Re: RFR (M) 8201655: Add thread-enabled support for the Heap Sampling

Mon, 05 Nov 2018 18:55:02 -0800

Hi Jc,

Okay, I see your point: the change in memAllocator.cpp is for performance.
Do you have any measurements showing a performance difference?
Also, do you need me to submit a mach5 test run?

Thanks,
Serguei


On 11/5/18 15:14, JC Beyler wrote:
Hi Serguei,

First off, thanks as always for looking at this :-) Let me inline my answers:

I actually "struggled" with this part of the change. My change is correct semantically and if you care about performance for when sampling a given thread. 
Your change will work semantically but the performance is the same as the global sampling. 

What I mean by working semantically is that that the tests and the code will work. However, this means that all threads will be doing the sampling work but when the code will post the event here:

(which is why your suggestion works, the change in jvmtiExport basically ensures only the threads requested are posting events)

The code will check that we actually only post for threads we care about. The code above ensures that only threads that were requested to be sampling are the ones that are sampling internally.

Note: I REALLY prefer your suggestion for two reasons:
  - We do not change the runtime/GC code at all, it remains "simple"
  - The overhead in the general case goes away and this is a NOP for my actual use-case from a performance point of view (sampling every thread)

But:
  - Then sampling per thread really is just telling the system don't pollute the callbacks, though internally you are doing all the work anyway.

Let me know which you prefer :)

 
 Also, do you see that enabling the sampling events globally still works?
Yes, otherwise HeapMonitorThreadTest.java would fail since it checks that.
 
http://cr.openjdk.java.net/%7Ejcbeyler/8201655/webrev.02/test/hotspot/jtreg/serviceability/jvmti/HeapMonitor/libHeapMonitorTest.c.frames.html

 A couple of places where err is declared as int instead of jvmtiError:
  
 714   int err;
 742   int err;

 Should not be silent in a case of JVMTI error:

 744   err = (*jvmti)->GetThreadInfo(jvmti, thread, &info);
 745   if (err != JVMTI_ERROR_NONE) {
 746     return;


Done and done, I added a fprintf on stderr saying the GetThreadInfo failed and the test is ignoring the add count.

Thanks again for looking and let me know what you think,
Jc

On Mon, Nov 5, 2018 at 2:25 PM [email protected] <[email protected]> wrote:
Hi Jc,

It looks good in general but I have some comments below.


http://cr.openjdk.java.net/%7Ejcbeyler/8201655/webrev.02/src/hotspot/share/gc/shared/memAllocator.cpp.udiff.html

+static bool thread_enabled_for_one_jvmti_env() {
+  JavaThread *thread  = JavaThread::current();
+  JvmtiThreadState *state = thread->jvmti_thread_state();
+  if (state == NULL) {
+    return false;
+  }
+
+  JvmtiEnvThreadStateIterator it(state);
+  for (JvmtiEnvThreadState* ets = it.first(); ets != NULL; ets = it.next(ets)) {
+    if (ets->is_enabled(JVMTI_EVENT_SAMPLED_OBJECT_ALLOC)) {
+      return true;
+    }
+  }
+
+  return false;
+}
+
 void MemAllocator::Allocation::notify_allocation_jvmti_sampler() {
   // support for JVMTI VMObjectAlloc event (no-op if not enabled)
   JvmtiExport::vm_object_alloc_event_collector(obj());
 
   if (!JvmtiExport::should_post_sampled_object_alloc()) {
     // Sampling disabled
     return;
   }
 
+  // Sampling is enabled for at least one thread, is it this one?
+  if (!thread_enabled_for_one_jvmti_env()) {
+    return;
+  }
+

 I don't think you need this change as this condition already does it:
   if (!JvmtiExport::should_post_sampled_object_alloc()) {
     // Sampling disabled
     return;
   }

 Please, look at the following line in the jvmtiEventController.cpp:
    JvmtiExport::set_should_post_sampled_object_alloc((any_env_thread_enabled & SAMPLED_OBJECT_ALLOC_BIT) != 0);

 I hope, testing will prove my suggestion is correct.
 Also, do you see that enabling the sampling events globally still works?


http://cr.openjdk.java.net/%7Ejcbeyler/8201655/webrev.02/test/hotspot/jtreg/serviceability/jvmti/HeapMonitor/libHeapMonitorTest.c.frames.html

 A couple of places where err is declared as int instead of jvmtiError:
  
 714   int err;
 742   int err;

 Should not be silent in a case of JVMTI error:

 744   err = (*jvmti)->GetThreadInfo(jvmti, thread, &info);
 745   if (err != JVMTI_ERROR_NONE) {
 746     return;

Thanks,
Serguei


On 10/26/18 10:48, JC Beyler wrote:
Hi all,

When working on the heap sampling, I had promised to do the per thread event so here it is! 

Could I get a review for this:

I was thinking of adding GC-dev for the memAllocator change once I get favorable reviews for the rest of the change.

I've done a bit of performance testing and on the Dacapo benchmark I see no change in performance when turned off (logical, any code change is behind a flag check already in place) and when turned on it is comparable to the current performance.

(More information is: I see a very slight degradation if we are doing 512k sampling but no degradation at 2MB). 

Thanks,
Jc



--

Thanks,
Jc

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