How does Java 7 decide on the max value of heap memory allocated (-Xmx) on OSX

Question

How does Java 7 decide on the max value of heap memory allocated (-Xmx) if not specified on an OSX bundle, I've read the manual page and it gave no indication. It seems to be allocated more than the default on Java 6 and I wonder if it varies with the memory available on the machine, that would be very useful to me because my application is memory bound, but I cannot set the default too high because then the application would fail to run at all on lower specification machines.

Answer 1

default heap size from openjdk source code hotspot/src/share/vm/runtime/arguments.cpp

copy comment from source

  // If the maximum heap size has not been set with -Xmx,
  // then set it as fraction of the size of physical memory,
  // respecting the maximum and minimum sizes of the heap.

Fraction = 4

  product(uintx, MaxRAMFraction, 4,                                         \
          "Maximum fraction (1/n) of real memory used for maximum heap "    \
          "size")                                                           \
                                                                            \
  product(uintx, DefaultMaxRAMFraction, 4,                                  \
          "Maximum fraction (1/n) of real memory used for maximum heap "    \
          "size; deprecated: to be renamed to MaxRAMFraction")              \

full code about mx

void Arguments::set_heap_size() {
  if (!FLAG_IS_DEFAULT(DefaultMaxRAMFraction)) {
    // Deprecated flag
    FLAG_SET_CMDLINE(uintx, MaxRAMFraction, DefaultMaxRAMFraction);
  }

  const julong phys_mem =
    FLAG_IS_DEFAULT(MaxRAM) ? MIN2(os::physical_memory(), (julong)MaxRAM)
                            : (julong)MaxRAM;

  // If the maximum heap size has not been set with -Xmx,
  // then set it as fraction of the size of physical memory,
  // respecting the maximum and minimum sizes of the heap.
  if (FLAG_IS_DEFAULT(MaxHeapSize)) {
    julong reasonable_max = phys_mem / MaxRAMFraction;

    if (phys_mem <= MaxHeapSize * MinRAMFraction) {
      // Small physical memory, so use a minimum fraction of it for the heap
      reasonable_max = phys_mem / MinRAMFraction;
    } else {
      // Not-small physical memory, so require a heap at least
      // as large as MaxHeapSize
      reasonable_max = MAX2(reasonable_max, (julong)MaxHeapSize);
    }
    if (!FLAG_IS_DEFAULT(ErgoHeapSizeLimit) && ErgoHeapSizeLimit != 0) {
      // Limit the heap size to ErgoHeapSizeLimit
      reasonable_max = MIN2(reasonable_max, (julong)ErgoHeapSizeLimit);
    }
    if (UseCompressedOops) {
      // Limit the heap size to the maximum possible when using compressed oops
      julong max_coop_heap = (julong)max_heap_for_compressed_oops();
      if (HeapBaseMinAddress + MaxHeapSize < max_coop_heap) {
        // Heap should be above HeapBaseMinAddress to get zero based compressed oops
        // but it should be not less than default MaxHeapSize.
        max_coop_heap -= HeapBaseMinAddress;
      }
      reasonable_max = MIN2(reasonable_max, max_coop_heap);
    }
    reasonable_max = os::allocatable_physical_memory(reasonable_max);

    if (!FLAG_IS_DEFAULT(InitialHeapSize)) {
      // An initial heap size was specified on the command line,
      // so be sure that the maximum size is consistent.  Done
      // after call to allocatable_physical_memory because that
      // method might reduce the allocation size.
      reasonable_max = MAX2(reasonable_max, (julong)InitialHeapSize);
    }

    if (PrintGCDetails && Verbose) {
      // Cannot use gclog_or_tty yet.
      tty->print_cr("  Maximum heap size " SIZE_FORMAT, reasonable_max);
    }
    FLAG_SET_ERGO(uintx, MaxHeapSize, (uintx)reasonable_max);
  }

  // If the initial_heap_size has not been set with InitialHeapSize
  // or -Xms, then set it as fraction of the size of physical memory,
  // respecting the maximum and minimum sizes of the heap.
  if (FLAG_IS_DEFAULT(InitialHeapSize)) {
    julong reasonable_minimum = (julong)(OldSize + NewSize);

    reasonable_minimum = MIN2(reasonable_minimum, (julong)MaxHeapSize);

    reasonable_minimum = os::allocatable_physical_memory(reasonable_minimum);

    julong reasonable_initial = phys_mem / InitialRAMFraction;

    reasonable_initial = MAX2(reasonable_initial, reasonable_minimum);
    reasonable_initial = MIN2(reasonable_initial, (julong)MaxHeapSize);

    reasonable_initial = os::allocatable_physical_memory(reasonable_initial);

    if (PrintGCDetails && Verbose) {
      // Cannot use gclog_or_tty yet.
      tty->print_cr("  Initial heap size " SIZE_FORMAT, (uintx)reasonable_initial);
      tty->print_cr("  Minimum heap size " SIZE_FORMAT, (uintx)reasonable_minimum);
    }
    FLAG_SET_ERGO(uintx, InitialHeapSize, (uintx)reasonable_initial);
    set_min_heap_size((uintx)reasonable_minimum);
  }
}