src/hotspot/share/gc/z/zHeuristics.cpp

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#include "gc/shared/gc_globals.hpp"
#include "gc/shared/gcLogPrecious.hpp"
#include "gc/z/zCPU.inline.hpp"
#include "gc/z/zGlobals.hpp"
#include "gc/z/zHeuristics.hpp"
#include "runtime/globals.hpp"
#include "runtime/os.hpp"
#include "utilities/align.hpp"
#include "utilities/globalDefinitions.hpp"
#include "utilities/powerOfTwo.hpp"

void ZHeuristics::set_medium_page_size() {
  // Set ZPageSizeMedium so that a medium page occupies at most 3.125% of the
  // max heap size. ZPageSizeMedium is initially set to 0, which means medium
  // pages are effectively disabled. It is adjusted only if ZPageSizeMedium
  // becomes larger than ZPageSizeSmall.
  const size_t min = ZGranuleSize;
  const size_t max = ZGranuleSize * 16;
  const size_t unclamped = (size_t)(MaxHeapSize * 0.03125);
  const size_t clamped = clamp(unclamped, min, max);
  const size_t size = round_down_power_of_2(clamped);

  if (size > ZPageSizeSmall) {
    // Enable medium pages
    ZPageSizeMediumMax          = size;
    ZPageSizeMediumMaxShift     = log2i_exact(ZPageSizeMediumMax);
    ZObjectSizeLimitMedium      = ZPageSizeMediumMax / 8;
    ZObjectAlignmentMediumShift = ZPageSizeMediumMaxShift - 13;
    ZObjectAlignmentMedium      = 1 << ZObjectAlignmentMediumShift;
    ZPageSizeMediumEnabled      = true;
    ZPageSizeMediumMin          = ZUseMediumPageSizeRange
                                ? align_up(ZObjectSizeLimitMedium, ZGranuleSize)
                                : ZPageSizeMediumMax;
  }
}

size_t ZHeuristics::relocation_headroom() {
  // Calculate headroom needed to avoid in-place relocation. Each worker will try
  // to allocate a small page, and all workers will share a single medium page.
  return (ConcGCThreads * ZPageSizeSmall) + ZPageSizeMediumMax;
}

bool ZHeuristics::use_per_cpu_shared_small_pages() {
  // Use per-CPU shared small pages only if these pages don't have a significant
  // heap overhead. Otherwise fall back to using a single shared small
  // page. This is useful when using small heaps on large machines.
  const size_t per_cpu_share = significant_heap_overhead() / ZCPU::count();
  return per_cpu_share >= ZPageSizeSmall;
}

static uint nworkers_based_on_ncpus(double cpu_share_in_percent) {
  return (uint)ceil(os::initial_active_processor_count() * cpu_share_in_percent / 100.0);
}

static uint nworkers_based_on_heap_size(double heap_share_in_percent) {
  return (uint)(MaxHeapSize * (heap_share_in_percent / 100.0) / ZPageSizeSmall);
}

static uint nworkers(double cpu_share_in_percent) {
  // Cap number of workers so that they don't use more than 2% of the max heap
  // during relocation. This is useful when using small heaps on large machines.
  return MIN2(nworkers_based_on_ncpus(cpu_share_in_percent),
              nworkers_based_on_heap_size(2.0));
}

uint ZHeuristics::nparallel_workers() {
  // Use 60% of the CPUs, rounded up. We would like to use as many threads as
  // possible to increase parallelism. However, using a thread count that is
  // close to the number of processors tends to lead to over-provisioning and
  // scheduling latency issues. Using 60% of the active processors appears to
  // be a fairly good balance.
  return MAX2(nworkers(60.0), 1u);
}

uint ZHeuristics::nconcurrent_workers() {
  // The number of concurrent threads we would like to use heavily depends
  // on the type of workload we are running. Using too many threads will have
  // a negative impact on the application throughput, while using too few
  // threads will prolong the GC cycle and we then risk being out-run by the
  // application.
  return MAX2(nworkers(25.0), 1u);
}

size_t ZHeuristics::significant_heap_overhead() {
  return (size_t)(MaxHeapSize * (ZFragmentationLimit / 100));
}

size_t ZHeuristics::significant_young_overhead() {
  return (size_t)(MaxHeapSize * (ZYoungCompactionLimit / 100));
}