ZGC gc策略及回收过程-源码分析

源码文件：/src/hotspot/share/gc/z/zDirector.cpp

 一、回收策略

main入口函数：

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);
    }
  }
}

ZMetronome::wait_for_tick 是zgc定义的一个循环时钟函数，sample_allocation_rate函数则用于rule_allocation_rate策略估算可能oom的时间。重点关注：make_gc_decision函数，在判断从make_gc_decision函数返回的结果不是no_gc后，zgc将进行一次gc。

 

make_gc_decision函数：

GCCause::Cause ZDirector::make_gc_decision() const {
  // Rule 0: Timer
  if (rule_timer()) {
    return GCCause::_z_timer;
  }

  // Rule 1: Warmup
  if (rule_warmup()) {
    return GCCause::_z_warmup;
  }

  // Rule 2: Allocation rate
  if (rule_allocation_rate()) {
    return GCCause::_z_allocation_rate;
  }

  // Rule 3: Proactive
  if (rule_proactive()) {
    return GCCause::_z_proactive;
  }

  // No GC
  return GCCause::_no_gc;
}

make_gc_decision一共提供了4种被动gc策略：

rule 1：固定间隔时间

通过配置ZCollectionInterval参数，可以控制zgc在一个固定的时间间隔进行gc，默认值为0，表示不采用该策略，否则则判断从上次gc到现在的时间间隔是否大于ZCollectionInterval秒，是则gc。源码如下：

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;
}

rule 2：预热规则

is_warm函数判断gc次数是否已超过3次，是则不使用该策略。

注释说的很清楚，当gc次数少于3时，判断堆使用率达到10%/20%/30%时，使用该策略

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;
}

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

// 位置：ZStat.cpp
uint64_t ZStatCycle::ncycles() {
  return _ncycles; // gc次数
}

rule 3：分配速率预估

is_first函数判断如果是首次gc，则直接返回false。

ZAllocationSpikeTolerance默认值为2，分配速率策略采用正态分布模型预测内存分配速率，加上ZAllocationSpikeTolerance修正因子，可以覆盖超过99.9%的内存分配速率的可能性

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;
}

bool ZDirector::is_first() const {
  return ZStatCycle::ncycles() == 0;
}

rule 4：积极回收策略

通过ZProactive可启用积极回收策略，is_warm函数判断启用该策略必须是在预热之后(gc次数超过3次)

自上一次gc后，堆使用率达到xmx的10%或者已过了5分钟，这个参数是弥补第三个规则中没有覆盖的场景，从上述分析可以得到第三个条件更多的覆盖分配速率比较高的场景。

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;
}

最后，当所有策略都不满足时，返回_no_gc，表示不进行gc

二、回收过程

gc整个周期：

彩色指针示意图：

• (STW)Pause Mark Start，开始标记，这个阶段只会标记(Mark0)由root引用的object，组成Root Set
• Concurrent Mark，并发标记，从Root Set出发，并发遍历Root Set object的引用链并标记(Mark1)
• (STW)Pause Mark End，检查是否已经并发标记完成，如果不是，需要进行多一次Concurrent Mark
• Concurrent Process Non-Strong References，并发处理弱引用
• Concurrent Reset Relocation Set
• Concurrent Destroy Detached Pages
• Concurrent Select Relocation Set，并发选择Relocation Set；
• Concurrent Prepare Relocation Set，并发预处理Relocation Set
• (STW)Pause Relocate Start，开始转移对象，依然是遍历root引用
• Concurrent Relocate，并发转移，将需要回收的Page里的对象转移到Relocation Set，然后回收Page给系统重新利用

run_gc_cycle函数（/src/hotspot/share/gc/z/zDriver.cpp）：

void ZDriver::run_gc_cycle(GCCause::Cause cause) {
  ZDriverCycleScope scope(cause);

  // Phase 1: Pause Mark Start
  {
    ZMarkStartClosure cl;
    vm_operation(&cl);
  }

  // Phase 2: Concurrent Mark
  {
    ZStatTimer timer(ZPhaseConcurrentMark);
    ZHeap::heap()->mark();
  }

  // Phase 3: Pause Mark End
  {
    ZMarkEndClosure cl;
    while (!vm_operation(&cl)) {
      // Phase 3.5: Concurrent Mark Continue
      ZStatTimer timer(ZPhaseConcurrentMarkContinue);
      ZHeap::heap()->mark();
    }
  }

  // Phase 4: Concurrent Process Non-Strong References
  {
    ZStatTimer timer(ZPhaseConcurrentProcessNonStrongReferences);
    ZHeap::heap()->process_non_strong_references();
  }

  // Phase 5: Concurrent Reset Relocation Set
  {
    ZStatTimer timer(ZPhaseConcurrentResetRelocationSet);
    ZHeap::heap()->reset_relocation_set();
  }

  // Phase 6: Concurrent Destroy Detached Pages
  {
    ZStatTimer timer(ZPhaseConcurrentDestroyDetachedPages);
    ZHeap::heap()->destroy_detached_pages();
  }

  // Phase 7: Concurrent Select Relocation Set
  {
    ZStatTimer timer(ZPhaseConcurrentSelectRelocationSet);
    ZHeap::heap()->select_relocation_set();
  }

  // Phase 8: Concurrent Prepare Relocation Set
  {
    ZStatTimer timer(ZPhaseConcurrentPrepareRelocationSet);
    ZHeap::heap()->prepare_relocation_set();
  }

  // Phase 9: Pause Relocate Start
  {
    ZRelocateStartClosure cl;
    vm_operation(&cl);
  }

  // Phase 10: Concurrent Relocate
  {
    ZStatTimer timer(ZPhaseConcurrentRelocated);
    ZHeap::heap()->relocate();
  }
}

未完待续