ParNew - how does it decide if Full GC is needed

Thu May 8 19:34:11 UTC 2014

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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