ParNew - how does it decide if Full GC is needed

Thu May 8 22:57:57 UTC 2014

Jon,

Thanks.  So ParNew behavior of not triggering a full gc preemptively seems
a better fit for my usecase.  In fact, we will not have another young gc in
our setup, allocation rate, and workload.  What's the purpose of doing a
preemptive full gc (with all the baggage it comes with) in parallel old?
Why not just wait until the next young collection (if that even happens)
and take the full gc hit then? I'm failing to see the advantage of taking
that hit eagerly, even after reading Peter's description.  Is it to avoid
promotion failure that it thinks will happen next time? And if so, it
thinks doing the preemptive full gc is faster than handling a promotion
failure next time?

Thanks guys

Sent from my phone

On 05/08/2014 01:24 PM, Srinivas Ramakrishna wrote:

The 98% old gen occupancy triggered one of my two neurons.
I think there was gc policy code (don't know if it;s still there) that
would proactiively precipitate a full gc when it realized (based on
recent/historical promotion volume stats) that the next minor gc would not
be able to promote its survivors into the head room remaining in old.
(Don't ask me why it;s better to do it now rather than the next time the
young gen fills up and just rely on the same check). Again I am not looking
at the code (as it takes some effort to get to the box where I keep a copy
of the hotspot/openjdk code.)

The UseParallelGC collector will do a full GC after a young GC if the
UseParallelGC
thinks the next young GC will not succeed (per Peter's explanation).  I
don't think
the ParNew GC will do that.   I looked for that code but did not find it.
I looked in
the do_collection() code and the ParNew::collect() code.

The only case I could find where a full GC followed a young GC with ParNew
was
if the collection failed to free enough space for the allocation.  Given
the amount
of free space in the young gen after the collection, that's unlikely.  Or
course, there
could be a bug.

Jon

 Hopefully Jon &co. will quickly confirm or shoot down the imaginations o
my foggy memory!
-- ramki

On Thu, May 8, 2014 at 12:55 PM, Vitaly Davidovich <vitalyd at gmail.com>wrote:

