[crac] RFR: Wait until G1 GC has finished before creating a snapshot. [v5]

[Sergey Nazarkin]

On Fri, 28 Jul 2023 14:42:00 GMT, Jan Kratochvil <jkratochvil at openjdk.org> wrote:

>> @rvansa did report his snapshots are about 2x-3x bigger than they should be. He then also found it only happens if the snapshot is done too quickly after GC should have been run.
>> 
>> One can reproduce the race case by:
>> 
>> --- a/src/hotspot/share/gc/g1/g1UncommitRegionTask.hpp
>> +++ b/src/hotspot/share/gc/g1/g1UncommitRegionTask.hpp
>> @@ -35,7 +35,7 @@ class G1UncommitRegionTask : public G1ServiceTask {
>>    // is short, while still making reasonable progress.
>>    static const uint UncommitSizeLimit = 128 * M;
>>    // Initial delay in milliseconds after GC before the regions are uncommitted.
>> -  static const uint UncommitInitialDelayMs = 100;
>> +  static const uint UncommitInitialDelayMs = 10*1000;
>>    // The delay between two uncommit task executions.
>>    static const uint UncommitTaskDelayMs = 10;
>
> Jan Kratochvil has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Remove accidental change of UncommitInitialDelayMs.
>    - bugreported by Radim Vansa

src/hotspot/share/gc/g1/g1UncommitRegionTask.cpp line 143:

> 141:   assert(!Thread::current()->is_ConcurrentGC_thread(), "deadlock prevention");
> 142: 
> 143:   while (_active) {

Is this cycle safe against infinite loop? _active is not atomic or volatile.

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PR Review Comment: https://git.openjdk.org/crac/pull/93#discussion_r1277755070