diff -r f99a36499b8c -r 7383557659bd src/share/vm/gc_implementation/g1/heapRegion.cpp
--- a/src/share/vm/gc_implementation/g1/heapRegion.cpp	Tue Aug 21 10:05:57 2012 -0700
+++ b/src/share/vm/gc_implementation/g1/heapRegion.cpp	Tue Aug 21 14:10:39 2012 -0700
@@ -384,10 +384,17 @@
 }
 
 void HeapRegion::calc_gc_efficiency() {
+  // GC efficiency is the ratio of how much space would be
+  // reclaimed over how long we predict it would take to reclaim it.
   G1CollectedHeap* g1h = G1CollectedHeap::heap();
   G1CollectorPolicy* g1p = g1h->g1_policy();
-  _gc_efficiency = (double) reclaimable_bytes() /
-                            g1p->predict_region_elapsed_time_ms(this, false);
+
+  // Retrieve a prediction of the elapsed time for this region for
+  // a mixed gc because the region will only be evacuated during a
+  // mixed gc.
+  double region_elapsed_time_ms =
+    g1p->predict_region_elapsed_time_ms(this, false /* for_young_gc */);
+  _gc_efficiency = (double) reclaimable_bytes() / region_elapsed_time_ms;
 }
 
 void HeapRegion::set_startsHumongous(HeapWord* new_top, HeapWord* new_end) {