[8] RFR: Shenandoah: specialize String Table scans for better pause performance

[rkennke at redhat.com]

Looks good to me!

Thanks,
Roman

> This is sh/jdk8 specific fix. String Table scans dominate some
> workloads, notably Cassandra. But
> they are also visible on simple hello world scenarios. The underlying
> reason is that parallel
> StringTable scans use a very small chunk size, so on decently-sized
> tables we hit the contended
> atomic very often. Larger chunk size is needed to improve
> performance.
> 
> Since that chunk size is hard-coded and entrenched in the 8u upstream
> code, I chose to copy-paste
> the relevant method and specialize it for Shenandoah. 11u and later
> code handles it better with
> OopStorage and concurrent hash tables, so this specialization is only
> needed for 8u.
> 
> 8u webrev:
>   
> https://cr.openjdk.java.net/~shade/shenandoah/8u-stringtable-scan/webrev.01/
> 
> This yields the impressive improvements on Cassandra perf tests, see
> example GC cycles:
> 
> === Baseline:
> 
> Pause Init Mark (N)           3646 us
>   Accumulate Stats              18 us
>   Make Parsable                  4 us
>   Update Region States          86 us
>   Scan Roots                  3478 us
>     S: <total>               26681 us
>     S: Thread Roots           1521 us, workers (us): 468, 392, 111,
> 122, 113, 106, 111,  98,
>     S: Universe Roots            3 us, workers (us):   3, ---, ---,
> ---, ---, ---, ---, ---,
>     S: JNI Handles Roots         3 us, workers (us):   3, ---, ---,
> ---, ---, ---, ---, ---,
>     S: JNI Weak Roots           25 us, workers (us): ---, ---, -
> --,  25, ---, ---, ---, ---,
>     S: String Table Roots    24644 us, workers (us): 2857, 2935,
> 3141, 3122, 3148, 3148, 3148, 3145,
>     S: Synchronizer Roots        3 us, workers
> (us):   2,   0,   0,   0,   0,   0,   0,   0,
>     S: Flat Profiler Roots       2 us, workers (us): ---, ---,   2,
> ---, ---, ---, ---, ---,
>     S: Management Roots          2 us, workers (us):   2, ---, ---,
> ---, ---, ---, ---, ---,
>     S: System Dict Roots        10 us, workers (us): ---, ---,  10,
> ---, ---, ---, ---, ---,
>     S: CLDG Roots              458 us, workers
> (us):  12,  13,  75,  72,  74,  75,  65,  72,
>     S: JVMTI Roots              10 us, workers (us): ---,  10, ---,
> ---, ---, ---, ---, ---,
>   Resize TLABs                  11 us
> 
> === Patched:
> Pause Init Mark (N)            999 us
>   Accumulate Stats              17 us
>   Make Parsable                  4 us
>   Update Region States          92 us
>   Scan Roots                   824 us
>     S: <total>                5494 us
>     S: Thread Roots           1584 us, workers (us): 121, 613, 114,
> 131, 301, 110, 102,  93,
>     S: Universe Roots            2 us, workers (us):   2, ---, ---,
> ---, ---, ---, ---, ---,
>     S: JNI Handles Roots         4 us, workers (us): ---,   4, ---,
> ---, ---, ---, ---, ---,
>     S: JNI Weak Roots           20 us, workers (us): ---, ---, ---,
> ---, ---,  20, ---, ---,
>     S: String Table Roots     3374 us, workers (us): 506,  81, 479,
> 487, 372, 482, 473, 494,
>     S: Synchronizer Roots        4 us, workers
> (us):   0,   0,   3,   0,   0,   0,   0,   0,
>     S: Flat Profiler Roots       5 us, workers (us): ---,   5, ---,
> ---, ---, ---, ---, ---,
>     S: Management Roots          1 us, workers (us):   1, ---, ---,
> ---, ---, ---, ---, ---,
>     S: System Dict Roots        10 us, workers (us): ---, ---,  10,
> ---, ---, ---, ---, ---,
>     S: CLDG Roots              478 us, workers
> (us):  79,  12,  77,  69,  13,  75,  75,  78,
>     S: JVMTI Roots              10 us, workers (us):  10, ---, ---,
> ---, ---, ---, ---, ---,
>   Resize TLABs                  10 us
> 
> 
> Testing: hotspot_gc_shenandoah, Cassandra benches, eyeballing gc logs
>