G1 STW phases and FGC column in jstat

kirk at kodewerk.com kirk at kodewerk.com
Wed Mar 2 13:29:18 UTC 2016


Hi Yasumasa,

It all sounds reasonable and of course the impact on downstream tooling vs the value of the change must be evaluated. In this case I think adding a CGC counter makes sense especially in prep for G1 becoming default in 9.

Regards,
Kirk

> On Mar 2, 2016, at 6:58 AM, Yasumasa Suenaga <yasuenag at gmail.com> wrote:
> 
> Hi Kirk,
> 
> Agree to you.
> However, I use occasional FGC counter at CMS as below:
> 
> 1. Check major collection occurrence
>      Some production systems have large memory as Java heap,
>      and they are not set GC log.
>      If their CPU usage becomes high, I want to check GC execution.
>      (Of course, we have to check any other points :-) )
> 
> 2. Core image analysis
>      If JVM is crashed, I want to check PerfCounter to know situation.
>      (In the past, I sometimes encountered crash at GC worker thread.)
> 
> I guess that I will want to check them at G1.
> 
> Thus, at least, I want to add PerfCounter for CGC (and add JVMTI event hook).
> However, this proposal will affect to jstat spec.
> So I want to discuss about it before filing to JBS.
> 
> 
> Thanks,
> 
> Yasumasa
> 
> 
> On 2016/03/02 19:02, kirk at kodewerk.com wrote:
>> Hi Yasumasa,
>> 
>> Good question. I’ve never considered CMS to be a Full GC. This implies that there should be separate performance counters for CMS pause phases as it is possible to have FGC. Of course a FGC during CMS maybe user triggered, triggered outside a CMS cycle, interrupts a CMS cycle, or interrupts a CMS phase. I’m not sure how much of a distinction one needs to make here as that could be a quick broader discussion. Certainly the purpose isn’t to recreate the GC logs in these performance counters. But at the very least not having a distinction between full and a STW-CMS phase is kind of misleading in my opinion.
>> 
>> Regards,
>> Kirk
>> 
>>> On Mar 1, 2016, at 5:03 PM, Yasumasa Suenaga <yasuenag at gmail.com> wrote:
>>> 
>>> Hi Kirk,
>>> 
>>>> It is also incorrect to count initial mark and remark in CMS as a FGC.
>>> 
>>> Though, how can we check execution of major collection without GC log?
>>> Should we add new PerfCounter for CGC (and add CGC column to jstat output)?
>>> 
>>> 
>>> Yasumasa
>>> 
>>> 
>>> On 2016/03/02 6:35, kirk at kodewerk.com wrote:
>>>> Hi,
>>>> 
>>>> I think it is incorrect to count remark and cleanup as FGC. They are not full collections. It is also incorrect to count initial mark and remark in CMS as a FGC. It is unfortunate that this is counted this way.
>>>> 
>>>> Regards,
>>>> Kirk
>>>> 
>>>>> On Mar 1, 2016, at 8:56 AM, Yasumasa Suenaga <yasuenag at gmail.com> wrote:
>>>>> 
>>>>> Hi all,
>>>>> 
>>>>> I wonder that STW phases (Remark and Cleanup) at G1 are not counted in jstat FGC column.
>>>>> For example, Initial Mark and Remark at CMS are counted as FGC.
>>>>> 
>>>>> For consistency, I think that G1 STW phases should be counted as FGC.
>>>>> What do you think about it?
>>>>> 
>>>>> If it is accepted, I will file it to JBS and will upload webrev.
>>>>> 
>>>>> 
>>>>> suggested fix:
>>>>> ----------------------
>>>>> diff -r 8a103ba9a7b2 src/share/vm/gc/g1/g1MonitoringSupport.cpp
>>>>> --- a/src/share/vm/gc/g1/g1MonitoringSupport.cpp	Mon Feb 29 22:54:24 2016 +0900
>>>>> +++ b/src/share/vm/gc/g1/g1MonitoringSupport.cpp	Tue Mar 01 23:43:30 2016 +0900
>>>>> @@ -103,7 +103,7 @@
>>>>>   //   name "collector.1".  In a generational collector this would be the
>>>>>   // old generation collection.
>>>>>   _full_collection_counters =
>>>>> -    new CollectorCounters("G1 stop-the-world full collections", 1);
>>>>> +    new CollectorCounters("G1 stop-the-world phase", 1);
>>>>> 
>>>>>   // timer sampling for all counters supporting sampling only update the
>>>>>   // used value.  See the take_sample() method.  G1 requires both used and
>>>>> diff -r 8a103ba9a7b2 src/share/vm/gc/g1/vm_operations_g1.cpp
>>>>> --- a/src/share/vm/gc/g1/vm_operations_g1.cpp	Mon Feb 29 22:54:24 2016 +0900
>>>>> +++ b/src/share/vm/gc/g1/vm_operations_g1.cpp	Tue Mar 01 23:43:30 2016 +0900
>>>>> @@ -230,6 +230,8 @@
>>>>>   G1CollectedHeap* g1h = G1CollectedHeap::heap();
>>>>>   GCTraceTime(Info, gc) t(_printGCMessage, g1h->gc_timer_cm(), GCCause::_no_gc, true);
>>>>>   IsGCActiveMark x;
>>>>> +  SvcGCMarker sgcm(SvcGCMarker::OTHER);
>>>>> +  TraceCollectorStats tcs(g1h->g1mm()->full_collection_counters());
>>>>>   _cl->do_void();
>>>>> }
>>>>> 
>>>>> ----------------------
>>>>> 
>>>>> 
>>>>> Thanks,
>>>>> 
>>>>> Yasumasa
>>>>> 
>>>>> 
>>>> 
>> 

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