Performance of pooling virtual threads vs. semaphores

[Attila Kelemen]

I vaguely remember Alan or Ron saying that timed waits for VTs are not yet
as efficient as they want them to be (though I'm not sure if that is the
case for JDK 23 still). So, I would expect that the major performance
difference you are seeing might be because of the sleep calls (and in
realistic scenarios, I'm guessing the same would be true for other timed
waits).

Liam Miller-Cushon <cushon at google.com> ezt írta (időpont: 2024. máj. 29.,
Sze, 0:44):

> Hello,
>
> JEP 444 [1] and the docs in [2] mention that virtual threads should not be
> pooled, and suggest semaphores as one possible alternative.
>
> My colleague Chi Wang has been investigating virtual thread performance,
> and has found that creating one virtual thread per task and synchronizing
> on a semaphore can result in worse performance on machines with large
> numbers of cores.
>
> A benchmark run on a 128 core machine is included below. It submits
> numTasks tasks to the executor determined by the strategy. The task itself
> is mixed with CPU and I/O work (simulated by fibonacci and sleep). The
> parallelism is set to 600 for all strategies.
>
> * Strategy 1 is the baseline where it just submits all the tasks to a
> ForkJoinPool whose pool size is 600.
> * Strategy 2 uses the method suggested by JEP 444.
> * Strategy 3 uses a fixed thread pool of 600 backed by virtual threads.
>
> Note that, with 100K and 1M tasks, strategy 2 has a CPU time regression
> that seems to increase with the number of cores. This result can be
> reproduced on the real-world program that is being migrated to a
> virtual-thread-per-task model.
>
> Diffing the cpu profile between strategy 1 and strategy 2 showed that most
> of the CPU regression comes from method
> `java.util.concurrent.ForkJoinPool.scan(java.util.concurrent.ForkJoinPool$WorkQueue,
> long, int)`.
>
> Are there any ideas for why the semaphore strategy uses more CPU than
> pooling virtual threads on machines with a large number of cores?
>
> Thanks,
> Liam
>
> [1] https://openjdk.org/jeps/444#Do-not-pool-virtual-threads
> [2]
> https://docs.oracle.com/en/java/javase/22/core/virtual-threads.html#GUID-2BCFC2DD-7D84-4B0C-9222-97F9C7C6C521
>
> import java.util.concurrent.ExecutorService;
> import java.util.concurrent.Executors;
> import java.util.concurrent.ForkJoinPool;
> import java.util.concurrent.Semaphore;
>
> public class Main {
>
>   private static Semaphore semaphore = null;
>
>   public static void main(String[] args) {
>     int strategy = 0;
>     int parallelism = 600;
>     int numTasks = 10000;
>
>     if (args.length > 1) {
>       strategy = Integer.parseInt(args[1]);
>     }
>
>     if (args.length > 2) {
>       numTasks = Integer.parseInt(args[2]);
>     }
>
>     ExecutorService executor;
>     switch (strategy) {
>       case 1 -> {
>         executor = new ForkJoinPool(parallelism);
>       }
>       case 2 -> {
>         executor = Executors.newVirtualThreadPerTaskExecutor();
>         semaphore = new Semaphore(parallelism);
>       }
>       case 3 -> {
>         executor = Executors.newFixedThreadPool(parallelism,
> Thread.ofVirtual().factory());
>       }
>       default -> {
>         throw new IllegalArgumentException();
>       }
>     }
>
>     try (executor) {
>       for (var i = 0; i < numTasks; ++i) {
>         executor.execute(Main::task);
>       }
>     }
>   }
>
>   private static void task() {
