RFR: 8338146: Improve Exchanger performance with VirtualThreads

[Doug Lea]

On Mon, 12 Aug 2024 17:07:42 GMT, Doug Lea <dl at openjdk.org> wrote:

> The Exchanger class uses spin-waits that are hostile to some uses of VirtualThreads. Improving this requires a means of estimating whether there are many VirtualThreads with few carriers, which can be supported by adding a method in class ForkJoinWorkerThread. This enables a reworking of the exchange method, and can also be used to deal with similar issues in LinkedTransferQueue and possibly elsewhere. We leave for now open whether this method (hasKnownQueuedWork) should be public, which would allow users to use it in similar contexts, at the possible expense of revealing too much about current VT implementation

I had forgotten that there was already a stripped-down jtreg version of an Exchanger test in test/jdk/java/util/concurrent/Exchanger. So I updated it to include virtual, platform and mixes. (Also including enough iterations and stages to reduce noisiness. Here's a sample run:
Warmup...
Threads: 2 PLATFORM	:       195 ns per transfer
Threads: 3 PLATFORM	:       226 ns per transfer
Threads: 4 PLATFORM	:       172 ns per transfer
Threads: 5 PLATFORM	:       148 ns per transfer
Threads: 6 PLATFORM	:       123 ns per transfer
Threads: 7 PLATFORM	:       100 ns per transfer
Threads: 8 PLATFORM	:        96 ns per transfer
Threads: 9 PLATFORM	:        78 ns per transfer
Threads: 2 VIRTUAL	:       185 ns per transfer
Threads: 3 VIRTUAL	:       223 ns per transfer
Threads: 4 VIRTUAL	:       194 ns per transfer
Threads: 5 VIRTUAL	:       139 ns per transfer
Threads: 6 VIRTUAL	:       121 ns per transfer
Threads: 7 VIRTUAL	:       118 ns per transfer
Threads: 8 VIRTUAL	:       190 ns per transfer
Threads: 9 VIRTUAL	:        79 ns per transfer
Threads: 2 MIXED	:       161 ns per transfer
Threads: 3 MIXED	:       196 ns per transfer
Threads: 4 MIXED	:       164 ns per transfer
Threads: 5 MIXED	:       146 ns per transfer
Threads: 6 MIXED	:       123 ns per transfer
Threads: 7 MIXED	:       106 ns per transfer
Threads: 8 MIXED	:       102 ns per transfer
Threads: 9 MIXED	:        87 ns per transfer

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PR Comment: https://git.openjdk.org/jdk/pull/20554#issuecomment-2302143653