jdk-24/hotspot/src/share/vm/gc/g1/g1EvacStats.cpp

/*
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 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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 * This code is free software; you can redistribute it and/or modify it
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 *
 * This code is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
 *
 * You should have received a copy of the GNU General Public License version
 * 2 along with this work; if not, write to the Free Software Foundation,
 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
 *
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#include "precompiled.hpp"
#include "gc/g1/g1EvacStats.hpp"
#include "gc/shared/gcId.hpp"
#include "trace/tracing.hpp"

void G1EvacStats::adjust_desired_plab_sz() {
  if (PrintPLAB) {
    gclog_or_tty->print(" (allocated = " SIZE_FORMAT " wasted = " SIZE_FORMAT " "
                        "unused = " SIZE_FORMAT " used = " SIZE_FORMAT " "
                        "undo_waste = " SIZE_FORMAT " region_end_waste = " SIZE_FORMAT " "
                        "regions filled = %u direct_allocated = " SIZE_FORMAT " "
                        "failure_used = " SIZE_FORMAT " failure_waste = " SIZE_FORMAT ") ",
                        _allocated, _wasted, _unused, used(), _undo_wasted, _region_end_waste,
                        _regions_filled, _direct_allocated, _failure_used, _failure_waste);
  }

  if (ResizePLAB) {

    assert(is_object_aligned(max_size()) && min_size() <= max_size(),
           "PLAB clipping computation may be incorrect");

    if (_allocated == 0) {
      assert((_unused == 0),
             err_msg("Inconsistency in PLAB stats: "
                     "_allocated: " SIZE_FORMAT ", "
                     "_wasted: " SIZE_FORMAT ", "
                     "_region_end_waste: " SIZE_FORMAT ", "
                     "_unused: " SIZE_FORMAT ", "
                     "_used  : " SIZE_FORMAT,
                     _allocated, _wasted, _region_end_waste, _unused, used()));
      _allocated = 1;
    }
    // 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 = (size_t)((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());
    plab_sz = MIN2(max_size(), plab_sz);
    plab_sz = align_object_size(plab_sz);
    // Latch the result
    _desired_net_plab_sz = plab_sz;
    if (PrintPLAB) {
      gclog_or_tty->print_cr(" (plab_sz = " SIZE_FORMAT " desired_plab_sz = " SIZE_FORMAT ") ", cur_plab_sz, plab_sz);
    }
  }
  // Clear accumulators for next round.
  reset();
}