RFR: 8321242: Enable WorkerThreads to run tasks in caller thread

[Aleksey Shipilev]

On Mon, 4 Dec 2023 21:46:05 GMT, Kim Barrett <kbarrett at openjdk.org> wrote:

>> `WorkerThreads` is a generic abstraction that allows running the task in multiple threads. There are cases, however, when we only ask for a single thread to carry the computation, and we can accept the caller running the computation instead of ping-ponging with the worker thread. As rendezvous with worker thread is a potential latency hiccup, we would like to avoid delegating work to worker threads unnecessarily. This improves latency on critical paths: the usual round-trip takes about 10..50us on systems I tried.
>> 
>> Nominally, we would like to handle the case for 1 worker requested. But I would argue we would also like to do this even if we are requesting N (N > 1) threads to carry the compute. We can submit (N-1) tasks to workers, and execute the other task in the caller. For lower N-s, this removes additional rendezvous point with workers, and allows caller to complete the bulk of the work while workers are waking up. This would also simplify testing: if caller path is always taken, this would verify the task can indeed be executed by caller.
>> 
>> We cannot, however, do this optimization unconditionally, because the caller thread might not be set up in the same way as workers are, and executing the code in caller might cause bugs. Therefore, it would be nice to have the opt-in option that allows running in caller thread. Since this looks to be the property of the task, I added it to task definition.
>> 
>> This PR is only the infrastructure code additions, without product code behavior changes. New option is sanity-tested by new gtest. Other PRs can then opt-in tasks into this, for example #16882.
>> 
>> Additional testing:
>>  - [x] New gtest
>>  - [x] Linux x86_64 server fastdebug, `tier{1,2,3}`
>>  - [x] Linux AArch64 server fastdebug, `tier{1,2,3}`
>
> src/hotspot/share/gc/shared/workerThread.cpp line 63:
> 
>> 61:   if (use_caller) {
>> 62:     // Execute task in caller.
>> 63:     task->work(0);
> 
> WorkerThreads have their priority set to NearMaxPriority, while the calling thread has whatever priority it has. 
> Priorities don't seem to matter much on unix-based platforms, but I _think_ do matter on Windows.  Should we
> temporarily adjust the calling thread's priority here?

Yes, priorities. Adjusted to the same priority the worker threads would run with. `ThreadPriorityAdjuster` can be moves somewhere else. Not sure where, though. Considered to put it in `os.hpp`, but that exposes us to definition circularities with `Thread::current`,

(Tangentially, this opens a question for me if we should make worker priorities configurable as well; the priority of concurrent GC threads might not need to be `NearMaxPriority`)

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PR Review Comment: https://git.openjdk.org/jdk/pull/16945#discussion_r1415293358