> I captured some usage and capacity stats via jstat right after that full
> gc that started this email thread.  It showed 0% usage of survivor spaces
> (which makes sense now that I know that a full gc empties that out
> irrespective of tenuring threshold and object age); eden usage went down to
> like 10%; tenured usage was very high, 98%.  Last gc cause was recorded as
> "Allocation Failure".  So it's true that the tenured doesn't have much
> breathing room here, but what prompted this email is I don't understand why
> that even matters considering young gen got cleaned up quite nicely.
>
>
> On Thu, May 8, 2014 at 3:36 PM, Srinivas Ramakrishna <ysr1729 at gmail.com>wrote:
>
>>
>> By the way, as others have noted, -XX:+PrintGCDetails at max verbosity
>> level would be your friend to get more visibility into this. Include
>> -XX:+PrintHeapAtGC for even better visibility. For good measure, after the
>> puzzling full gc happens (and hopefully before another GC happens) capture
>> jstat data re the heap (old gen), for direct allocation visibility.
>>
>>  -- ramki
>>
>>
>> On Thu, May 8, 2014 at 12:34 PM, Srinivas Ramakrishna <ysr1729 at gmail.com>wrote:
>>
>>> Hi Vitaly --
>>>
>>>
>>>  On Thu, May 8, 2014 at 11:38 AM, Vitaly Davidovich <vitalyd at gmail.com>wrote:
>>>
>>>> Hi Jon,
>>>>
>>>>  Nope, we're not using CMS here; this is the throughput/parallel
>>>> collector setup.
>>>>
>>>>  I was browsing some of the gc code in openjdk, and noticed a few
>>>> places where each generation attempts to decide (upfront from what I can
>>>> tell, i.e. before doing the collection) whether it thinks it's "safe" to
>>>> perform the collection (and if it's not, it punts to the next generation)
>>>> and also whether some amount of promoted bytes will fit.
>>>>
>>>>  I didn't dig too much yet, but a cursory scan of that code leads me
>>>> to think that perhaps the defNew generation is asking the next gen (i.e.
>>>> tenured) whether it could handle some estimated promotion amount, and given
>>>> the large imbalance between Young and Tenured size, tenured is reporting
>>>> that things won't fit -- this then causes a full gc.  Is that at all
>>>> possible from what you know?
>>>>
>>>
>>>  If that were to happen, you wouldn't see the minor gc that precedes
>>> the full gc in the log snippet you posted.
>>>
>>>  The only situation I know where a minor GC is followed immediately by
>>> a major is when a minor gc didn't manage to fit an allocation request in
>>> the space available. But, thinking more about that, it can't be because one
>>> would expect that Eden knows the largest object it can allocate, so if the
>>> request is larger than will fit in young, the allocator would just go look
>>> for space in the older generation. If that didn't fit, the old gen would
>>> precipitate a gc which would collect the entire heap (all this should be
>>> taken with a dose of salt as I don't have the code in front of me as I
>>> type, and I haven't looked at the allocation policy code in ages).
>>>
>>>
>>>>
>>>>  On your first remark about compaction, just to make sure I
>>>> understand, you're saying that a full GC prefers to move all live objects
>>>> into tenured (this means taking objects out of survivor space and eden),
>>>> irrespective of whether their tenuring threshold has been exceeded? If that
>>>> compaction/migration of objects into tenured overflows tenured, then it
>>>> attempts to compact the young gen, with overflow into survivor space from
>>>> eden.  So basically, this generation knows how to perform compaction and
>>>> it's not just a copying collection?
>>>>
>>>
>>>  That is correct. A full gc does in fact move all survivors from young
>>> gen into the old gen. This is a limitation (artificial nepotism can ensue
>>> because of "too young" objects that will soon die, getting artificially
>>> dragged into the old generation) that I had been lobbying to fix for a
>>> while now. I think there's even an old, perhaps still open, bug for this.
>>>
>>>
>>>>  Is there a way to get the young gen to print an age table of objects
>>>> in its survivor space? I couldn't find one, but perhaps I'm blind.
>>>>
>>>
>>>  +PrintTenuringDistribution (for ParNew/DefNew, perhaps also G1?)
>>>
>>>
>>>>
>>>>  Also, as a confirmation, System.gc() always invokes a full gc with
>>>> the parallel collector, right? I believe so, but just wanted to double
>>>> check while we're on the topic.
>>>>
>>>
>>>  Right. (Not sure what happens if JNI critical section is in force --
>>> whether it's skipped or we wait for the JNI CS to exit/complete; hopefully
>>> others can fill in the blanks/inaccuracies in my comments above, since they
>>> are based on things that used to be a while ago in code I haven't looked at
>>> recently.)
>>>
>>>  -- ramki
>>>
>>>
>>>>
>>>>  Thanks
>>>>
>>>>
>>>> On Thu, May 8, 2014 at 1:39 PM, Jon Masamitsu <jon.masamitsu at oracle.com
>>>> > wrote:
>>>>
>>>>>
>>>>> On 05/07/2014 05:55 PM, Vitaly Davidovich wrote:
>>>>>
>>>>> Yes, I know :) This is some cruft that needs to be cleaned up.
>>>>>
>>>>> So my suspicion is that full gc is triggered precisely because old gen
>>>>> occupancy is almost 100%, but I'd appreciate confirmation on that.  What's
>>>>> surprising is that even though old gen is almost full, young gen has lots
>>>>> of room now.  In fact, this system is restarted daily so we never see
>>>>> another young gc before the restart.
>>>>>
>>>>> The other odd observation is that survivor spaces are completely empty
>>>>> after this full gc despite tenuring threshold not being adjusted.
>>>>>
>>>>>
>>>>>  The full gc algorithm used compacts everything (old gen and young
>>>>> gen) into
>>>>> the old gen unless it does not all fit.   If the old gen overflows,
>>>>> the young gen
>>>>> is compacted into itself.  Live in the young gen is compacted into
>>>>> eden first and
>>>>> then into the survivor spaces.
>>>>>
>>>>>  My intuitive thinking is that there was no real reason for the full
>>>>> gc to occur; whatever allocation failed in young could now succeed and
>>>>> whatever was tenured fit, albeit very tightly.
>>>>>
>>>>>
>>>>>  Still puzzling about the full GC.  Are you using CMS?  If you have
>>>>> PrintGCDetails output,
>>>>> that might help.
>>>>>
>>>>> Jon
>>>>>
>>>>>  Sent from my phone
>>>>> On May 7, 2014 8:40 PM, "Bernd Eckenfels" <bernd-2014 at eckenfels.net>
>>>>> wrote:
>>>>>
>>>>>> Am Wed, 7 May 2014 19:34:20 -0400
>>>>>> schrieb Vitaly Davidovich <vitalyd at gmail.com>:
>>>>>>
>>>>>> > The vm args are:
>>>>>> >
>>>>>> > -Xms16384m -Xmx16384m -Xmn16384m -XX:NewSize=12288m
>>>>>> > -XX:MaxNewSize=12288m -XX:SurvivorRatio=10
>>>>>>
>>>>>> Hmm... you have confliciting arguments here, MaxNewSize overwrites
>>>>>> Xmn.
>>>>>> You will get 16384-12288=4gb old size, thats quite low. As you can see
>>>>>> in your FullGC the steady state after FullGC has filled it nearly
>>>>>> completely.
>>>>>>
>>>>>> Gruss
>>>>>> Bernd
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>>>>>
>>>>>
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>
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