472d767f99b86815ef2984be5f..2d413cb43847903da9a26ed0cdb3718bdd252919 100644
--- a/gpu/command_buffer/client/query_tracker.cc
+++ b/gpu/command_buffer/client/query_tracker.cc
@@ -93,7 +93,7 @@ QueryTracker::Query::Query(GLuint id, GLenum target,
       state_(kUninitialized),
       submit_count_(0),
       token_(0),
-      flushed_(false),
+      last_flush_(0),
       client_begin_time_us_(0),
       result_(0) {
     }
@@ -140,6 +140,7 @@ void QueryTracker::Query::End(GLES2Implementation* gl) {
       }
     }
   }
+  last_flush_ = gl->helper()->flush_generation();
   gl->helper()->EndQueryEXT(target(), submit_count());
   MarkAsPending(gl->helper()->InsertToken());
 }
@@ -168,13 +169,10 @@ bool QueryTracker::Query::CheckResultsAvailable(
       }
       state_ = kComplete;
     } else {
-      if (!flushed_) {
-        // TODO(gman): We could reduce the number of flushes by having a
-        // flush count, recording that count at the time we insert the
-        // EndQuery command and then only flushing here if we've have not
-        // passed that count yet.
-        flushed_ = true;
+      uint32 flush_generation = helper->flush_generation();
+      if (!(flush_generation>last_flush_)) {
         helper->Flush();
+        last_flush_ = helper->flush_generation();

[Zhenyao Mo, 2014/04/08 21:13:56]

My apology for not catching this earlier, but this makes situation worse, not
better.  Now if you keep calling CheckResultsAvailable(), very likely
flush_generation == last_flush_, and you will flush everytime.

So instead, once you set last_flush_, you don't wanna update its value here. 
last_flush_ is also a misleading name, maybe just flush_count_.

[rptr, 2014/04/08 22:44:16]

Done.
Problem was updating flush_count_ here. we should update that only when EndQuery
is called.

       } else {
         // Insert no-ops so that eventually the GPU process will see more work.
         helper->Noop(1);