src/hotspot/share/gc/shenandoah/shenandoahMetrics.cpp

/*
 * Copyright (c) 2013, 2019, Red Hat, Inc. All rights reserved.
 * Copyright (c) 2025, Oracle and/or its affiliates. All rights reserved.
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 *
 * This code is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 only, as
 * published by the Free Software Foundation.
 *
 * 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.
 *
 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
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#include "gc/shenandoah/shenandoahFreeSet.hpp"
#include "gc/shenandoah/shenandoahHeap.inline.hpp"
#include "gc/shenandoah/shenandoahHeapRegion.hpp"
#include "gc/shenandoah/shenandoahMetrics.hpp"

ShenandoahMetricsSnapshot::ShenandoahMetricsSnapshot() {
  _heap = ShenandoahHeap::heap();
}

void ShenandoahMetricsSnapshot::snap_before() {
  _used_before = _heap->used();
  _if_before = _heap->free_set()->internal_fragmentation();
  _ef_before = _heap->free_set()->external_fragmentation();
}
void ShenandoahMetricsSnapshot::snap_after() {
  _used_after = _heap->used();
  _if_after = _heap->free_set()->internal_fragmentation();
  _ef_after = _heap->free_set()->external_fragmentation();
}

// For degenerated GC, generation is Young in generational mode, Global in non-generational mode.
// For full GC, generation is always Global.
//
// Note that the size of the chosen collection set is proportional to the relevant generation's collection set.
// Note also that the generation size may change following selection of the collection set, as a side effect
// of evacuation.  Evacuation may promote objects, causing old to grow and young to shrink.  Or this may be a
// mixed evacuation.  When old regions are evacuated, this typically allows young to expand.  In all of these
// various scenarios, the purpose of asking is_good_progress() is to determine if there is enough memory available
// within young generation to justify making an attempt to perform a concurrent collection.  For this reason, we'll
// use the current size of the generation (which may not be different than when the collection set was chosen) to
// assess how much free memory we require in order to consider the most recent GC to have had good progress.

bool ShenandoahMetricsSnapshot::is_good_progress(ShenandoahGeneration* generation) {
  // Under the critical threshold?
  ShenandoahFreeSet* free_set = _heap->free_set();
  size_t free_actual   = free_set->available();
  assert(free_actual != ShenandoahFreeSet::FreeSetUnderConstruction, "Avoid this race");

  // ShenandoahCriticalFreeThreshold is expressed as a percentage.  We multiple this percentage by 1/100th
  // of the generation capacity to determine whether the available memory within the generation exceeds the
  // critical threshold.
  size_t free_expected = (generation->max_capacity() / 100) * ShenandoahCriticalFreeThreshold;

  bool prog_free = free_actual >= free_expected;
  log_info(gc, ergo)("%s progress for free space: %zu%s, need %zu%s",
                     prog_free ? "Good" : "Bad",
                     byte_size_in_proper_unit(free_actual),   proper_unit_for_byte_size(free_actual),
                     byte_size_in_proper_unit(free_expected), proper_unit_for_byte_size(free_expected));
  if (!prog_free) {
    return false;
  }

  // Freed up enough?
  size_t progress_actual   = (_used_before > _used_after) ? _used_before - _used_after : 0;
  size_t progress_expected = ShenandoahHeapRegion::region_size_bytes();
  bool prog_used = progress_actual >= progress_expected;
  log_info(gc, ergo)("%s progress for used space: %zu%s, need %zu%s",
                     prog_used ? "Good" : "Bad",
                     byte_size_in_proper_unit(progress_actual),   proper_unit_for_byte_size(progress_actual),
                     byte_size_in_proper_unit(progress_expected), proper_unit_for_byte_size(progress_expected));
  if (prog_used) {
    return true;
  }

  // Internal fragmentation is down?
  double if_actual = _if_before - _if_after;
  double if_expected = 0.01; // 1% should be enough
  bool prog_if = if_actual >= if_expected;
  log_info(gc, ergo)("%s progress for internal fragmentation: %.1f%%, need %.1f%%",
                     prog_if ? "Good" : "Bad",
                     if_actual * 100, if_expected * 100);
  if (prog_if) {
    return true;
  }

  // External fragmentation is down?
  double ef_actual = _ef_before - _ef_after;
  double ef_expected = 0.01; // 1% should be enough
  bool prog_ef = ef_actual >= ef_expected;
  log_info(gc, ergo)("%s progress for external fragmentation: %.1f%%, need %.1f%%",
                     prog_ef ? "Good" : "Bad",
                     ef_actual * 100, ef_expected * 100);
  if (prog_ef) {
    return true;
  }

  // Nothing good had happened.
  return false;
}