RFR (S): JDK-8027440: DefNew does not log heap change information when a promotion failure occurs

[Bengt Rutisson]

Hi Tao,

On 11/1/13 6:53 PM, Tao Mao wrote:
> Hi Bengt,
>
> Thank you for looking at this.
>
> Can you explain why, in this particular case, the used heap size got 
> larger after GC?

It is because of the way the used value is calculated. The used value 
for the DefNew generation is implemented as:

size_t DefNewGeneration::used() const {
   return eden()->used()
        + from()->used();      // to() is only used during scavenge
}

Where, for both eden and from, used() is implemented as:

   size_t used() const            { return byte_size(bottom(), top());

Since we got a promotion failure the top value is not reset. There are 
still objects in the eden and from spaces. This means that the used 
value after the GC is whatever we managed to promote plus the full young 
gen (except the to space). This is more than the used before the GC, 
since that was just what was already in the old gen plus the full young gen.

This is the same behavior as with the other collectors.

Thanks,
Bengt

>
> Thanks.
> Tao
>
> On 11/1/13 8:50 AM, Bengt Rutisson wrote:
>>
>> Hi all,
>>
>> Can I have a couple of reviews for this small change?
>>
>> http://cr.openjdk.java.net/~brutisso/8027440/webrev.00/
>>
>> https://bugs.openjdk.java.net/browse/JDK-8027440
>>
>> DefNew was not printing the heap change information when a promotion 
>> failure occured. Before this change the logging for a DefNew 
>> collection that got promotion failure looked like this:
>>
>> [GC (Allocation Failure) , 0.0543440 secs]
>>
>> After my change it looks like:
>>
>> [GC (Allocation Failure)  81920K->120831K(120832K), 0.0620180 secs]
>>
>> Thanks,
>> Bengt