RFR: 8259231: Fix the chance to allocate failure and improve the speed and quality of EpsilonHeap::allocate_work

[Aleksey Shipilev]

On Wed, 16 Dec 2020 01:29:55 GMT, Lehua Ding <github.com+13173904+lhtin at openjdk.org> wrote:

> Hi all,
> 
> The `EpsilonHeap::allocate_work` method maybe can be fixed and improved by this:
> 1. it can prevent allocate failure by retry `_space->par_allocate` before expanding virtual space,  when there not enough virtual space but another thread expanding succeeded just and has enough space.
> 2. it can reduce the lock time by move `res = _space->par_allocate(size);` out of lock scope.
> 
> Test on macosx-x86_64-server-{release, fastdebug, slowdebug} with current test case.

Good find! Comments below:

src/hotspot/share/gc/epsilon/epsilon_globals.hpp line 67:

> 65:                                                                             \
> 66:   product(bool, EpsilonElasticTLABDecay, true, EXPERIMENTAL,                \
> 67:           "Use timed decays to shrink TLAB sizes. This conserves memory "   \

Let's not do the typo fixes in this PR. Maybe there are other spelling problems elsewhere in `gc/epsilon` that we could fix wholesale in another PR?

src/hotspot/share/gc/epsilon/epsilonHeap.cpp line 113:

> 111:   while (res == NULL) {
> 112:     // Allocation failed, attempt expansion, and retry:
> 113:     {

I see what you are trying to do, and it makes sense. I believe this form would be cleaner:

HeapWord* res = NULL;
while (true) {
  // Try to allocate, assume space is available
  res = par_allocate(size);
  if (res != NULL) {
    break;
  }

  MutexLocker ml(Heap_Lock);

  // Try to allocate under the lock, assume another thread was able to expand
  res = par_allocate(size);
  if (res != NULL) {
    break;
  }

  // Expand and loop back if space is available
  size_t space_left = max_capacity() - capacity();
  size_t want_space = MAX2(size, EpsilonMinHeapExpand);
   ...
}

-------------

Changes requested by shade (Reviewer).

PR: https://git.openjdk.java.net/jdk/pull/1794