Request for review (XXS): 7193157: G1: Make some develpflags available in product builds

[John Cuthbertson]

Hi Ramki,

Once we're happy with the policies, and default values and G1 is mostly 
meeting peoples' expectations, we definitely should work a tuning guide. 
Any outside help and suggestions (hint, hint) will be more than welcome. 
The perf team has been working some what-if scenarios and that could be 
used as a basis. There has also been mention of contributing to a future 
edition of Charlie Hunt's excellent book (if there is one).

IMO we need to improve the mixed GC times. During a typical mixed GC 
sequence the pause times are typically higher than the typical young GC 
and, in the latter stages of mixed GC sequence, the pause times go up 
significantly as the cost of collecting the regions goes up. I think we 
can reduce this through changing default flag settings (for example a 
90% occupancy might result in too many "expensive" candidate regions). 
For increased RSet updating times - which is typically the case with a 
mixed GC because of the increased number of filled updated buffers by 
the GC itself - it might be worth investigating starting the concurrent 
refine threads more aggressively to process the higher number of 
deferred updates. I also think there may be an issue in the prediction 
logic for the RSet scanning time based upon RSet size - I don't think it 
handles coarse RSet entries very well.

Also there were a few times where we weren't meeting the pause time goal 
because we couldn't size the young generation small enough - we were 
running into the 20% lower limit of the heap.

As a result of Bengt's change we have provided a few more knobs for 
people to play with.

Thanks,

JohnC

On 08/22/12 01:21, Srinivas Ramakrishna wrote:
> Looks good to me. I am hoping the perf and gc teams will together also 
> publish a G1 tuning white-paper soon!
>
> -- ramki
>
> On Wed, Aug 22, 2012 at 12:46 AM, Bengt Rutisson 
> <bengt.rutisson at oracle.com <mailto:bengt.rutisson at oracle.com>> wrote:
>
>
>     Hi all,
>
>     Could I please have a couple of reviews for this really small fix?
>     http://cr.openjdk.java.net/~brutisso/7193157/webrev.00/
>     <http://cr.openjdk.java.net/%7Ebrutisso/7193157/webrev.00/>
>
>     We need to make some of the G1 develop flags available in product
>     builds to allow the performance team to tune G1 properly. We
>     decided to make the flags experimental to start with. After the
>     performance team has evaluated the flags we might make some of
>     them product flags. The current prime candidates for becoming
>     product flags are:
>
>     G1DefaultMinNewGenPercent
>     G1DefaultMaxNewGenPercent
>     G1OldCSetRegionLiveThresholdPercent
>
>     Thanks,
>     Bengt
>
>

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