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List: openjdk-hotspot-dev
Subject: Re: RFR (XXS) JDK-8153578,Default NewRatio is ignored when UseConcMarkSweepGC is used as GC algorith
From: Jon Masamitsu <jon.masamitsu () oracle ! com>
Date: 2016-04-20 22:29:35
Message-ID: 571802CF.7010000 () oracle ! com
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On 04/20/2016 12:35 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.
>
> 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...
The intent of this code was to control the young pause times by limiting
the size of
the young gen. The preferred size scaling with
young_gen_per_worker * ParallelGCThreads
was meant to take into account the fact you could have approximately the
same pauses with larger heaps as the number of GC workers increases.
I didn't add this code but I'm pretty sure it was added as a result of
customer interaction.
Jon
>
> 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
>>
>>
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