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src/hotspot/share/gc/shenandoah/heuristics/shenandoahAdaptiveHeuristics.cpp

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*** 25,12 ***
  #include "precompiled.hpp"
  
  #include "gc/shenandoah/heuristics/shenandoahAdaptiveHeuristics.hpp"
  #include "gc/shenandoah/shenandoahCollectionSet.hpp"
  #include "gc/shenandoah/shenandoahFreeSet.hpp"
! #include "gc/shenandoah/shenandoahHeap.inline.hpp"
  #include "gc/shenandoah/shenandoahHeapRegion.inline.hpp"
  #include "logging/log.hpp"
  #include "logging/logTag.hpp"
  #include "utilities/quickSort.hpp"
  
  // These constants are used to adjust the margin of error for the moving
--- 25,13 ---
  #include "precompiled.hpp"
  
  #include "gc/shenandoah/heuristics/shenandoahAdaptiveHeuristics.hpp"
  #include "gc/shenandoah/shenandoahCollectionSet.hpp"
  #include "gc/shenandoah/shenandoahFreeSet.hpp"
! #include "gc/shenandoah/shenandoahGeneration.hpp"
  #include "gc/shenandoah/shenandoahHeapRegion.inline.hpp"
+ #include "gc/shenandoah/shenandoahYoungGeneration.hpp"
  #include "logging/log.hpp"
  #include "logging/logTag.hpp"
  #include "utilities/quickSort.hpp"
  
  // These constants are used to adjust the margin of error for the moving

*** 52,12 ***
  // that the true value of our estimate is outside the interval. These are used
  // as bounds on the adjustments applied at the outcome of a GC cycle.
  const double ShenandoahAdaptiveHeuristics::MINIMUM_CONFIDENCE = 0.319; // 25%
  const double ShenandoahAdaptiveHeuristics::MAXIMUM_CONFIDENCE = 3.291; // 99.9%
  
! ShenandoahAdaptiveHeuristics::ShenandoahAdaptiveHeuristics() :
!   ShenandoahHeuristics(),
    _margin_of_error_sd(ShenandoahAdaptiveInitialConfidence),
    _spike_threshold_sd(ShenandoahAdaptiveInitialSpikeThreshold),
    _last_trigger(OTHER) { }
  
  ShenandoahAdaptiveHeuristics::~ShenandoahAdaptiveHeuristics() {}
--- 53,12 ---
  // that the true value of our estimate is outside the interval. These are used
  // as bounds on the adjustments applied at the outcome of a GC cycle.
  const double ShenandoahAdaptiveHeuristics::MINIMUM_CONFIDENCE = 0.319; // 25%
  const double ShenandoahAdaptiveHeuristics::MAXIMUM_CONFIDENCE = 3.291; // 99.9%
  
! ShenandoahAdaptiveHeuristics::ShenandoahAdaptiveHeuristics(ShenandoahGeneration* generation) :
!   ShenandoahHeuristics(generation),
    _margin_of_error_sd(ShenandoahAdaptiveInitialConfidence),
    _spike_threshold_sd(ShenandoahAdaptiveInitialSpikeThreshold),
    _last_trigger(OTHER) { }
  
  ShenandoahAdaptiveHeuristics::~ShenandoahAdaptiveHeuristics() {}

*** 82,53 ***
    // Therefore, we start by sorting the regions by garbage. Then we unconditionally add the best candidates
    // before we meet min_garbage. Then we add all candidates that fit with a garbage threshold before
    // we hit max_cset. When max_cset is hit, we terminate the cset selection. Note that in this scheme,
    // ShenandoahGarbageThreshold is the soft threshold which would be ignored until min_garbage is hit.
  
