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But that’s just how HotSpot works, and virtual threads didn’t change that. There are other memory-related optimisations in the JIT, too. For example, every `new Foo`, when running in the interpreter, will allocate memory in the heap. But when compiled, the
object could be allocated on the stack (scalar-replaced) or even not at all (which means there can be cases where JITted code consumes zero heap memory, while the same code, when interpreted, consumes an unbounded amount, i.e. will OOME at any heap size).
So the memory profile of Java applications running on OpenJDK can be very different depending on JIT behaviour. This might become even more pronounced with Valhalla.
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<div class="">In this case, the behaviour has nothing to do with virtual threads or with “capture.” The optimising compiler simply analyses the method and sees that the bigBuffer variable is unused after the call to slowIO so it optimises it away (and with
the local gone, the GC will not find the buffer and it will be collected).</div>
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<div class="">If the interpreter were to perform this or other similar optimisations based on analysing the method before interpreting it, then it wouldn’t be doing its job as the mechanism that quickly starts execution or the mechanism used when debugging.
Even when some optimisations are possible in the interpreter, it’s rarely worth it to invest much time in that because the interpreter runs little code over the program’s overall execution.</div>
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<div class="">It does, however, mean that microbenchmarks need to be done carefully.</div>
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<div class="">— Ron<br class="">
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<blockquote type="cite" class="">
<div class="">On 11 Nov 2022, at 14:01, Arnaud Masson <<a href="mailto:arnaud.masson@fr.ibm.com" class="">arnaud.masson@fr.ibm.com</a>> wrote:</div>
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<span class="">Ron,<o:p class=""></o:p></span></div>
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<span class=""><o:p class=""> </o:p></span></div>
<div style="margin: 0cm; font-size: 11pt; font-family: Calibri, sans-serif;" class="">
<span lang="EN-US" class="">Indeed, it’s different with normal warmup as you suspected.<o:p class=""></o:p></span></div>
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<span lang="EN-US" class="">The threshold is between 500 and 600 loops (not fully reproducible).<o:p class=""></o:p></span></div>
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<span lang="EN-US" class="">After around 600 iterations, the bigBuffer is not retained, so it’s good.<o:p class=""></o:p></span></div>
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<span lang="EN-US" class="">Test code below.<o:p class=""></o:p></span></div>
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<span lang="EN-US" class=""><o:p class=""> </o:p></span></div>
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<span lang="EN-US" class="">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.)<o:p class=""></o:p></span></div>
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<span lang="EN-US" class=""><o:p class=""> </o:p></span></div>
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<span lang="EN-US" class="">thanks<o:p class=""></o:p></span></div>
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<span lang="EN-US" class="">Arnaud<o:p class=""></o:p></span></div>
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<span lang="EN-US" class=""><o:p class=""> </o:p></span></div>
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<span lang="EN-US" class="">------<o:p class=""></o:p></span></div>
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<span lang="EN-US" class=""><o:p class=""> </o:p></span></div>
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<span lang="EN-US" class="">import java.lang.management.ManagementFactory;<o:p class=""></o:p></span></div>
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<span lang="EN-US" class="">import java.util.Timer;<o:p class=""></o:p></span></div>
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<span lang="EN-US" class="">import java.util.TimerTask;<o:p class=""></o:p></span></div>
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<span lang="EN-US" class="">import java.util.concurrent.ExecutionException;<o:p class=""></o:p></span></div>
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<span lang="EN-US" class="">import java.util.concurrent.ExecutorService;<o:p class=""></o:p></span></div>
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<span lang="EN-US" class="">import java.util.concurrent.Executors;<o:p class=""></o:p></span></div>
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<span lang="EN-US" class=""><o:p class=""> </o:p></span></div>
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<span lang="EN-US" class="">public class Main1b {<o:p class=""></o:p></span></div>
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<span lang="EN-US" class=""> static int iter = 0;<o:p class=""></o:p></span></div>
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<span lang="EN-US" class=""> static final ExecutorService executor = Executors.newVirtualThreadPerTaskExecutor();<o:p class=""></o:p></span></div>
