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

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#include "gc/g1/g1EvacStats.hpp"
#include "gc/shared/gc_globals.hpp"
#include "gc/shared/gcId.hpp"
#include "logging/log.hpp"
#include "memory/allocation.inline.hpp"
#include "runtime/globals.hpp"

void G1EvacStats::log_plab_allocation() {
  log_debug(gc, plab)("%s PLAB allocation: "
                      "allocated: %zuB, "
                      "wasted: %zuB, "
                      "unused: %zuB, "
                      "used: %zuB, "
                      "undo waste: %zuB, ",
                      _description,
                      _allocated * HeapWordSize,
                      _wasted * HeapWordSize,
                      _unused * HeapWordSize,
                      used() * HeapWordSize,
                      _undo_wasted * HeapWordSize);
  log_debug(gc, plab)("%s other allocation: "
                      "region end waste: %zuB, "
                      "regions filled: %u, "
                      "num plab filled: %zu, "
                      "direct allocated: %zuB, "
                      "num direct allocated: %zu, "
                      "failure used: %zuB, "
                      "failure wasted: %zuB",
                      _description,
                      _region_end_waste * HeapWordSize,
                      _regions_filled,
                      _num_plab_filled,
                      _direct_allocated * HeapWordSize,
                      _num_direct_allocated,
                      _failure_used * HeapWordSize,
                      _failure_waste * HeapWordSize);
}

void G1EvacStats::log_sizing(size_t calculated_words, size_t net_desired_words) {
  log_debug(gc, plab)("%s sizing: "
                      "calculated: %zuB, "
                      "actual: %zuB",
                      _description,
                      calculated_words * HeapWordSize,
                      net_desired_words * HeapWordSize);
}

size_t G1EvacStats::compute_desired_plab_size() const {
  // 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 (G1LastPLABAverageOccupancy),
  // the new desired PLAB size is set to 20 words.
  //
  // (This also implies that we expect (100-G1LastPLABAverageOccupancy)/TargetPLABWastePct
  // number of refills during allocation).
  //
  // 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;
  return (size_t)((double)total_waste_allowed / (100 - G1LastPLABAverageOccupancy));
}

G1EvacStats::G1EvacStats(const char* description, size_t default_per_thread_plab_size, unsigned wt) :
  PLABStats(description),
  _default_plab_size(default_per_thread_plab_size),
  _desired_net_plab_size(default_per_thread_plab_size * ParallelGCThreads),
  _net_plab_size_filter(wt),
  _region_end_waste(0),
  _regions_filled(0),
  _num_plab_filled(0),
  _direct_allocated(0),
  _num_direct_allocated(0),
  _failure_used(0),
  _failure_waste(0) {
}

// Calculates plab size for current number of gc worker threads.
size_t G1EvacStats::desired_plab_size(uint no_of_gc_workers) const {
  if (!ResizePLAB) {
    // There is a circular dependency between the heap and PLAB initialization,
    // so _default_plab_size can have an unaligned value.
    return align_object_size(_default_plab_size);
  }
  return align_object_size(clamp(_desired_net_plab_size / no_of_gc_workers, min_size(), max_size()));
}

void G1EvacStats::adjust_desired_plab_size() {
  log_plab_allocation();

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

    assert(_allocated != 0 || _unused == 0,
           "Inconsistency in PLAB stats: "
           "_allocated: %zu, "
           "_wasted: %zu, "
           "_unused: %zu, "
           "_undo_wasted: %zu",
           _allocated, _wasted, _unused, _undo_wasted);

    size_t plab_size = compute_desired_plab_size();
    // Take historical weighted average
    _net_plab_size_filter.sample(plab_size);
    _desired_net_plab_size = MAX2(min_size(), (size_t)_net_plab_size_filter.average());

    log_sizing(plab_size, _desired_net_plab_size);
  }
  // Clear accumulators for next round
  reset();
}