src/share/vm/gc/g1/g1EvacStats.cpp

rev 8875 : [mq]: 8067341-modify-plab-sizing-algorithm-to-waste-less
rev 8876 : [mq]: jon-eric-reviews
rev 8877 : [mq]: more-reviews

@@ -52,21 +52,46 @@
                      "_unused: "SIZE_FORMAT", "
                      "_used  : "SIZE_FORMAT,
                      _allocated, _wasted, _region_end_waste, _unused, used()));
       _allocated = 1;
     }
-    // We account region end waste fully to PLAB allocation. This is not completely fair,
-    // but is a conservative assumption because PLABs may be sized flexibly while we
-    // cannot adjust direct allocations.
-    // In some cases, wasted_frac may become > 1 but that just reflects the problem
-    // with region_end_waste.
-    double wasted_frac    = (double)(_unused + _wasted + _region_end_waste) / (double)_allocated;
-    size_t target_refills = (size_t)((wasted_frac * TargetSurvivorRatio) / TargetPLABWastePct);
-    if (target_refills == 0) {
-      target_refills = 1;
-    }
-    size_t cur_plab_sz = used() / target_refills;
+    // Calculate the new PLAB size as the amount of space that could be wasted to
+    // keep TargetPLABWastePct given latest memory usage and that the last buffer
+    // will be G1PLABPercent full.
+    //
+    // E.g. assume that if in the current GC 100 words were allocated and a
+    // TargetPLABWastePct of 10 had been set.
+    //
+    // So we could waste up to 10 words to meet that percentage. Given that we
+    // also assume that that buffer is typically half-full, the new desired PLAB
+    // size is 20 words.
+    //
+    // The amount of allocation performed is we can spend is independent of the
+    // number of threads, so is the maximum waste we can spend in total. So if we used
+    // n threads to allocate, each of them can spend maximum waste/n words in
+    // a first rough approximation. The number of threads only comes into play later
+    // when actually retrieving the actual desired PLAB size.
+    //
+    // After calculating this optimal PLAB size the algorithm applies the usual
+    // exponential decaying average over this value to guess the next PLAB size.
+    //
+    // We account region end waste fully to PLAB allocation (in the calculation of
+    // what we consider as "used_for_waste_calculation" below). This is not
+    // completely fair, but is a conservative assumption because PLABs may be sized
+    // flexibly while we cannot adjust inline allocations.
+    // Allocation during GC will try to minimize region end waste so this impact
+    // should be minimal.
+    
+    // We need to cover overflow when calculating the amount of space actually used
+    // by objects in PLABs when subtracting the region end waste.
+    // This is a possible situation if many threads do not allocate anything but a
+    // few rather large objects. In this degenerate case the PLAB size would simply quickly
+    // tend to minimum PLAB size, which is an okay reaction.
+    size_t const used_for_waste_calculation = used() > _region_end_waste ? used() - _region_end_waste : 0;
+
+    size_t const total_waste_allowed = used_for_waste_calculation * TargetPLABWastePct;
+    size_t const cur_plab_sz = (double)total_waste_allowed / G1LastPLABAverageOccupancy;
     // Take historical weighted average
     _filter.sample(cur_plab_sz);
     // Clip from above and below, and align to object boundary
     size_t plab_sz;
     plab_sz = MAX2(min_size(), (size_t)_filter.average());