RFR: 8280139: Report more detailed statistics about task stealing in task queue stats

[Kim Barrett]

On Wed, 19 Jan 2022 19:55:46 GMT, Kim Barrett <kbarrett at openjdk.org> wrote:

>> Hi all,
>> 
>>   can I have reviews that improves task queue statistics?
>> 
>> This is a step to see problems with task queue in general and measure future task stealing improvements in GC; particularly contention on steal information has shown to be correlated to performance, so information about basic occurrences during stealing (how many empty, how many contended, how many contended in a row, max contended accesses in a row) in the task queue is very interesting for further performance work.
>> 
>> Old output of `gc+tasks+stats=trace` after compiling in these statistics:
>> 
>> thr      qpush       qpop     qpop-s   qattempt     qsteal      opush       omax
>> --- ---------- ---------- ---------- ---------- ---------- ---------- ----------
>>   0    3777115    3776816       3872      17434       9630          0          0
>> 
>> And new:
>> 
>> thr        push         pop    pop-slow  st-attempt    st-empty     st-ctdd  st-success st-ctdd-max st-biasdrop  ovflw-push   ovflw-max
>> --- ----------- ----------- ----------- ----------- ----------- ----------- ----------- ----------- ----------- ----------- -----------
>>   0     4616264     4616218        3608       12945           1        1650       11294           5        9405           0           0
>> [...]
>> 
>> The first four columns stayed the same, then the `qsteal` has been split up into the various steal results (empty/contended/success). `st_ctdd_max` gives the maximum amount of contended steal attempts in a row. `st_biasdrop` shows how many times the steal bias has been dropped (reset).
>> `ovflw-push` and `ovfl-max` are the same as before, just renamed a bit to use the available space.
>> 
>> There has been no intention to change anything about the algorithm, so there is no need for perf testing (I can see). Later changes will do that.
>> 
>> Thanks,
>>   Thomas
>
> src/hotspot/share/gc/shared/taskqueue.hpp line 366:
> 
>> 364:   // recently pushed).
>> 365:   // The result value order of this correspond to the order in the corresponding StatId.
>> 366:   enum PopResult {
> 
> I think the `PopResult` enum should be in `TaskQueueSuper`.  I think that will allow some of the references to be simplified.  Others might be simplified by being brought into scope via `using`.

Also, consider making `PopResult` an enum class.

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PR: https://git.openjdk.java.net/jdk/pull/7143