Request for review (S): 7113021 G1: automatically enable young gen size auto-tuning when -Xms==-Xmx

[Bengt Rutisson]

Hi everyone,

Could I have a couple of reviews for this change?
http://cr.openjdk.java.net/~brutisso/7113021/webrev.02/

The idea is to give G1 some room to allow it to use its heuristics for 
calculating the young gen size even when the heap size is fixed.

CR:
7113021 G1: automatically enable young gen size auto-tuning when -Xms==-Xmx
http://monaco.us.oracle.com/detail.jsf?cr=7113021
http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=7113021

I introduce two new develop flags, G1DefaultMinNewGenPercent and 
G1DefaultMaxNewGenPercent. I will try to get the performance team to 
evaluate some good default values for these flags. Right now they are 20 
and 50 respectively. This might change if we find that some other values 
are more appropriate.

Background:

There are three command line options related to the young gen size: 
NewSize, MaxNewSize and NewRatio (There is also -Xmn, but that is just a 
short form for NewSize==MaxNewSize). G1 will use its internal heuristics 
to calculate the actual young gen size, so these options basically only 
limit the range within which G1 can pick a young gen size. Also, these 
are general options taking byte sizes. G1 will internally work with a 
number of regions instead. So, some rounding will occur.

The idea is that my fix should work pretty much as follows:

If nothing related to the the young gen size is set on the command line 
we should allow the young gen to be between 20% and 50% of the heap 
size. This means that every time the heap size changes the limits for 
the young gen size will be updated.

If only -XX:NewSize is set we should use the specified value as the 
minimum size for young gen. Still using 50% of the heap as maximum.

If only -XX:MaxNewSize is set we should use the specified value as the 
maximum size for young gen. Still using 20% of the heap as minimum.

If -XX:NewSize and -XX:MaxNewSize are both specified we use these 
values. No updates when the heap size changes. There is a special case 
when NewSize==MaxNewSize. This is interpreted as "fixed" and will use a 
different heuristic for calculating the collection set when we do mixed 
collection.

If only -XX:NewRatio is set we should use the specified ratio of the 
heap as both min and max. This will be interpreted as "fixed" just like 
the NewSize==MaxNewSize case above. But we will update the min and max 
everytime the heap size changes.

NewSize and MaxNewSize override NewRatio. So, NewRatio is ignored if it 
is combined with either NewSize or MaxNewSize. (A warning message is 
printed.)

Thanks,
Bengt