NewRatio: to twiddle or not to twiddle

[Charles Oliver Nutter]

Cross-posting to hotspot-gc-use...

I guess a certain portion of the challenge for language impls like JRuby 
is similar to the challenge you all face as JVM implementers: finding 
typical applications, typical loads, and appropriate defaults when our 
heads are usually buried two or three levels of abstraction beneath such 
applications.

-Xmn is a good one I had forgotten about, and answers my question about 
how to increase NewRatio beyond 1. I guess there's really two tasks I'm 
looking into right now:

1. Discovering appropriate flags to tweak and "more appropriate" 
defaults dynlangs might want to try
2. Exploring real-world dynlang applications and loads to refine those 
better/best-guess defaults

I'd say this round of emails is focused mostly on 1, since 2 is going to 
vary more across languages. And I think we can only start to explore 2 
iff we know 1. So -Xmn goes on the list alongside NewRatio.

Perhaps a brief illustration of the adaptive capabilities of the other 
collectors would be useful here? Obviously for server applications we'd 
hope HotSpot "does the right thing" for us already. Then perhaps we can 
learn from that (and please, teach me how to learn, including better 
tools and switches I can use to get the necessary GC metrics) and apply 
what we've learned to command-line/client runs (which will be more 
transient but not asimilar to the server runs in a language like Ruby).

- Charlie

Peter B. Kessler wrote:
> Why limit yourself to NewRatio?  The best you can get that way is half 
> the heap for the young generation.  If you really want to a big young 
> generation (to give your temporary objects time to die without even 
> being looked at by the collector), use -Xmn (or -XX:NewSize= and 
> -XX:MaxNewSize=) to set it directly.  Figure out what your live data 
> size is and use that as the base size for the old generation.  Then 
> figure out what kinds of pauses the young generation collections impose, 
> and how much they promote, then amortize the eventual old generation 
> collection time over as many young generation collections as you can 
> give space to in the old generation.  Then make your total heap (-Xmx) 
> as big as you can afford to get as big a young generation as that will 
> allow.
> 
>             ... peter
> 
> Charles Oliver Nutter wrote:
>> I've ben playing with JRuby on various benchmarks recently and found 
>> that several object-intensive scripts run better if I set NewRatio=1. 
>> Ruby, even more than Java, tends to generate lots and lots of objects, 
>> especially considering that there's no unboxed primitive numeric types 
>> (and no fixnums on the JVM...ahem ahem). So my general theory is that:
>>
>> 1. A NewRatio of 1 allows all those extra transient objects to get 
>> collected more quickly.
>> 2. Too small a "new" generation causes transient objects to get shoved 
>> into older generations, potentially snowballing and forcing more 
>> comprehensive GC runs as time goes on.
>>
>> I'm curious whether this theory sounds reasonable, whether there's a 
>> better way I can adapt hotspot to the memory demands of a dynamic 
>> language like Ruby, and what other implications there are in setting 
>> NewRatio to 1.
>>
>> Thoughts?
>>
>> (And please let me know if there's a better list to post this sort of 
>> question to)
>>
>> - Charlie
>