RFR (XXS) JDK-8153578,Default NewRatio is ignored when UseConcMarkSweepGC is used as GC algorithm

[Jesper Wilhelmsson]

Hi Joe,

If I understand the bug description correctly the problem is that NewSize 
becomes too small. According to the bug the VM ignores the NewRatio setting.

Your change removes the setting of MaxNewSize in the case where NewSize has the 
default value. It's not obvious to me how that is related to the bug.

There is an if statement enclosing the code you are changing. It has a comment 
that I find interesting:

1755   // If either MaxNewSize or NewRatio is set on the command line,
1756   // assume the user is trying to set the size of the young gen.
1757   if (FLAG_IS_DEFAULT(MaxNewSize) && FLAG_IS_DEFAULT(NewRatio)) {

The interesting part is that the comment says "MaxNewSize OR NewRatio" but the 
code says "MaxNewSize AND NewRatio". This could be a typo in the comment, or it 
could be related to your bug.

I don't think the fix here is to ignore the calculated preferred_max_new_size, 
but rather to figure out why it has the wrong value. preferred_max_new_size is 
calculated a few lines up, and it is based on NewRatio. The number of threads 
seems to be involved as well. Should it be? Usually things based on the number 
of threads tend to be wrong...

1748     MIN2(max_heap/(NewRatio+1), ScaleForWordSize(young_gen_per_worker * 
ParallelGCThreads));

young_gen_per_worker is CMSYoungGenPerWorker which defaults to things like 16M 
or 64M. ParallelGCThreads is usually just a handful, 8 on my machine. Since we 
take the smallest number of this thread based thing and the NewRatio 
calculation, I would guess the threads will limit the MaxNewSize quite a lot. I 
wonder if this isn't the bug you are looking for. It would make more sense to me 
if it was MAX of the two instead of MIN. You should probably consult whoever 
wrote this code.

/Jesper


Den 20/4/16 kl. 20:08, skrev Joseph Provino:
> Please review this tiny change.  It only affects ParNew.  Are there any
> unintended consequences?
>
> Passes JPRT.
>
> CR: JDK-8153578 Default NewRatio is ignored when UseConcMarkSweepGC is used as
> GC algorithm <https://bugs.openjdk.java.net/browse/JDK-8153578>
>
> Webrev:  http://cr.openjdk.java.net/~jprovino/8153578/webrev.00
>
>