!   size_t capacity    = ShenandoahHeap::heap()->soft_max_capacity();
!   size_t max_cset    = (size_t)((1.0 * capacity / 100 * ShenandoahEvacReserve) / ShenandoahEvacWaste);
-   size_t free_target = (capacity / 100 * ShenandoahMinFreeThreshold) + max_cset;
-   size_t min_garbage = (free_target > actual_free ? (free_target - actual_free) : 0);
  
!   log_info(gc, ergo)("Adaptive CSet Selection. Target Free: " SIZE_FORMAT "%s, Actual Free: "
!                      SIZE_FORMAT "%s, Max CSet: " SIZE_FORMAT "%s, Min Garbage: " SIZE_FORMAT "%s",
!                      byte_size_in_proper_unit(free_target), proper_unit_for_byte_size(free_target),
!                      byte_size_in_proper_unit(actual_free), proper_unit_for_byte_size(actual_free),
                       byte_size_in_proper_unit(max_cset),    proper_unit_for_byte_size(max_cset),
!                      byte_size_in_proper_unit(min_garbage), proper_unit_for_byte_size(min_garbage));
  
    // Better select garbage-first regions
    QuickSort::sort<RegionData>(data, (int)size, compare_by_garbage, false);
  
    size_t cur_cset = 0;
!   size_t cur_garbage = 0;
  
    for (size_t idx = 0; idx < size; idx++) {
      ShenandoahHeapRegion* r = data[idx]._region;
  
      size_t new_cset    = cur_cset + r->get_live_data_bytes();
-     size_t new_garbage = cur_garbage + r->garbage();
  
!     if (new_cset > max_cset) {
!       break;
!     }
! 
!     if ((new_garbage < min_garbage) || (r->garbage() > garbage_threshold)) {
        cset->add_region(r);
        cur_cset = new_cset;
!       cur_garbage = new_garbage;
      }
    }
  }
  
  void ShenandoahAdaptiveHeuristics::record_cycle_start() {
    ShenandoahHeuristics::record_cycle_start();
    _allocation_rate.allocation_counter_reset();
  }
  
! void ShenandoahAdaptiveHeuristics::record_success_concurrent() {
!   ShenandoahHeuristics::record_success_concurrent();
  
    size_t available = ShenandoahHeap::heap()->free_set()->available();
  
    _available.add(available);
    double z_score = 0.0;
--- 83,77 ---
    // Therefore, we start by sorting the regions by garbage. Then we unconditionally add the best candidates
    // before we meet min_garbage. Then we add all candidates that fit with a garbage threshold before
    // we hit max_cset. When max_cset is hit, we terminate the cset selection. Note that in this scheme,
    // ShenandoahGarbageThreshold is the soft threshold which would be ignored until min_garbage is hit.
  
!   size_t max_cset    = (ShenandoahHeap::heap()->get_young_evac_reserve() / ShenandoahEvacWaste);
!   size_t capacity    = ShenandoahHeap::heap()->young_generation()->soft_max_capacity();
  
!   // As currently implemented, we are not enforcing that new_garbage > min_garbage
!   // size_t free_target = (capacity / 100) * ShenandoahMinFreeThreshold + max_cset;
!   // size_t min_garbage = (free_target > actual_free ? (free_target - actual_free) : 0);
! 
+   log_info(gc, ergo)("Adaptive CSet Selection. Max CSet: " SIZE_FORMAT "%s, Actual Free: " SIZE_FORMAT "%s.",
                       byte_size_in_proper_unit(max_cset),    proper_unit_for_byte_size(max_cset),
!                      byte_size_in_proper_unit(actual_free), proper_unit_for_byte_size(actual_free));
  
    // Better select garbage-first regions
    QuickSort::sort<RegionData>(data, (int)size, compare_by_garbage, false);
  
    size_t cur_cset = 0;
!   // size_t cur_garbage = 0;
+ 
+   // In generational mode, the sort order within the data array is not strictly descending amounts of garbage.  In
+   // particular, regions that have reached tenure age will be sorted into this array before younger regions that contain
+   // more garbage.  This represents one of the reasons why we keep looking at regions even after we decide, for example,
+   // to exclude one of the regions because it might require evacuation of too much live data.
  
    for (size_t idx = 0; idx < size; idx++) {
      ShenandoahHeapRegion* r = data[idx]._region;
+     size_t biased_garbage = data[idx]._garbage;
  
      size_t new_cset    = cur_cset + r->get_live_data_bytes();
  