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<span lang="EN-US" class=""> <o:p class=""></o:p></span></div>
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<span lang="EN-US" class=""> public static void main(String[] args) throws ExecutionException, InterruptedException {<o:p class=""></o:p></span></div>
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<span lang="EN-US" class=""> var timer = new Timer();<o:p class=""></o:p></span></div>
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<span lang="EN-US" class=""> var memBean = ManagementFactory.getMemoryMXBean();<o:p class=""></o:p></span></div>
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<span lang="EN-US" class=""> timer.schedule(new TimerTask() {<o:p class=""></o:p></span></div>
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<span lang="EN-US" class=""> @Override<o:p class=""></o:p></span></div>
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<span lang="EN-US" class=""> public void run() {<o:p class=""></o:p></span></div>
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<span lang="EN-US" class=""> System.gc();<o:p class=""></o:p></span></div>
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<span lang="EN-US" class=""> System.out.println("--- " + memBean.getHeapMemoryUsage());<o:p class=""></o:p></span></div>
<div style="margin: 0cm; font-size: 11pt; font-family: Calibri, sans-serif;" class="">
<span lang="EN-US" class=""> }<o:p class=""></o:p></span></div>
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<span lang="EN-US" class=""> }, 5_000, 5_000);<o:p class=""></o:p></span></div>
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<span lang="EN-US" class=""> <o:p class=""></o:p></span></div>
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<span lang="EN-US" class=""> for (int i=0; i<600; i++) // ******* warmup ******<o:p class=""></o:p></span></div>
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<span lang="EN-US" class=""> myApp(0, 0);<o:p class=""></o:p></span></div>
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<span lang="EN-US" class=""> <o:p class=""></o:p></span></div>
<div style="margin: 0cm; font-size: 11pt; font-family: Calibri, sans-serif;" class="">
<span lang="EN-US" class=""> myApp(Long.MAX_VALUE, 1024 * 1024 * 1024);<o:p class=""></o:p></span></div>
<div style="margin: 0cm; font-size: 11pt; font-family: Calibri, sans-serif;" class="">
<span lang="EN-US" class=""> }<o:p class=""></o:p></span></div>
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<span lang="EN-US" class=""><o:p class=""> </o:p></span></div>
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<span lang="EN-US" class=""> private static void myApp(long durationMs, int buffSize) throws ExecutionException, InterruptedException {<o:p class=""></o:p></span></div>
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<span lang="EN-US" class=""><o:p class=""> </o:p></span></div>
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<span lang="EN-US" class=""> var future = executor.submit(() -> {<o:p class=""></o:p></span></div>
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<span lang="EN-US" class=""> try {<o:p class=""></o:p></span></div>
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<span lang="EN-US" class=""> iter ++;<o:p class=""></o:p></span></div>
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<span lang="EN-US" class=""> System.out.println("Starting work, iter #" + iter);<o:p class=""></o:p></span></div>
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<span lang="EN-US" class=""> var bigBuffer = new byte[buffSize];<o:p class=""></o:p></span></div>
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<span lang="EN-US" class=""> System.out.println("bigBuffer size: " + bigBuffer.length);<o:p class=""></o:p></span></div>
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<span lang="EN-US" class=""> // bigBuffer = null;<o:p class=""></o:p></span></div>
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<span lang="EN-US" class=""> slowIO(durationMs);<o:p class=""></o:p></span></div>
<div style="margin: 0cm; font-size: 11pt; font-family: Calibri, sans-serif;" class="">
<span lang="EN-US" class=""> } catch (InterruptedException e) {<o:p class=""></o:p></span></div>
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<span lang="EN-US" class=""> throw new RuntimeException(e);<o:p class=""></o:p></span></div>
<div style="margin: 0cm; font-size: 11pt; font-family: Calibri, sans-serif;" class="">
<span lang="EN-US" class=""> }<o:p class=""></o:p></span></div>
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<span lang="EN-US" class=""> });<o:p class=""></o:p></span></div>
<div style="margin: 0cm; font-size: 11pt; font-family: Calibri, sans-serif;" class="">
<span lang="EN-US" class=""> future.get();<o:p class=""></o:p></span></div>
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<span lang="EN-US" class=""> }<o:p class=""></o:p></span></div>
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<span lang="EN-US" class=""><o:p class=""> </o:p></span></div>
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<span lang="EN-US" class=""> private static void slowIO(long durationMs) throws InterruptedException {<o:p class=""></o:p></span></div>
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<span lang="EN-US" class=""> System.out.println("Starting slowIO");<o:p class=""></o:p></span></div>
