ParallelOldGC : Single threaded with one cpu 100% during full gc

lohit lohit.vijayarenu at
Mon Nov 5 19:15:49 UTC 2012

Thanks for the information Srinivas.

2012/11/4 Srinivas Ramakrishna <ysr1729 at>

> CMS might help if you are able to tune it to avoid the full gc pauses
> entirely by doing all the
> collection work mostly concurrently. With CMS, a few cpu's will be busy
> for a while as CMS runs
> concurrently with the application in certain phases. If CMS "loses the
> race" (or there is excessive
> fragmentation), the stop-world full gc will be pretty long and will be
> single-threaded.
> $ /usr/lib/jvm/jdk1.7.0_05/bin/java -XX:+PrintFlagsFinal -version | grep
> Compact
>      bool CMSCompactWhenClearAllSoftRefs            = true
> {product}
>     uintx CMSFullGCsBeforeCompaction                = 0
> {product}
>      bool CompactFields                             = true
> {product}
>     uintx HeapFirstMaximumCompactionCount           = 3
> {product}
>     uintx HeapMaximumCompactionInterval             = 20
> {product}
>      intx MarkSweepAlwaysCompactCount               = 4
> {product}
>      bool UseCMSCompactAtFullCollection             = true
> {product}
>      bool UseMaximumCompactionOnSystemGC            = true
> {product}
> java version "1.7.0_05"
> Java(TM) SE Runtime Environment (build 1.7.0_05-b06)
> Java HotSpot(TM) 64-Bit Server VM (build 23.1-b03, mixed mode)
> The options we used were HeapFirstMaximumCompactionCount  and
> HeapMaximumCompactionInterval, both  set to  very large values.
> For our use case, this worked reasonably well, but I have seen cases of
> other applications where this isn't effective because
> of the demographics of large oop-rich objects (such as large hash maps and
> such, for example) as they "churn through" the
> old generation rather than sedimenting in the dense prefix. We didn't need
> to, and I think you won't need to either, worry
> about space lost to deadwood in the dense prefix, because the dense prefix
> computation doesn't waste much space. I don't
> recall the computation, but John would be able to help.
> -- ramki
> On Sun, Nov 4, 2012 at 11:34 AM, lohit <lohit.vijayarenu at> wrote:
>> Thanks Srinivas and Krystal for the pointers and information.
>> Would CMS help in this case? Because I see similar behavior in CMS as
>> well, where in one cpu gets busy for quite some time.
>> I could try to grab similar stack trace if that helps.
>> From what I read there is no way around it other than disabling max
>> compaction (I could not find any switch to deferring it forever, yet not
>> disable it).
>> Downside of that is the memory gets fragmented and application
>> performance degrade or causes OOM.
>> Any further thoughts on this?
>> Thanks again!
>> 2012/11/4 Srinivas Ramakrishna <ysr1729 at>
>>> This is the deferred pointer updates phase which is unfortunately
>>> single-threaded, and occasionally greatly dominates the pause times.
>>> I called in a bug for this late last year and there was some discussion
>>> on how this could be fixed. I was planning to work on
>>> a patch, but never quite got around to it.  We worked around it
>>> (although the workaround may not always work, see a parallel (hmm)
>>> ongoing discussion) by deferring the "maximum compaction" forever,
>>> always relying on the dense prefix to demarcate the
>>> boundary below which we would never choose to compact. In typical cases
>>> I have seen large oop-rich objects (which in our
>>> case sedimented to the bottom of the heap) were the cause of this slow
>>> down.
>>> I don't recall the bug id tracking this, but someone on the list may
>>> know. I'll try and find the old discussion of this, and send a pointer.
>>> -- ramki
>>> On Sun, Nov 4, 2012 at 9:54 AM, lohit <lohit.vijayarenu at>wrote:
>>>> Hello Devs,
>>>> We are trying to profile Full GC performance on our Java server.
>>>> Heap Size is about 80G, running on CentOS 5.5. As of now we use
>>>> ParallelOldGC for old generation compactions.
>>>> When we trigger fullGC by hand we see that it takes more than 2
>>>> minutes.
>>>> Running mpstat showed that for about half of the time only one CPU was
>>>> spinning 100% and all others were idle. (Box has 24 CPUs)
>>>> While this was happening we took stack trace and see that all threads
>>>> are waiting behind one thread whose trace look like below.
>>>> Question is the below expected. Even though documentation says
>>>> parallelOldGC uses multiple threads, are there cases when this kind of
>>>> serialization happens where all threads wait behind single thread.
>>>> I might have given very little information about problem, but let me
>>>> know if I could add any more information to know about this
>>>> Thread 18 (Thread 0x419f2940 (LWP 55314)):^M
>>>> #0  0x00007f051735e3e0 in BitMap::get_next_one_offset_inline_aligned_right(unsigned long, unsigned long) const ()^M
>>>>    from /usr/java/jdk1.6.0_24/jre/lib/amd64/server/^M
>>>> #1  0x00007f051735e09e in ParMarkBitMap::live_words_in_range(HeapWord*, oopDesc*) const ()^M
>>>>    from /usr/java/jdk1.6.0_24/jre/lib/amd64/server/^M
>>>> #2  0x00007f0517398848 in ParallelCompactData::calc_new_pointer(HeapWord*) ()^M
>>>>    from /usr/java/jdk1.6.0_24/jre/lib/amd64/server/^M
>>>> #3  0x00007f05173413ec in objArrayKlass::oop_update_pointers(ParCompactionManager*, oopDesc*) ()^M
>>>>    from /usr/java/jdk1.6.0_24/jre/lib/amd64/server/^M
>>>> #4  0x00007f051739d0a7 in PSParallelCompact::update_deferred_objects(ParCompactionManager*, PSParallelCompact::SpaceId) ()^M
>>>>    from /usr/java/jdk1.6.0_24/jre/lib/amd64/server/^M
>>>> #5  0x00007f051739c6c7 in PSParallelCompact::compact() () from /usr/java/jdk1.6.0_24/jre/lib/amd64/server/^M
>>>> #6  0x00007f051739aeca in PSParallelCompact::invoke_no_policy(bool) ()^M
>>>>    from /usr/java/jdk1.6.0_24/jre/lib/amd64/server/^M
>>>> #7  0x00007f051739a845 in PSParallelCompact::invoke(bool) () from /usr/java/jdk1.6.0_24/jre/lib/amd64/server/^M
>>>> #8  0x00007f05174a0bb0 in VM_ParallelGCSystemGC::doit() () from /usr/java/jdk1.6.0_24/jre/lib/amd64/server/^M
>>>> #9  0x00007f05174ada5a in VM_Operation::evaluate() () from /usr/java/jdk1.6.0_24/jre/lib/amd64/server/^M
>>>> (More stack frames follow...)^M
>>>> --
>>>> Have a Nice Day!
>>>> Lohit
>> --
>> Have a Nice Day!
>> Lohit

Have a Nice Day!
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