!     // As currently implemented, we are not enforcing that new_garbage > min_garbage
!     // size_t new_garbage = cur_garbage + r->garbage();
! 
!     // Note that live data bytes within a region is not the same as heap_region_size - garbage.  This is because
!     // each region contains a combination of used memory (which is garbage plus live) and unused memory, which has not
+     // yet been allocated.  It may be the case that the region on this iteration has too much live data to be added to
+     // the collection set while one or more regions seen on subsequent iterations of this loop can be added to the collection
+     // set because they have smaller live memory, even though they also have smaller garbage (and necessarily a larger
+     // amount of unallocated memory).
+ 
+     // BANDAID: In an earlier version of this code, this was written:
+     //   if ((new_cset <= max_cset) && ((new_garbage < min_garbage) || (r->garbage() > garbage_threshold)))
+     // The problem with the original code is that in some cases the collection set would include hundreds of regions,
+     // each with less than 100 bytes of garbage.  Evacuating these regions is counterproductive.
+ 
+     // TODO: Think about changing the description and defaults for ShenandoahGarbageThreshold and ShenandoahMinFreeThreshold.
+     // If "customers" want to evacuate regions with smaller amounts of garbage contained therein, they should specify a lower
+     // value of ShenandoahGarbageThreshold.  As implemented currently, we may experience back-to-back collections if there is
+     // not enough memory to be reclaimed.  Let's not let pursuit of min_garbage drive us to make poor decisions.  Maybe we
+     // want yet another global parameter to allow a region to be placed into the collection set if
+     // (((new_garbage < min_garbage) && (r->garbage() > ShenandoahSmallerGarbageThreshold)) || (r->garbage() > garbage_threshold))
+ 
+     if ((new_cset <= max_cset) && ((r->garbage() > garbage_threshold) || (r->age() >= InitialTenuringThreshold))) {
        cset->add_region(r);
        cur_cset = new_cset;
!       // cur_garbage = new_garbage;
+     } else if (biased_garbage == 0) {
+       break;
      }
    }
  }
  
  void ShenandoahAdaptiveHeuristics::record_cycle_start() {
    ShenandoahHeuristics::record_cycle_start();
    _allocation_rate.allocation_counter_reset();
  }
  
! void ShenandoahAdaptiveHeuristics::record_success_concurrent(bool abbreviated) {
!   ShenandoahHeuristics::record_success_concurrent(abbreviated);
  
    size_t available = ShenandoahHeap::heap()->free_set()->available();
  
    _available.add(available);
    double z_score = 0.0;

*** 194,38 ***
  static double saturate(double value, double min, double max) {
    return MAX2(MIN2(value, max), min);
  }
  
  bool ShenandoahAdaptiveHeuristics::should_start_gc() {
!   ShenandoahHeap* heap = ShenandoahHeap::heap();
!   size_t max_capacity = heap->max_capacity();
!   size_t capacity = heap->soft_max_capacity();
!   size_t available = heap->free_set()->available();
!   size_t allocated = heap->bytes_allocated_since_gc_start();
  
    // Make sure the code below treats available without the soft tail.
    size_t soft_tail = max_capacity - capacity;
    available = (available > soft_tail) ? (available - soft_tail) : 0;
  
    // Track allocation rate even if we decide to start a cycle for other reasons.
    double rate = _allocation_rate.sample(allocated);
    _last_trigger = OTHER;
  
    size_t min_threshold = capacity / 100 * ShenandoahMinFreeThreshold;
    if (available < min_threshold) {
!     log_info(gc)("Trigger: Free (" SIZE_FORMAT "%s) is below minimum threshold (" SIZE_FORMAT "%s)",
                   byte_size_in_proper_unit(available),     proper_unit_for_byte_size(available),
                   byte_size_in_proper_unit(min_threshold), proper_unit_for_byte_size(min_threshold));
      return true;
    }
  
    const size_t max_learn = ShenandoahLearningSteps;
    if (_gc_times_learned < max_learn) {
      size_t init_threshold = capacity / 100 * ShenandoahInitFreeThreshold;
      if (available < init_threshold) {
!       log_info(gc)("Trigger: Learning " SIZE_FORMAT " of " SIZE_FORMAT ". Free (" SIZE_FORMAT "%s) is below initial threshold (" SIZE_FORMAT "%s)",
!                    _gc_times_learned + 1, max_learn,
                     byte_size_in_proper_unit(available),      proper_unit_for_byte_size(available),
                     byte_size_in_proper_unit(init_threshold), proper_unit_for_byte_size(init_threshold));
        return true;
      }
    }
--- 219,44 ---
  static double saturate(double value, double min, double max) {
    return MAX2(MIN2(value, max), min);
  }
  
  bool ShenandoahAdaptiveHeuristics::should_start_gc() {
!   size_t max_capacity = _generation->max_capacity();
!   size_t capacity = _generation->soft_max_capacity();
!   size_t available = _generation->available();
!   size_t allocated = _generation->bytes_allocated_since_gc_start();
! 
+   log_debug(gc)("should_start_gc (%s)? available: " SIZE_FORMAT ", soft_max_capacity: " SIZE_FORMAT
+                 ", max_capacity: " SIZE_FORMAT ", allocated: " SIZE_FORMAT,
+                 _generation->name(), available, capacity, max_capacity, allocated);
  
