G1 discovers same garbage again?

Florian Binder java at java4.info
Mon Dec 12 19:08:17 UTC 2011


Tony,

As you have seen at the jconsole-screenshot the app does make very few 
garbage. It is optimized to make no garbage. But suddenly there seems to 
be every 90 seconds about 4 gb of (new) garbage (even the load of the 
app does not change). Therefore I thought it might be the g1-collector. 
I will try to reproduce this senario an have a closure look on it.

Flo

Am 12.12.2011 15:46, schrieb Tony Printezis:
> Florian,
>
> Not sure what you mean by "the same GC". It is because the space 
> transition looks like: 16394M->12914M(20480M)? This is normal. Heap 
> occupancy reaches 16394M, GC happens, heap occupancy goes down to 
> 12914M. Next time it happens, the heap will have new objects. This is 
> how GCs work.
>
> Tony
>
> On 12/09/2011 09:08 PM, Florian Binder wrote:
>> In the gc log it seems that there is always running the same garbage 
>> collection.
>> For example:
>> $ zcat out_err.log.gz | grep 20480M | tail
>> 2011-11-28T19:13:19.482+0100: [GC cleanup 14515M->14515M(20480M), 
>> 0.1370060 secs]
>>    [ 16394M->12914M(20480M)]
>>    [ 16394M->12914M(20480M)]
>> 2011-11-28T19:17:12.509+0100: [GC cleanup 15582M->15582M(20480M), 
>> 0.1387230 secs]
>>    [ 16394M->12914M(20480M)]
>>    [ 16394M->12914M(20480M)]
>>    [ 16394M->12914M(20480M)]
>> 2011-11-28T19:21:06.089+0100: [GC cleanup 12978M->12978M(20480M), 
>> 0.1344170 secs]
>>    [ 16394M->12914M(20480M)]
>>    [ 16394M->12914M(20480M)]
>>
>> Therefore I assume this might be a bug ;-)
>> You can download the whole log at: http://java4.info/g1/out_err.log.gz
>>
>> I don't think that we still have the logs of that very long gc, but I 
>> will have a look for it at monday.
>> Furthermore I do not think that we have logged there much details of 
>> the gc. But I know that this happend on a very special server, which 
>> contains more the 30gb of references (yes, just references to other 
>> objects). If we run it with CompressedOops we reduce the memory 
>> useage to nearly 50%.
>>
>> Flo
>>
>>
>>
>> Am 10.12.2011 01:34, schrieb Srinivas Ramakrishna:
>>>
>>>
>>> A couple of things caught my eye....
>>>
>>>     On 12/05/2011 02:19 PM, Florian Binder wrote:
>>>     >> I assume you know when the data will be dropped and
>>>     reinitialized,
>>>     >> right? Can you do a Full GC (with System.gc()) after you
>>>     re-initialize
>>>     >> the heap. This typically helps a lot with CMS.
>>>     > Yes, this is exactly what we are trying at this time ;-)
>>>     > Either this Full-GC pauses are very long (60-300 seconds) they
>>>     are much
>>>     > shorter than after a CMS-failure (when they take sometimes
>>>     more than
>>>     > 4000 seconds ;-)) and we can define when they occure.
>>>
>>>
>>> A full GC that takes an hour is definitely a bug. Have you logged 
>>> that bug?
>>> Or at least share the GC log? What's the version of the JDK that 
>>> this behaviour
>>> was seen with?
>>>
>>> ...
>>>
>>>     > Furthermore this should not increase the total-heap-space:
>>>     > http://java4.info/g1/all.png
>>>
>>>
>>>  Remember that jconsole asynchronously samples the heap, whose size is
>>> read "with possible glitches". Rather, you should probably rely on 
>>> the GC log in order to assess
>>> the heap size after each GC event, rather than the asynchronous 
>>> samples from
>>> jconsole. I myself have not had the chance to look at yr GC logs to see
>>> what that indicated wrt the size of Eden and of the Heap.
>>>
>>> -- ramki
>>
>>
>>
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