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.gaisba...@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 < > jcbey...@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 < >> jcbey...@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.gaisba...@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 >> >
