RFR (M) 8201655: Add thread-enabled support for the Heap Sampling
JC Beyler
jcbeyler at google.com
Wed Nov 7 04:03:12 UTC 2018
Hi Serguei,
You are right, I should have reverted the memAllocator.cpp file, sorry
about that.
Here is the new webrev:
http://cr.openjdk.java.net/~jcbeyler/8201655/webrev.04/
I think we are good by testing standards, like I
said HeapMonitorThreadTest.java tests multiple threads. I did test an
example with a thousand threads and I get the samples from 1000 threads so
it seems to work there too.
Per thread is tested via the new HeapMonitorThreadDisabledTest.java so I
think we are good there too.
I would recommend a mach-5 testing just in case for this one if you can, it
will be better to have that reinsurance.
Thanks for your help,
Jc
On Tue, Nov 6, 2018 at 4:29 PM <serguei.spitsyn at oracle.com> wrote:
> Hi Jc,
>
> Not sure, I understand a motivation for this change:
>
> - if (JvmtiExport::should_post_sampled_object_alloc()) {+ {
>
> Also, I'm not sure this is still needed:
>
> +#include "prims/jvmtiEventController.inline.hpp"+#include "prims/jvmtiThreadState.inline.hpp"
>
> I expected you'd just revert all the changes in the memAllocator.cpp.
>
> Also, it is up to you to make a decision if these performance-related fix
> is needed or not.
>
> But it needs to be well tested so that both global+thread event management
> works correctly.
>
> Thanks,
> Serguei
>
>
> On 11/6/18 9:42 AM, JC Beyler wrote:
>
> Hi Serguei,
>
> Yes exactly it was an optimization. When using a 512k sampling rate, I
> don't see a no real difference (the overhead is anyway low for that
> sampling rate), I imagine there would be a difference if trying to sample
> every allocation or with a low sampling interval. But because you are right
> and it is an optimization of the system and not a functional need, I've
> reverted it and now the webrev is updated here:
>
> Webrev: http://cr.openjdk.java.net/~jcbeyler/8201655/webrev.03/
> Bug: https://bugs.openjdk.java.net/browse/JDK-8201655
>
> The incremental webrev is here:
> http://cr.openjdk.java.net/~jcbeyler/8201655/webrev.02_03/
>
> Let me know what you think,
> Jc
>
> On Mon, Nov 5, 2018 at 6:51 PM serguei.spitsyn at oracle.com <
> serguei.spitsyn at oracle.com> wrote:
>
>> 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:
>> ->
>> http://cr.openjdk.java.net/~jcbeyler/8201655/webrev.02/src/hotspot/share/prims/jvmtiExport.cpp.udiff.html
>>
>> (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 serguei.spitsyn at oracle.com <
>> serguei.spitsyn at oracle.com> 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:
>>> Webrev: http://cr.openjdk.java.net/~jcbeyler/8201655/webrev.02/
>>> Bug: https://bugs.openjdk.java.net/browse/JDK-8201655
>>>
>>> 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
>>
>>
>>
>
> --
>
> Thanks,
> Jc
>
>
>
--
Thanks,
Jc
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