RFR: 8150518: G1 GC crashes at G1CollectedHeap::do_collection_pause_at_safepoint(double)

[Jon Masamitsu]

On 3/10/2016 12:46 PM, Thomas Schatzl wrote:
> Hi,
>
> On Thu, 2016-03-10 at 15:42 -0500, Kim Barrett wrote:
>>> On Mar 10, 2016, at 12:21 PM, Jon Masamitsu <
>>> jon.masamitsu at oracle.com> wrote:
>>>
>>>
>>>
> [...]
>> The retry setting to parallel_worker_threads() again code dates back
>> to the initial G1 checkin.  Hard to know what was intended.
>>
>> The current jdk9 code does not have that: it looks like
>>
>>    FLAG_SET_DEFAULT(ParallelGCThreads,
>> Abstract_VM_Version::parallel_worker_threads());
>>    if (ParallelGCThreads == 0) {
>>      assert(!FLAG_IS_DEFAULT(ParallelGCThreads), "The default value
>> for ParallelGCThreads should not be 0.");
>>      vm_exit_during_initialization("The flag -XX:+UseG1GC can not be
>> combined with -XX:ParallelGCThreads=0", NULL);
>>    }
>>
>> This proposed change is for jdk8u-something, correct?
>>
>> In jdk9 G1 does not support ParallelGCThreads == 0 at all, as can be
>> seen above.  Making use of that decision, a cleanup pass was made
>> that eliminated a whole bunch of “if (ParallelGCThreads == 0) then
>> pretend it’s 1 or do some other special thing” code.  The backport of
>> 8017462 to 8u72 (i.e. 8149347) looks like it might not have taken
>> that into account.  For example,
> just some historical note: in 8u, ParallelGCThreads == 0 means "use the
> VM thread, single threaded", and ParallelGCThreads == 1 means use a
> single worker thread.

By "VM thread, single threaded" I believe you mean that we used the serial
version (no threading code) and let the VM execute it.  I recall this 
being the
case with some ParNew  code.

> In jdk9 we removed that option to use the vm thread in a single
> threaded way.
>
> We did not backport that change because of potential performance impact
> (whatever that might be).

This can be fixed while leaving the ParallelGCThreads == 0 as an option 
for G1.
In the 8u it is no longer an option for ParNew  nor ParallelGC (both 
prohibit
ParallelGCThreads == 0 in the argument processing).

How about we make G1 like the other collectors in 8u and prohibit
ParallelGCThreads == 0.   That would be the simplest and most
consistent thing to do (in my opinion).

Jon

>
> Thanks,
>    Thomas
>