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

[Lehua Ding]

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.

Tencent

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