Request for review (S): 8005972: ParNew should not update the tenuring threshold when promotion failed has occurred

Bengt Rutisson bengt.rutisson at oracle.com
Fri Jan 11 12:57:06 UTC 2013


Hi Vitaly,

On 1/11/13 1:45 PM, Vitaly Davidovich wrote:
>
> Hi Bengt,
>
> Regarding the benchmark score, are you saying ParNew has longer 
> cumulative GC time or just the average is higher? If it's just 
> average, maybe the total # of them (and cumulative time) is less.  I 
> don't know the characteristics of this particular specjbb benchmark, 
> but perhaps having fewer total GCs is better because of the overhead 
> of getting all threads to a safe point, going go the OS to suspend 
> them, and then restarting them.  After they're restarted, the CPU 
> cache may be cold for it because the GC thread polluted it.  Or I'm 
> entirely wrong in my speculation ... :).
>

You have a good point about the number of GCs. The problem in my runs is 
that ParNew does more GCs than DefNew. So there are both more of them 
and their average time is higher, but the score is still better. That 
ParNew does more GCs is not that strange. It has a higher score, which 
means that it had higher throughput and had time to create more objects. 
So, that is kind of expected. But I don't understand how it can have 
higher throughput when the GCs take longer. My current guess is that it 
does something differently with how objects are copied in a way that is 
beneficial for the execution time between GCs.

It also seems like ParNew keeps more objects alive for each GC. That is 
either the reason why it does more and more frequent GCs than DefNew, or 
it is an effect of the fact that more objects are created due to the 
higher throughput. This is the reason I started looking at the tenuring 
threshold.

Bengt

> Thanks
>
> Sent from my phone
>
> On Jan 11, 2013 6:02 AM, "Bengt Rutisson" <bengt.rutisson at oracle.com 
> <mailto:bengt.rutisson at oracle.com>> wrote:
>
>
>     Hi Ramki,
>
>     Thanks for looking at this!
>
>     On 1/10/13 9:28 PM, Srinivas Ramakrishna wrote:
>>     Hi Bengt --
>>
>>     The change looks reasonable, but I have a comment and a follow-up
>>     question.
>>
>>     Not your change, but I'd elide the "half the real survivor size"
>>     since it's really a configurable parameter based on
>>     TargetSurvivorRatio with default half.
>>     I'd leave the comment as "set the new tenuring threshold and
>>     desired survivor size".
>
>     I'm fine with removing this from the comment, but I thought the
>     "half the real survivor size" aimed at the fact that we pass only
>     the "to" capacity and not the "from" capacity in to
>     compute_tenuring_threshold(). With that interpretation I think the
>     comment is correct.
>
>     Would you like me to remove it anyway? Either way is fine with me.
>
>>     I'm curious though, as to what performance data prompted this change,
>     Good point. This change was preceded by an internal discussion in
>     the GC team, so I should probably have explained the background
>     more in my review request to the open.
>
>     I was comparing the ParNew and DefNew implementation since I am
>     seeing some strange differences in some SPECjbb2005 results. I am
>     running ParNew with a single thread and get much better score than
>     with DefNew. But I also get higher average GC times. So, I was
>     trying to figure out what DefNew and ParNew does differently.
>
>     When I was looking at DefNewGeneration::collect() and
>     ParNewGeneration::collect() I saw that they contain a whole lot of
>     code duplication. It would be tempting to try to extract the
>     common code out into DefNewGeneration since it is the super class.
>     But there are some minor differences. One of them was this issue
>     with how they handle the tenuring threshold.
>
>     We tried to figure out if there is a reason for ParNew and DefNew
>     to behave different in this regard. We could not come up with any
>     good reason for that. So, we needed to figure out if we should
>     change ParNew or DefNew to make them consistent. The decision to
>     change ParNew was based on two things. First, it seems wrong to
>     use the data from a collection that got promotion failure. This
>     collection will not have allowed the tenuring threshold to fulfill
>     its purpose. Second, ParallelScavenge works the same way as DefNew.
>
>     BTW, the difference between DefNew and ParNew seems to have been
>     there from the start. So, there is no bug or changeset in
>     mercurial or TeamWare to explain why the difference was introduced.
>
>     (Just to be clear, this difference was not the cause of my
>     performance issue. I still don't have a good explanation for how
>     ParNew can have longer GC times but better SPECjbb score.)
>
>>     and whether it might make sense, upon a promotion failure to do
>>     something about the tenuring threshold for the next scavenge
>>     (i.e. for example make the tenuring threshold half of its current
>>     value as a reaction to the fact that promotion failed). Is it
>>     currently left at its previous value or is it asjusted back to
>>     the default max value (which latter may be the wrong thing to do)
>>     or something else?
>
>     As far as I can tell the tenuring threshold is left untouched if
>     we get a promotion failure. It is probably a good idea to update
>     it in some way. But I would prefer to handle that as a separate
>     bug fix.
>
>     This change is mostly a small cleanup to make
>     DefNewGeneration::collect() and ParNewGeneration::collect() be
>     more consistent. We've done the thinking so, it's good to make the
>     change in preparation for the next person that comes a long and
>     has a few cycles over and would like to merge the two collect()
>     methods in some way.
>
>     Thanks again for looking at this!
>     Bengt
>
>>
>>     -- ramki
>>
>>     On Thu, Jan 10, 2013 at 1:30 AM, Bengt Rutisson
>>     <bengt.rutisson at oracle.com <mailto:bengt.rutisson at oracle.com>> wrote:
>>
>>
>>         Hi everyone,
>>
>>         Could I have a couple of reviews for this small change to
>>         make DefNew and ParNew be more consistent in the way they
>>         treat the tenuring threshold:
>>
>>         http://cr.openjdk.java.net/~brutisso/8005972/webrev.00/
>>         <http://cr.openjdk.java.net/%7Ebrutisso/8005972/webrev.00/>
>>
>>         Thanks,
>>         Bengt
>>
>>
>

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