1 /*
2 * Copyright (c) 2015, Oracle and/or its affiliates. All rights reserved.
3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
4 *
5 * This code is free software; you can redistribute it and/or modify it
6 * under the terms of the GNU General Public License version 2 only, as
7 * published by the Free Software Foundation.
8 *
9 * This code is distributed in the hope that it will be useful, but WITHOUT
10 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
11 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
12 * version 2 for more details (a copy is included in the LICENSE file that
13 * accompanied this code).
14 *
15 * You should have received a copy of the GNU General Public License version
16 * 2 along with this work; if not, write to the Free Software Foundation,
17 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
18 *
19 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
20 * or visit www.oracle.com if you need additional information or have any
21 * questions.
22 *
23 */
24
25 #include "precompiled.hpp"
26 #include "gc/g1/g1EvacStats.hpp"
27 #include "gc/shared/gcId.hpp"
28 #include "trace/tracing.hpp"
29
30 void G1EvacStats::adjust_desired_plab_sz() {
31 if (PrintPLAB) {
32 gclog_or_tty->print(" (allocated = " SIZE_FORMAT " wasted = " SIZE_FORMAT " "
33 "unused = " SIZE_FORMAT " used = " SIZE_FORMAT " "
34 "undo_waste = " SIZE_FORMAT " region_end_waste = " SIZE_FORMAT " "
35 "regions filled = %u direct_allocated = " SIZE_FORMAT " "
36 "failure_used = " SIZE_FORMAT " failure_waste = " SIZE_FORMAT ") ",
37 _allocated, _wasted, _unused, used(), _undo_wasted, _region_end_waste,
38 _regions_filled, _direct_allocated, _failure_used, _failure_waste);
39 }
40
41 if (ResizePLAB) {
42
43 assert(is_object_aligned(max_size()) && min_size() <= max_size(),
44 "PLAB clipping computation may be incorrect");
45
46 if (_allocated == 0) {
47 assert((_unused == 0),
48 err_msg("Inconsistency in PLAB stats: "
49 "_allocated: "SIZE_FORMAT", "
50 "_wasted: "SIZE_FORMAT", "
51 "_region_end_waste: "SIZE_FORMAT", "
52 "_unused: "SIZE_FORMAT", "
53 "_used : "SIZE_FORMAT,
54 _allocated, _wasted, _region_end_waste, _unused, used()));
55 _allocated = 1;
56 }
57 // Calculate the new PLAB size as the amount of space that could be wasted to
58 // keep TargetPLABWastePct given latest memory usage and that the last buffer
59 // will be G1PLABPercent full.
60 //
61 // E.g. assume that if in the current GC 100 words were allocated and a
62 // TargetPLABWastePct of 10 had been set.
63 //
64 // So we could waste up to 10 words to meet that percentage. Given that we
65 // also assume that that buffer is typically half-full, the new desired PLAB
66 // size is 20 words.
67 //
68 // The amount of allocation performed is we can spend is independent of the
69 // number of threads, so is the maximum waste we can spend in total. So if we used
70 // n threads to allocate, each of them can spend maximum waste/n words in
71 // a first rough approximation. The number of threads only comes into play later
72 // when actually retrieving the actual desired PLAB size.
73 //
74 // After calculating this optimal PLAB size the algorithm applies the usual
75 // exponential decaying average over this value to guess the next PLAB size.
76 //
77 // We account region end waste fully to PLAB allocation (in the calculation of
78 // what we consider as "used_for_waste_calculation" below). This is not
79 // completely fair, but is a conservative assumption because PLABs may be sized
80 // flexibly while we cannot adjust inline allocations.
81 // Allocation during GC will try to minimize region end waste so this impact
82 // should be minimal.
83
84 // We need to cover overflow when calculating the amount of space actually used
85 // by objects in PLABs when subtracting the region end waste.
86 // This is a possible situation if many threads do not allocate anything but a
87 // few rather large objects. In this degenerate case the PLAB size would simply quickly
88 // tend to minimum PLAB size, which is an okay reaction.
89 size_t const used_for_waste_calculation = used() > _region_end_waste ? used() - _region_end_waste : 0;
90
91 size_t const total_waste_allowed = used_for_waste_calculation * TargetPLABWastePct;
92 size_t const cur_plab_sz = (double)total_waste_allowed / G1LastPLABAverageOccupancy;
93 // Take historical weighted average
94 _filter.sample(cur_plab_sz);
95 // Clip from above and below, and align to object boundary
96 size_t plab_sz;
97 plab_sz = MAX2(min_size(), (size_t)_filter.average());
98 plab_sz = MIN2(max_size(), plab_sz);
99 plab_sz = align_object_size(plab_sz);
100 // Latch the result
101 _desired_net_plab_sz = plab_sz;
102 if (PrintPLAB) {
103 gclog_or_tty->print_cr(" (plab_sz = " SIZE_FORMAT " desired_plab_sz = " SIZE_FORMAT ") ", cur_plab_sz, plab_sz);
104 }
105 }
106 // Clear accumulators for next round.
107 reset();
108 }
109