    // Make sure the code below treats available without the soft tail.
    size_t soft_tail = max_capacity - capacity;
    available = (available > soft_tail) ? (available - soft_tail) : 0;
  
    // Track allocation rate even if we decide to start a cycle for other reasons.
    double rate = _allocation_rate.sample(allocated);
    _last_trigger = OTHER;
  
    size_t min_threshold = capacity / 100 * ShenandoahMinFreeThreshold;
+ 
    if (available < min_threshold) {
!     log_info(gc)("Trigger (%s): Free (" SIZE_FORMAT "%s) is below minimum threshold (" SIZE_FORMAT "%s)",
+                  _generation->name(),
                   byte_size_in_proper_unit(available),     proper_unit_for_byte_size(available),
                   byte_size_in_proper_unit(min_threshold), proper_unit_for_byte_size(min_threshold));
      return true;
    }
  
+   // Check if we need to learn a bit about the application
    const size_t max_learn = ShenandoahLearningSteps;
    if (_gc_times_learned < max_learn) {
      size_t init_threshold = capacity / 100 * ShenandoahInitFreeThreshold;
      if (available < init_threshold) {
!       log_info(gc)("Trigger (%s): Learning " SIZE_FORMAT " of " SIZE_FORMAT ". Free (" SIZE_FORMAT "%s) is below initial threshold (" SIZE_FORMAT "%s)",
!                    _generation->name(), _gc_times_learned + 1, max_learn,
                     byte_size_in_proper_unit(available),      proper_unit_for_byte_size(available),
                     byte_size_in_proper_unit(init_threshold), proper_unit_for_byte_size(init_threshold));
        return true;
      }
    }

*** 233,21 ***
    // Check if allocation headroom is still okay. This also factors in:
    //   1. Some space to absorb allocation spikes
    //   2. Accumulated penalties from Degenerated and Full GC
    size_t allocation_headroom = available;
  
    size_t spike_headroom = capacity / 100 * ShenandoahAllocSpikeFactor;
    size_t penalties      = capacity / 100 * _gc_time_penalties;
  
    allocation_headroom -= MIN2(allocation_headroom, spike_headroom);
    allocation_headroom -= MIN2(allocation_headroom, penalties);
  
    double avg_cycle_time = _gc_time_history->davg() + (_margin_of_error_sd * _gc_time_history->dsd());
    double avg_alloc_rate = _allocation_rate.upper_bound(_margin_of_error_sd);
    if (avg_cycle_time > allocation_headroom / avg_alloc_rate) {
!     log_info(gc)("Trigger: Average GC time (%.2f ms) is above the time for average allocation rate (%.0f %sB/s) to deplete free headroom (" SIZE_FORMAT "%s) (margin of error = %.2f)",
!                  avg_cycle_time * 1000,
                   byte_size_in_proper_unit(avg_alloc_rate), proper_unit_for_byte_size(avg_alloc_rate),
                   byte_size_in_proper_unit(allocation_headroom), proper_unit_for_byte_size(allocation_headroom),
                   _margin_of_error_sd);
  
      log_info(gc, ergo)("Free headroom: " SIZE_FORMAT "%s (free) - " SIZE_FORMAT "%s (spike) - " SIZE_FORMAT "%s (penalties) = " SIZE_FORMAT "%s",
--- 264,102 ---
    // Check if allocation headroom is still okay. This also factors in:
    //   1. Some space to absorb allocation spikes
    //   2. Accumulated penalties from Degenerated and Full GC
    size_t allocation_headroom = available;
  
