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

[Markus Gaisbauer]

Please forget about the Windows thing. I forgot that my colleague saw the
same 0xf1f1f1f1f1f1f1f1 also on Linux.

Markus

On Wed, Jun 17, 2020 at 9:57 AM Markus Gaisbauer <markus.gaisbauer at gmail.com>
wrote:

> Hi Jean,
>
> Thank you for having a look at this.
>
> I attached the code of my basic JVMTI agent. I ran my tests on Windows.
> Maybe this 0xf1f1f1f1f1f1f1f1 is a Windows thing and Linux initializes the
> memory to all zeros.
>
> Regards,
> Markus
>
>
> On Tue, Jun 16, 2020 at 2:25 AM Jean Christophe Beyler <
> jcbeyler at google.com> wrote:
>
>> Hi Markus,
>>
>> I played around adding your Java code in the testing framework and I
>> don't get exactly the same failure as you do. Basically, I get about 5%
>> samples compared to the number of threads, whereas you seem to get a sample
>> for each element. Could you add the code you used for the agent so I can
>> see if you are doing something different than I am in that regard?
>>
>> This doesn't change the issue, I'm just curious why you seem to be
>> exposing it more. I'm still digging into what would be the right solution
>> for this.
>>
>> Thanks,
>> Jc
>>
>> On Mon, Jun 15, 2020 at 9:53 AM Jean Christophe Beyler <
>> jcbeyler at google.com> wrote:
>>
>>> 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
>>>
>>
>>
>> --
>>
>> Thanks,
>> Jc
>>
>
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