RFR: 8336640: Shenandoah: Parallel worker use in parallel_heap_region_iterate [v2]

[Xiaolong Peng]

> [parallel_heap_region_iterate](https://github.com/openjdk/jdk/blob/master/src/hotspot/share/gc/shenandoah/shenandoahHeap.cpp#L1726-L1734) is used to execute lightweight operations on heap regions, including ShenandoahPrepareForMarkClosure, ShenandoahInitMarkUpdateRegionStateClosure, ShenandoahFinalUpdateRefsUpdateRegionStateClosure, ShenandoahResetUpdateRegionStateClosure and ShenandoahFinalMarkUpdateRegionStateClosure. Since all the operations are very lightweight, in regular cases w/o large number of heap regions, the parallelism seems to be an overkill because the cost of multi-thread orchestrating could be more expensive; In most cases, single thread should be more efficient. Also, if multiple threading is needed, we should maximize the utilization of all active workers for best performance.
> 
> This PR includes proposed improvments addressing the known issues:
> 1. Change the default value of ShenandoahParallelRegionStride to 0, when it is 0, Shenandoah will auto derive the value of stride for best performance; 
> 2. if num_regions is <= 4096, not use worker threads at all to avoid the overhead of multi-threading;
> 3. When num_regions is more than 4096, use worker threads to parallelize the workload, derive the value of stride to evenly distribute the workload to all active workers.
> 4. When number of active workers is 1, don't bother the workers, it is faster to finish the workload in current thread(avoid overhead of multi-threads orchestration)
> 
> There are some time metrics I collected from test with TIP version(I added time metrics for parallel_heap_region_iterate):
> 
> JVM args: export JAVA_OPTS="-Xms8G -Xmx8G  -XX:+AlwaysPreTouch -XX:+UseShenandoahGC -XX:+UnlockExperimentalVMOptions -XX:ShenandoahParallelRegionStride=<stride> -XX:ShenandoahTargetNumRegions=<num_regions>  -Xlog:gc*"
> 
> |             | 1024 regions | 2048 regions | 4096 regions | 8192 regions |16384 regions |
> | ----------- | ------------ | ------------ | ------------ | ------------ |------------ |
> | 1024 stride | 5785 ns         | 22194 ns        | 20953 ns        | 23008 ns        |33013 ns       |
> | 2048 stride | N/A          | 6491 ns         | 22476  ns        | 25842 ns        |34378 ns        |
> | 4096 stride | N/A          | N/A          | 14034 ns        | 28425 ns        |36324 ns        |
> | 8192 stride | N/A          | N/A          | N/A          | 24359 ns        |45231 ns        |
> | 16384 stride | N/A          | N/A          | N/A          | N/A          |53679 ns        |
> 
> Basically w...

Xiaolong Peng has updated the pull request incrementally with one additional commit since the last revision:

  Add trailing space

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Changes:
  - all: https://git.openjdk.org/jdk/pull/20305/files
  - new: https://git.openjdk.org/jdk/pull/20305/files/01367942..dcc1d29f

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=20305&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=20305&range=00-01

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/20305.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20305/head:pull/20305

PR: https://git.openjdk.org/jdk/pull/20305