+   // ShenandoahAllocSpikeFactor is the percentage of capacity that we endeavor to assure to be free at the end of the GC
+   // cycle.
+   // TODO: Correct the representation of this quantity
+   //       (and dive deeper into _gc_time_penalties as this may also need to be corrected)
+   //
+   //       Allocation spikes are a characteristic of both the application ahd the JVM configuration.  On the JVM command line,
+   //       the application developer may want to supply a hint of the nature of spikes that are inherent in the application
+   //       workload, and this information would normally be independent of heap size (not a percentage thereof).  On the
+   //       other hand, some allocation spikes are correlated with JVM configuration.  For example, there are allocation
+   //       spikes at the starts of concurrent marking and evacuation to refresh all local allocation buffers.  The nature
+   //       of these spikes is determined by LAB min and max sizes and numbers of threads, but also on frequency of GC passes,
+   //       and on "periodic" behavior of these threads  If GC frequency is much higher than the periodic trigger for mutator
+   //       threads, then many of the mutator threads may be able to "sit out" of most GC passes.  Though the thread's stack
+   //       must be scanned, the thread does not need to refresh its LABs if it sits idle throughout the duration of the GC
+   //       pass.  The best prediction for this aspect of spikes in allocation patterns is probably recent past history.
+   //
+   //  Rationale:
+   //    The idea is that there is an average allocation rate and there are occasional abnormal bursts (or spikes) of
+   //    allocations that exceed the average allocation rate.  What do these spikes look like?
+   //
+   //    1. At certain phase changes, we may discard large amounts of data and replace it with large numbers of newly
+   //       allocated objects.  This "spike" looks more like a phase change.  We were in steady state at M bytes/sec
+   //       allocation rate and now we're in a "reinitialization phase" that looks like N bytes/sec.  We need the "spike"
+   //       accomodation to give us enough runway to recalibrate our "average allocation rate".
+   //
+   //   2. The typical workload changes.  "Suddenly", our typical workload of N TPS increases to N+delta TPS.  This means
+   //       our average allocation rate needs to be adjusted.  Once again, we need the "spike" accomodation to give us
+   //       enough runway to recalibrate our "average allocation rate".
+   //
+   //    3. Though there is an "average" allocation rate, a given workload's demand for allocation may be very bursty.  We
+   //       allocate a bunch of LABs during the 5 ms that follow completion of a GC, then we perform no more allocations for
+   //       the next 150 ms.  It seems we want the "spike" to represent the maximum divergence from average within the
+   //       period of time between consecutive evaluation of the should_start_gc() service.  Here's the thinking:
+   //
+   //       a) Between now and the next time I ask whether should_start_gc(), we might experience a spike representing
+   //          the anticipated burst of allocations.  If that would put us over budget, then we should start GC immediately.
+   //       b) Between now and the anticipated depletion of allocation pool, there may be two or more bursts of allocations.
+   //          If there are more than one of these bursts, we can "approximate" that these will be separated by spans of
+   //          time with very little or no allocations so the "average" allocation rate should be a suitable approximation
+   //          of how this will behave.
+   //
+   //    For cases 1 and 2, we need to "quickly" recalibrate the average allocation rate whenever we detect a change
+   //    in operation mode.  We want some way to decide that the average rate has changed.  Make average allocation rate
+   //    computations an independent effort.
+ 
    size_t spike_headroom = capacity / 100 * ShenandoahAllocSpikeFactor;
    size_t penalties      = capacity / 100 * _gc_time_penalties;
  
+   // TODO: Account for inherent delays in responding to GC triggers
+   //  1. It has been observed that delays of 200 ms or greater are common between the moment we return true from should_start_gc()
+   //     and the moment at which we begin execution of the concurrent reset phase.  Add this time into the calculation of
+   //     avg_cycle_time below.  (What is "this time"?  Perhaps we should remember recent history of this delay for the
+   //     running workload and use the maximum delay recently seen for "this time".)
+   //  2. The frequency of inquiries to should_start_gc() is adaptive, ranging between ShenandoahControlIntervalMin and
+   //     ShenandoahControlIntervalMax.  The current control interval (or the max control interval) should also be added into
+   //     the calculation of avg_cycle_time below.
+ 
    allocation_headroom -= MIN2(allocation_headroom, spike_headroom);
    allocation_headroom -= MIN2(allocation_headroom, penalties);
  
