Late cleanup of stack objects

[Arnaud Masson]

Ron,

Indeed, it’s different with normal warmup as you suspected.
The threshold is between 500 and 600 loops (not fully reproducible).
After around 600 iterations, the bigBuffer is not retained, so it’s good.
Test code below.

So it mitigates my concern of retained objects in sync code vs retained in async code, but I think it’s still maybe not ideal since method compilation is related to CPU cost and not memory usage. (“capture” behavior on suspended stack call should be more predictable and unrelated to JIT, like for lambda.)

thanks
Arnaud

------

import java.lang.management.ManagementFactory;
import java.util.Timer;
import java.util.TimerTask;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

public class Main1b {
                static int iter = 0;
                static final ExecutorService executor = Executors.newVirtualThreadPerTaskExecutor();

                public static void main(String[] args) throws ExecutionException, InterruptedException {
                               var timer = new Timer();
                               var memBean = ManagementFactory.getMemoryMXBean();
                               timer.schedule(new TimerTask() {
                                                               @Override
                                                               public void run() {
                                                                               System.gc();
                                                                               System.out.println("--- " + memBean.getHeapMemoryUsage());
                                                               }
                                               }, 5_000, 5_000);

                               for (int i=0; i<600; i++) // ******* warmup ******
                                               myApp(0, 0);

                               myApp(Long.MAX_VALUE, 1024 * 1024 * 1024);
                }

                private static void myApp(long durationMs, int buffSize) throws ExecutionException, InterruptedException {

                               var future = executor.submit(() -> {
                                               try {
                                                               iter ++;
                                                               System.out.println("Starting work, iter #" + iter);
                                                               var bigBuffer = new byte[buffSize];
                                                               System.out.println("bigBuffer size: " + bigBuffer.length);
                                                               // bigBuffer = null;
                                                               slowIO(durationMs);
                                               } catch (InterruptedException e) {
                                                               throw new RuntimeException(e);
                                               }
                               });
                               future.get();
                }

                private static void slowIO(long durationMs) throws InterruptedException {
                               System.out.println("Starting slowIO");
                               Thread.sleep(durationMs);
                }
}



>Can you test with C2 engaged normally through warmup?
>
>
>On 8 Nov 2022, at 13:56, Arnaud Masson <arnaud.masson at fr.ibm.com> wrote:
>
>
>> I will check if C2 helps, but afaik since it’s triggered when method is
>> used often (not because the method consumes significant memory), it’s
>> not ideal to count on it anyway. Moreover, I suspect it’s not documented
>> public behavior while lambda capture is well defined.
>
>
>I have checked the following example with (no debugger)
>
>--enable-preview
>-XX:-TieredCompilation
>-Xbatch
>-Xcomp
>-Xlog:nmethod+install
>
>… to force C2, but the bigBuffer is still retained.
>
>(Won’t C2 clear a stack ref only if it can be reused in the same scope for another java var?)
>
>Thanks
>Arnaud

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