JVMTI callback SampledObjectAlloc always fires for first allocation in a new thread

[Jean Christophe Beyler]

Hi Markus,

I created:
https://bugs.openjdk.java.net/browse/JDK-8247615

And I'll see what needs to be done for it :)
Jc

On Fri, Jun 5, 2020 at 3:45 AM Markus Gaisbauer <markus.gaisbauer at gmail.com>
wrote:

> Hi,
>
> JVMTI callback SampledObjectAlloc is currently always called for the first
> allocation of a thread. This generates a lot of bias in an application that
> regularly starts new threads.
>
> I tested this with latest Java 11 and Java 15.
>
> E.g. here is a sample that creates 100 threads and allocates one object in
> each thread.
>
>     public class AllocationProfilingBiasReproducer {
>         public static void main(String[] args) throws Exception {
>             for (int i = 0; i < 100; i++) {
>                 new Thread(new Task(), "Task " + i).start();
>                 Thread.sleep(1);
>             }
>             Thread.sleep(1000);
>         }
>         private static class Task implements Runnable {
>             @Override
>             public void run() {
>                 new A();
>             }
>         }
>         private static class A {
>         }
>     }
>
> I built a simple JVMTI agent that registers SampledObjectAlloc callback
> and sets interval to 1 MB with SetHeapSamplingInterval. The callback simply
> logs thread name and class name of allocated object.
>
> I see the following output:
>
> SampledObjectAlloc Ljava/lang/String; via Task 0
> SampledObjectAlloc LAllocationProfilingBiasReproducer$A; via Task 1
> SampledObjectAlloc LAllocationProfilingBiasReproducer$A; via Task 2
> SampledObjectAlloc LAllocationProfilingBiasReproducer$A; via Task 3
> SampledObjectAlloc LAllocationProfilingBiasReproducer$A; via Task 4
> SampledObjectAlloc LAllocationProfilingBiasReproducer$A; via Task 5
> SampledObjectAlloc LAllocationProfilingBiasReproducer$A; via Task 6
> SampledObjectAlloc LAllocationProfilingBiasReproducer$A; via Task 7
> SampledObjectAlloc LAllocationProfilingBiasReproducer$A; via Task 8
> SampledObjectAlloc LAllocationProfilingBiasReproducer$A; via Task 9
> SampledObjectAlloc LAllocationProfilingBiasReproducer$A; via Task 10
> ...
>
> This is not expected.
>
> I set a breakpoint in my SampledObjectAlloc callback and observed the
> following:
>
> In MemAllocator::Allocation::notify_allocation_jvmti_sampler() the local
> var bytes_since_last is always 0xf1f1f1f1f1f1f1f1 for first allocation of a
> thread. So first allocation is always reported to my agent.
>
> ThreadLocalAllocBuffer::_bytes_since_last_sample_point does not seem to be
> explicitly initialized before accessing it for the first time. I assume
> 0xf1f1f1f1f1f1f1f1  is a default value provided by some Hotspot allocator.
> Only after the first event fired, notify_allocation_jvmti_sampler
> calls ThreadLocalAllocBuffer::set_sample_end which initializes
> _bytes_since_last_sample_point to a proper value.
>
> I am looking for someone who could create a JIRA ticket for this.
>
> Regards,
> Markus
>


-- 

Thanks,
Jc
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