    double avg_cycle_time = _gc_time_history->davg() + (_margin_of_error_sd * _gc_time_history->dsd());
+ 
+   size_t last_live_memory = get_last_live_memory();
+   size_t penultimate_live_memory = get_penultimate_live_memory();
+   double original_cycle_time = avg_cycle_time;
+   if ((penultimate_live_memory < last_live_memory) && (penultimate_live_memory != 0)) {
+     // If the live-memory size is growing, our estimates of cycle time are based on lighter workload, so adjust.
+     // TODO: Be more precise about how to scale when live memory is growing.  Existing code is a very rough approximation
+     // tuned with very limited workload observations.
+     avg_cycle_time = (avg_cycle_time * 2 * last_live_memory) / penultimate_live_memory;
+   } else {
+     int degen_cycles = degenerated_cycles_in_a_row();
+     if (degen_cycles > 0) {
+       // If we've degenerated recently, we might be waiting too long between triggers so adjust trigger forward.
+       // TODO: Be more precise about how to scale when we've experienced recent degenerated GC.  Existing code is a very
+       // rough approximation tuned with very limited workload observations.
+       avg_cycle_time += degen_cycles * avg_cycle_time;
+     }
+   }
+ 
    double avg_alloc_rate = _allocation_rate.upper_bound(_margin_of_error_sd);
+   log_debug(gc)("%s: average GC time: %.2f ms, allocation rate: %.0f %s/s",
+     _generation->name(), avg_cycle_time * 1000, byte_size_in_proper_unit(avg_alloc_rate), proper_unit_for_byte_size(avg_alloc_rate));
+ 
    if (avg_cycle_time > allocation_headroom / avg_alloc_rate) {
!     if (avg_cycle_time > original_cycle_time) {
!       log_debug(gc)("%s: average GC time adjusted from: %.2f ms to %.2f ms because upward trend in live memory retention",
+                     _generation->name(), original_cycle_time, avg_cycle_time);
+     }
+ 
+     log_info(gc)("Trigger (%s): Average GC time (%.2f ms) is above the time for average allocation rate (%.0f %sB/s) to deplete free headroom (" SIZE_FORMAT "%s) (margin of error = %.2f)",
+                  _generation->name(), avg_cycle_time * 1000,
                   byte_size_in_proper_unit(avg_alloc_rate), proper_unit_for_byte_size(avg_alloc_rate),
                   byte_size_in_proper_unit(allocation_headroom), proper_unit_for_byte_size(allocation_headroom),
                   _margin_of_error_sd);
  
      log_info(gc, ergo)("Free headroom: " SIZE_FORMAT "%s (free) - " SIZE_FORMAT "%s (spike) - " SIZE_FORMAT "%s (penalties) = " SIZE_FORMAT "%s",

*** 260,14 ***
      return true;
    }
  
    bool is_spiking = _allocation_rate.is_spiking(rate, _spike_threshold_sd);
    if (is_spiking && avg_cycle_time > allocation_headroom / rate) {
!     log_info(gc)("Trigger: Average GC time (%.2f ms) is above the time for instantaneous allocation rate (%.0f %sB/s) to deplete free headroom (" SIZE_FORMAT "%s) (spike threshold = %.2f)",
!                  avg_cycle_time * 1000,
                   byte_size_in_proper_unit(rate), proper_unit_for_byte_size(rate),
                   byte_size_in_proper_unit(allocation_headroom), proper_unit_for_byte_size(allocation_headroom),
                   _spike_threshold_sd);
      _last_trigger = SPIKE;
      return true;
    }
  
--- 372,15 ---
      return true;
    }
  
    bool is_spiking = _allocation_rate.is_spiking(rate, _spike_threshold_sd);
    if (is_spiking && avg_cycle_time > allocation_headroom / rate) {
!     log_info(gc)("Trigger (%s): Average GC time (%.2f ms) is above the time for instantaneous allocation rate (%.0f %sB/s) to deplete free headroom (" SIZE_FORMAT "%s) (spike threshold = %.2f)",
!                  _generation->name(), avg_cycle_time * 1000,
                   byte_size_in_proper_unit(rate), proper_unit_for_byte_size(rate),
                   byte_size_in_proper_unit(allocation_headroom), proper_unit_for_byte_size(allocation_headroom),
+ 
                   _spike_threshold_sd);
      _last_trigger = SPIKE;
      return true;
    }
  
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