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<span lang="EN-US" class=""> Thread.sleep(durationMs);<o:p class=""></o:p></span></div>
<div style="margin: 0cm; font-size: 11pt; font-family: Calibri, sans-serif;" class="">
<span lang="EN-US" class=""> }<o:p class=""></o:p></span></div>
<div style="margin: 0cm; font-size: 11pt; font-family: Calibri, sans-serif;" class="">
<span lang="EN-US" class="">}<o:p class=""></o:p></span></div>
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<span lang="EN-US" class=""><o:p class=""> </o:p></span></div>
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<span lang="EN-US" class=""><o:p class=""> </o:p></span></div>
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<span lang="EN-US" class=""><o:p class=""> </o:p></span></div>
<div style="margin: 0cm; font-size: 11pt; font-family: Calibri, sans-serif;" class="">
<span lang="EN-US" class="">>Can you test with C2 engaged normally through warmup?<o:p class=""></o:p></span></div>
<div style="margin: 0cm; font-size: 11pt; font-family: Calibri, sans-serif;" class="">
<span lang="EN-US" class="">><o:p class=""> </o:p></span></div>
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<span lang="EN-US" class="">><o:p class=""> </o:p></span></div>
<div style="margin: 0cm; font-size: 11pt; font-family: Calibri, sans-serif;" class="">
<span lang="EN-US" class="">>On 8 Nov 2022, at 13:56, Arnaud Masson <<a href="mailto:arnaud.masson@fr.ibm.com" class="">arnaud.masson@fr.ibm.com</a>> wrote:<o:p class=""></o:p></span></div>
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<span lang="EN-US" class="">><o:p class=""> </o:p></span></div>
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<span lang="EN-US" class="">><span class="Apple-converted-space"> </span><o:p class=""></o:p></span></div>
<div style="margin: 0cm; font-size: 11pt; font-family: Calibri, sans-serif;" class="">
<span lang="EN-US" class="">>> I will check if C2 helps, but afaik since it’s triggered when method is<o:p class=""></o:p></span></div>
<div style="margin: 0cm; font-size: 11pt; font-family: Calibri, sans-serif;" class="">
<span lang="EN-US" class="">>> used often (not because the method consumes significant memory), it’s<o:p class=""></o:p></span></div>
<div style="margin: 0cm; font-size: 11pt; font-family: Calibri, sans-serif;" class="">
<span lang="EN-US" class="">>> not ideal to count on it anyway. Moreover, I suspect it’s not documented<o:p class=""></o:p></span></div>
<div style="margin: 0cm; font-size: 11pt; font-family: Calibri, sans-serif;" class="">
<span lang="EN-US" class="">>> public behavior while lambda capture is well defined.<o:p class=""></o:p></span></div>
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<span lang="EN-US" class="">><span class="Apple-converted-space"> </span><o:p class=""></o:p></span></div>
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<span lang="EN-US" class="">><span class="Apple-converted-space"> </span><o:p class=""></o:p></span></div>
<div style="margin: 0cm; font-size: 11pt; font-family: Calibri, sans-serif;" class="">
<span lang="EN-US" class="">>I have checked the following example with (no debugger)<o:p class=""></o:p></span></div>
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<span lang="EN-US" class="">><span class="Apple-converted-space"> </span><o:p class=""></o:p></span></div>
<div style="margin: 0cm; font-size: 11pt; font-family: Calibri, sans-serif;" class="">
<span lang="EN-US" class="">>--enable-preview<o:p class=""></o:p></span></div>
<div style="margin: 0cm; font-size: 11pt; font-family: Calibri, sans-serif;" class="">
<span lang="EN-US" class="">>-XX:-TieredCompilation<o:p class=""></o:p></span></div>
<div style="margin: 0cm; font-size: 11pt; font-family: Calibri, sans-serif;" class="">
<span lang="EN-US" class="">>-Xbatch<o:p class=""></o:p></span></div>
<div style="margin: 0cm; font-size: 11pt; font-family: Calibri, sans-serif;" class="">
<span lang="EN-US" class="">>-Xcomp<o:p class=""></o:p></span></div>
<div style="margin: 0cm; font-size: 11pt; font-family: Calibri, sans-serif;" class="">
<span lang="EN-US" class="">>-Xlog:nmethod+install<o:p class=""></o:p></span></div>
<div style="margin: 0cm; font-size: 11pt; font-family: Calibri, sans-serif;" class="">
<span lang="EN-US" class="">><span class="Apple-converted-space"> </span><o:p class=""></o:p></span></div>
<div style="margin: 0cm; font-size: 11pt; font-family: Calibri, sans-serif;" class="">
<span lang="EN-US" class="">>… to force C2, but the bigBuffer is still retained.<o:p class=""></o:p></span></div>
<div style="margin: 0cm; font-size: 11pt; font-family: Calibri, sans-serif;" class="">
<span lang="EN-US" class="">><span class="Apple-converted-space"> </span><o:p class=""></o:p></span></div>
<div style="margin: 0cm; font-size: 11pt; font-family: Calibri, sans-serif;" class="">
<span lang="EN-US" class="">>(Won’t C2 clear a stack ref only if it can be reused in the same scope for another java var?)<o:p class=""></o:p></span></div>
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><span class="Apple-converted-space"> </span><o:p class=""></o:p></div>
<div style="margin: 0cm; font-size: 11pt; font-family: Calibri, sans-serif;" class="">
>Thanks<o:p class=""></o:p></div>
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>Arnaud<o:p class=""></o:p></div>
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