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

[Joe Provino]

Hi Jesper, I had a feeling what looked like a simple fix wouldn't be 
that simple.  ;-)

On 04/20/2016 03:35 PM, Jesper Wilhelmsson wrote:
> 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.
If the code inside that "if" is executed, then the bug occurs.  So I 
think changing to OR won't fix the problem.
>
> 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.
Yeah, I don't know the reasoning behind this.
> You should probably consult whoever wrote this code.
That sounds like a good idea.  I'm not sure who that person is.

thanks.

joe

>
> /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
>>
>>