changeset 8930:be685a5356a4

8067341: Modify PLAB sizing algorithm to waste less Summary: Change the G1 PLAB resizing algorithm to waste less memory by being more conservative about the PLAB sizes. Reviewed-by: ecaspole, jmasa
author tschatzl
date Tue, 01 Sep 2015 10:53:29 +0200
parents da9eeb2c1cbd
children 6dad23e1c89d
files src/share/vm/gc/g1/g1EvacStats.cpp src/share/vm/gc/g1/g1_globals.hpp
diffstat 2 files changed, 45 insertions(+), 11 deletions(-) [+]
line wrap: on
line diff
--- a/src/share/vm/gc/g1/g1EvacStats.cpp	Mon Aug 31 11:51:29 2015 -0700
+++ b/src/share/vm/gc/g1/g1EvacStats.cpp	Tue Sep 01 10:53:29 2015 +0200
@@ -54,17 +54,46 @@
                      _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;
+    // The size of the PLAB caps the amount of space that can be wasted at the
+    // end of the collection. In the worst case the last PLAB could be completely
+    // empty.
+    // This allows us to calculate the new PLAB size to achieve the
+    // TargetPLABWastePct given the latest memory usage and that the last buffer
+    // will be G1LastPLABAverageOccupancy 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 set to 20 words.
+    //
+    // The amount of allocation performed should be independent of the number of
+    // threads, so should 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.
+    // Region end waste may be higher than actual allocation. This may occur 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
--- a/src/share/vm/gc/g1/g1_globals.hpp	Mon Aug 31 11:51:29 2015 -0700
+++ b/src/share/vm/gc/g1/g1_globals.hpp	Tue Sep 01 10:53:29 2015 +0200
@@ -84,6 +84,11 @@
           "If true, enable reference discovery during concurrent "          \
           "marking and reference processing at the end of remark.")         \
                                                                             \
+  experimental(double, G1LastPLABAverageOccupancy, 50.0,                    \
+               "The expected average occupancy of the last PLAB in "        \
+               "percent.")                                                  \
+               range(0.001, 100.0)                                          \
+                                                                            \
   product(size_t, G1SATBBufferSize, 1*K,                                    \
           "Number of entries in an SATB log buffer.")                       \
           range(1, max_uintx)                                               \