OpenJDK ZGC 源码分析（二）触发GC的时机

1. 简介

ZGC的回收周期触发时机与其他GC算法略有不同，VM内部有个线程轮询定期检查是否满足开始回收的条件，如果满足则开始回收。

ZGC提供四种策略，其中一种满足条件即触发回收：

• rule_timer，定时策略，距离上次GC时间超过interval即触发GC
• rule_warmup，VM启动后，如果从来没有发生过GC，则在堆内存使用超过10%、20%、30%时，分别触发一次GC，以收集GC数据
• rule_allocation_rate，根据对象分配速率决定是否GC
• rule_proactive，主动控制策略，根据距离上次GC增长的堆空间和距离上次GC的时间判断是否触发GC

如果对象分配过快，以至于以上四种策略均无法及时回收对象，则在到达阈值后，STW并行回收。

2. 代码分析

2.1 GC轮询线程

zDirector.cpp

void ZDirector::run_service() {
  // Main loop
  while (_metronome.wait_for_tick()) {
    sample_allocation_rate();
    const GCCause::Cause cause = make_gc_decision();
    if (cause != GCCause::_no_gc) {
      ZCollectedHeap::heap()->collect(cause);
    }
  }
}

• 调用wait_for_tick()等待100ms
• 调用sample_allocation_rate，抽样计算分配速率
• 判断是否触发GC，如是则执行collect函数

zStat.hpp

class ZStatAllocRate : public AllStatic {
private:
  static const ZStatUnsampledCounter _counter;
  static TruncatedSeq                _rate;     // B/s
  static TruncatedSeq                _rate_avg; // B/s

public:
  static const uint64_t sample_window_sec = 1; // seconds
  static const uint64_t sample_hz         = 10;

  static const ZStatUnsampledCounter& counter();
  static uint64_t sample_and_reset();

  static double avg();
  static double avg_sd();
};

• wait_for_tick轮询等待时间是sample_hz的倒数，即100毫秒

zStat.cpp

uint64_t ZStatAllocRate::sample_and_reset() {
  const ZStatCounterData bytes_per_sample = _counter.collect_and_reset();
  const uint64_t bytes_per_second = bytes_per_sample._counter * sample_hz;

  _rate.add(bytes_per_second);
  _rate_avg.add(_rate.avg());

  return bytes_per_second;
}

• 合计每个CPU自上次抽样到现在分配的字节数
• 分配的字节数除以抽样频率，可得分配速率

2.2 GC触发策略

2.2.1 rule_timer

bool ZDirector::rule_timer() const {
  if (ZCollectionInterval == 0) {
    // Rule disabled
    return false;
  }

  // Perform GC if timer has expired.
  const double time_since_last_gc = ZStatCycle::time_since_last();
  const double time_until_gc = ZCollectionInterval - time_since_last_gc;

  log_debug(gc, director)("Rule: Timer, Interval: %us, TimeUntilGC: %.3lfs",
                          ZCollectionInterval, time_until_gc);

  return time_until_gc <= 0;
}

• 如果没有设置JVM参数-XX:ZCollectionInterval，则返回，不会使用rule_timer策略
• 如果设置了ZCollectionInterval，则判断当前时间减上次gc时间是否大于ZCollectionInterval，如是则触发GC

2.2.2 rule_warmup

bool ZDirector::is_warm() const {
  return ZStatCycle::ncycles() >= 3;
}

bool ZDirector::rule_warmup() const {
  if (is_warm()) {
    // Rule disabled
    return false;
  }

  // Perform GC if heap usage passes 10/20/30% and no other GC has been
  // performed yet. This allows us to get some early samples of the GC
  // duration, which is needed by the other rules.
  const size_t max_capacity = ZHeap::heap()->current_max_capacity();
  const size_t used = ZHeap::heap()->used();
  const double used_threshold_percent = (ZStatCycle::ncycles() + 1) * 0.1;
  const size_t used_threshold = max_capacity * used_threshold_percent;

  log_debug(gc, director)("Rule: Warmup %.0f%%, Used: " SIZE_FORMAT "MB, UsedThreshold: " SIZE_FORMAT "MB",
                          used_threshold_percent * 100, used / M, used_threshold / M);

  return used >= used_threshold;
}

• 先判断系统是否已经预热，如果GC次数大于等于3，则不使用rule_warmup策略
• 每当堆空间占用率大于10%、20%、30%时，触发一次GC

2.2.3 rule_allocation_rate

bool ZDirector::rule_allocation_rate() const {
  if (is_first()) {
    // Rule disabled
    return false;
  }

  // Perform GC if the estimated max allocation rate indicates that we
  // will run out of memory. The estimated max allocation rate is based
  // on the moving average of the sampled allocation rate plus a safety
  // margin based on variations in the allocation rate and unforeseen
  // allocation spikes.