>     if (semaphore != null) {
>       try {
>         semaphore.acquire();
>       } catch (InterruptedException e) {
>         throw new IllegalStateException();
>       }
>     }
>
>     try {
>       fibonacci(20);
>       try {
>         Thread.sleep(10);
>       } catch (InterruptedException e) {
>       }
>       fibonacci(20);
>       try {
>         Thread.sleep(10);
>       } catch (InterruptedException e) {
>       }
>       fibonacci(20);
>     } finally {
>       if (semaphore != null) {
>         semaphore.release();
>       }
>     }
>   }
>
>   private static int fibonacci(int n) {
>     if (n == 0) {
>       return 0;
>     } else if (n == 1) {
>       return 1;
>     } else {
>       return fibonacci(n - 1) + fibonacci(n - 2);
>     }
>   }
> }
>
> # openjdk full version "23-ea+24-1995"
> # AMD Ryzen Threadripper PRO 3995WX, hyperthreading enabled
>
> $ hyperfine --parameter-scan strategy 1 3 --parameter-list numTasks
> 10000,100000,1000000 './jdk-23/bin/java Main -- {strategy} {numTasks}'
>
> Benchmark 1: ./jdk-23/bin/java Main -- 1 10000
>   Time (mean ± σ):     658.3 ms ±  24.4 ms    [User: 26808.8 ms, System:
> 493.7 ms]
>   Range (min … max):   613.1 ms … 702.0 ms    10 runs
>
> Benchmark 2: ./jdk-23/bin/java Main -- 2 10000
>   Time (mean ± σ):     486.9 ms ±  28.5 ms    [User: 14804.4 ms, System:
> 501.5 ms]
>   Range (min … max):   451.0 ms … 533.4 ms    10 runs
>
> Benchmark 3: ./jdk-23/bin/java Main -- 3 10000
>   Time (mean ± σ):     452.0 ms ±  10.8 ms    [User: 6598.1 ms, System:
> 335.8 ms]
>   Range (min … max):   435.6 ms … 470.6 ms    10 runs
>
> Benchmark 4: ./jdk-23/bin/java Main -- 1 100000
>   Time (mean ± σ):      3.668 s ±  0.028 s    [User: 38.469 s, System:
> 1.188 s]
>   Range (min … max):    3.628 s …  3.704 s    10 runs
>
> Benchmark 5: ./jdk-23/bin/java Main -- 2 100000
>   Time (mean ± σ):      3.612 s ±  0.042 s    [User: 65.924 s, System:
> 2.072 s]
>   Range (min … max):    3.563 s …  3.687 s    10 runs
>
> Benchmark 6: ./jdk-23/bin/java Main -- 3 100000
>   Time (mean ± σ):      3.503 s ±  0.008 s    [User: 27.791 s, System:
> 1.211 s]
>   Range (min … max):    3.492 s …  3.515 s    10 runs
>
> Benchmark 7: ./jdk-23/bin/java Main -- 1 1000000
>   Time (mean ± σ):     34.093 s ±  0.031 s    [User: 206.235 s, System:
> 14.313 s]
>   Range (min … max):   34.015 s … 34.120 s    10 runs
>
> Benchmark 8: ./jdk-23/bin/java Main -- 2 1000000
>   Time (mean ± σ):     34.354 s ±  0.063 s    [User: 330.215 s, System:
> 17.501 s]
>   Range (min … max):   34.267 s … 34.479 s    10 runs
>
> Benchmark 9: ./jdk-23/bin/java Main -- 3 1000000
>   Time (mean ± σ):     34.551 s ±  1.018 s    [User: 238.050 s, System:
> 10.258 s]
>   Range (min … max):   34.124 s … 37.420 s    10 runs
>
> Summary
>   ./jdk-23/bin/java Main -- 3 10000 ran
>     1.08 ± 0.07 times faster than ./jdk-23/bin/java Main -- 2 10000
>     1.46 ± 0.06 times faster than ./jdk-23/bin/java Main -- 1 10000
>     7.75 ± 0.19 times faster than ./jdk-23/bin/java Main -- 3 100000
>     7.99 ± 0.21 times faster than ./jdk-23/bin/java Main -- 2 100000
>     8.12 ± 0.20 times faster than ./jdk-23/bin/java Main -- 1 100000
>    75.43 ± 1.80 times faster than ./jdk-23/bin/java Main -- 1 1000000
>    76.01 ± 1.82 times faster than ./jdk-23/bin/java Main -- 2 1000000
>    76.44 ± 2.90 times faster than ./jdk-23/bin/java Main -- 3 1000000
>
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