RFR: 8336640: Shenandoah: Parallel worker use in parallel_heap_region_iterate

[Xiaolong Peng]

On Wed, 24 Jul 2024 15:42:50 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

>> [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   ...
>
> src/hotspot/share/gc/shenandoah/shenandoahHeap.cpp line 1697:
> 
>> 1695:   ShenandoahHeap* const _heap;
>> 1696:   ShenandoahHeapRegionClosure* const _blk;
>> 1697:   size_t _stride;
> 
> Should be `size_t const _stride;`?

Yes, it should be const

> src/hotspot/share/gc/shenandoah/shenandoahHeap.cpp line 1737:
> 
>> 1735:     // not use worker threads to avoid the overhead; otherwise cacluate the stride by num_regions/active_workers
>> 1736:     // to make sure every worker thread will have same amount of workload.
>> 1737:     stride = n_regions <= 4096 ? 4096 : checked_cast<size_t>(ceil(checked_cast<float>(n_regions) / checked_cast<float>(active_workers)));
> 
> I suggest writing it like this:
> 
> 
>   size_t stride = ShenandoahParallelRegionStride;
> 
>   if (stride == 0 && active_workers > 1) {
>     // Automatically derive the stride to balance the work between threads
>     // evenly. Do not try to split work if below the reasonable threshold.
>     const size_t threshold = 4096;
>     stride = (n_regions <= threshold) ?
>             threshold :
>             (n_regions + active_workers - 1) / active_workers;
>   }
> 
>   if (n_regions > stride && active_workers > 1) {

Neat! Thanks! I'll update the PR.

-------------

PR Review Comment: https://git.openjdk.org/jdk/pull/20305#discussion_r1690281649
PR Review Comment: https://git.openjdk.org/jdk/pull/20305#discussion_r1690281203