  // Calculate amount of free memory available to Java threads. Note that
  // the heap reserve is not available to Java threads and is therefore not
  // considered part of the free memory.
  const size_t max_capacity = ZHeap::heap()->current_max_capacity();
  const size_t max_reserve = ZHeap::heap()->max_reserve();
  const size_t used = ZHeap::heap()->used();
  const size_t free_with_reserve = max_capacity - used;
  const size_t free = free_with_reserve - MIN2(free_with_reserve, max_reserve);

  // Calculate time until OOM given the max allocation rate and the amount
  // of free memory. The allocation rate is a moving average and we multiply
  // that with an allocation spike tolerance factor to guard against unforeseen
  // phase changes in the allocate rate. We then add ~3.3 sigma to account for
  // the allocation rate variance, which means the probability is 1 in 1000
  // that a sample is outside of the confidence interval.
  const double max_alloc_rate = (ZStatAllocRate::avg() * ZAllocationSpikeTolerance) + (ZStatAllocRate::avg_sd() * one_in_1000);
  const double time_until_oom = free / (max_alloc_rate + 1.0); // Plus 1.0B/s to avoid division by zero

  // Calculate max duration of a GC cycle. The duration of GC is a moving
  // average, we add ~3.3 sigma to account for the GC duration variance.
  const AbsSeq& duration_of_gc = ZStatCycle::normalized_duration();
  const double max_duration_of_gc = duration_of_gc.davg() + (duration_of_gc.dsd() * one_in_1000);

  // Calculate time until GC given the time until OOM and max duration of GC.
  // We also deduct the sample interval, so that we don't overshoot the target
  // time and end up starting the GC too late in the next interval.
  const double sample_interval = 1.0 / ZStatAllocRate::sample_hz;
  const double time_until_gc = time_until_oom - max_duration_of_gc - sample_interval;

  log_debug(gc, director)("Rule: Allocation Rate, MaxAllocRate: %.3lfMB/s, Free: " SIZE_FORMAT "MB, MaxDurationOfGC: %.3lfs, TimeUntilGC: %.3lfs",
                          max_alloc_rate / M, free / M, max_duration_of_gc, time_until_gc);

  return time_until_gc <= 0;
}

• 如果VM启动后，从来发生过GC，则不使用rule_allocation_rate
• ZGC分配速率的计算与G1不同，采用的是正态分布，置信度为99.9%时，最大内存分配速率为((ZStatAllocRate::avg() * 1) + (ZStatAllocRate::avg_sd() * 3.290527))
• ZAllocationSpikeTolerance是个修正参数，默认2，加入该修正系数后，置信度远大于99.9%，计算公式为((ZStatAllocRate::avg() * ZAllocationSpikeTolerance) + (ZStatAllocRate::avg_sd() * 3.290527))
• 根据最大分配速率，可以计算出到达OOM的剩余时间
• 同样根据正太分布，取置信度99.9%，计算出最大GC持续时间
• 当time_until_oom - max_duration_of_gc - sample_interval小于0时，即触发GC

2.2.4 rule_proactive

bool ZDirector::rule_proactive() const {
  if (!ZProactive || !is_warm()) {
    // Rule disabled
    return false;
  }

  // Perform GC if the impact of doing so, in terms of application throughput
  // reduction, is considered acceptable. This rule allows us to keep the heap
  // size down and allow reference processing to happen even when we have a lot
  // of free space on the heap.

  // Only consider doing a proactive GC if the heap usage has grown by at least
  // 10% of the max capacity since the previous GC, or more than 5 minutes has
  // passed since the previous GC. This helps avoid superfluous GCs when running
  // applications with very low allocation rate.
  const size_t used_after_last_gc = ZStatHeap::used_at_relocate_end();
  const size_t used_increase_threshold = ZHeap::heap()->current_max_capacity() * 0.10; // 10%
  const size_t used_threshold = used_after_last_gc + used_increase_threshold;
  const size_t used = ZHeap::heap()->used();
  const double time_since_last_gc = ZStatCycle::time_since_last();
  const double time_since_last_gc_threshold = 5 * 60; // 5 minutes
  if (used < used_threshold && time_since_last_gc < time_since_last_gc_threshold) {
    // Don't even consider doing a proactive GC
    log_debug(gc, director)("Rule: Proactive, UsedUntilEnabled: " SIZE_FORMAT "MB, TimeUntilEnabled: %.3lfs",
                            (used_threshold - used) / M,
                            time_since_last_gc_threshold - time_since_last_gc);
    return false;
  }

  const double assumed_throughput_drop_during_gc = 0.50; // 50%
  const double acceptable_throughput_drop = 0.01;        // 1%
  const AbsSeq& duration_of_gc = ZStatCycle::normalized_duration();
  const double max_duration_of_gc = duration_of_gc.davg() + (duration_of_gc.dsd() * one_in_1000);
  const double acceptable_gc_interval = max_duration_of_gc * ((assumed_throughput_drop_during_gc / acceptable_throughput_drop) - 1.0);
  const double time_until_gc = acceptable_gc_interval - time_since_last_gc;

  log_debug(gc, director)("Rule: Proactive, AcceptableGCInterval: %.3lfs, TimeSinceLastGC: %.3lfs, TimeUntilGC: %.3lfs",
                          acceptable_gc_interval, time_since_last_gc, time_until_gc);

  return time_until_gc <= 0;
}

• 先判断JVM参数ZProactive（默认true），如果是false则不使用rule_proactive
• 如果VM还没有预热，则不使用rule_proactive
• 如果距离上次GC，堆内存占用增长小于10%且小于5分钟，则不使用rule_proactive
• 如果距离上次GC时间超过最大预测GC时间乘49，则触发GC

3. 引用

OpenJDK 12 源代码