DirectBufferAllocTest fails after replacement of Thread.sleep() with Thread.onSpinWait()

[David Holmes]

On 20/06/2022 7:43 pm, Сергей Цыпанов wrote:
> Hello,
> 
> while playing with Thread.onSpinWait() in busy-wait loops I've replaced Thread.sleep() with
> preferable (at least as it appears from the JavaDoc) Thread.onSpinWait() in java.nio.Bits.
> 
> Immediately after that DirectBufferAllocTest started to fail with OOME. This is surprising, as the change
> does not seem to change any program logic:

Do you get the OOME using Thread.yield() or just a busy-wait? The sleep 
limits the computation and ensures other threads can make progress; 
whereas a busy loop may not allow that and so you exhaust memory.

David

> diff --git a/src/java.base/share/classes/java/nio/Bits.java b/src/java.base/share/classes/java/nio/Bits.java
> --- a/src/java.base/share/classes/java/nio/Bits.java	(revision 7d4df6a83f6333e0e73686b807ee5d4b0ac10cd2)
> +++ b/src/java.base/share/classes/java/nio/Bits.java	(date 1655709575452)
> @@ -162,8 +162,7 @@
>                   }
>                   try {
>                       if (!jlra.waitForReferenceProcessing()) {
> -                        Thread.sleep(sleepTime);
> -                        sleepTime <<= 1;
> +                        Thread.onSpinWait();
>                           sleeps++;
>                       }
>                   } catch (InterruptedException e) {
> 
> Another surprise here is that when I copy the test as simple Java application
> it fails when I run it with the same JVM options (-XX:MaxDirectMemorySize=128m -XX:-ExplicitGCInvokesConcurrent),
> but doesn't if I put a breakpoint in IntelliJ at sleeps++ and stop there for 6-7 times while
> the test is running. No failure observed if I drop JVM options either.
> 
> Is it a bug somewhere in GC/runtime or am I doing something wrong?
> 
> Regards,
> Sergey Tsypanov