TenuringThreshold being adaptive when UseAdaptiveSizePolicy is off

Jon Masamitsu jon.masamitsu at oracle.com
Tue Mar 10 18:27:38 UTC 2015


On 3/10/2015 12:01 AM, Kirk Pepperdine wrote:
> Hi Jon,
>
> Slightly off topic.
>
> This question triggered me to once again take a closer look at 
> psAdaptiveSizePolicy and CMSAdaptiveSizePolicy. What I saw is that
>
> uint PSAdaptiveSizePolicy::compute_survivor_space_size_and_threshold(
> bool is_survivor_overflow,
>                                           uint tenuring_threshold,
>                                           size_t survivor_limit) {
>
> and
>
> uint CMSAdaptiveSizePolicy::compute_survivor_space_size_and_threshold(
> bool is_survivor_overflow,
>                                           uint tenuring_threshold,
>                                           size_t survivor_limit) {
>
> are cut and paste copies of each other. In fact the amount of common 
> code in both classes suggests they were a complete cut and paste copy 
> of each other. Yes there are a couple of minor differences but not 
> enough to warrant two separate classes. It’s also a bit scary to see 
> this level of cut and paste in the JVM source code. Maybe it was 
> intentional, if so it should have been justified with a comment but … 
> I’ll say no more on this topic.

That was a project that was never completed.   We're removing it in 9.
>
> As I’ve mentioned before, AdaptiveSize can get into this degenerative 
> pathological condition in low free heap conditions. It starts with 
> higher GC frequencies slowing the allocation rates from the mutator 
> threads. These lower allocation rates when feed back into adaptive 
> sizing, are a signal to shrink the size of heap which is exactly the 
> opposite of what you’d like to see in this condition. I have offered 
> to work on a patch to fix this but I’ve been told by several people 
> that they have a patch to correct this behavior so no need to do so.

If I was one of those people, I apologize but don't recall the 
conversation.  Was I?

>
> My question is, can we get one of these patches released so that we 
> can correct this behavior?
Generally speaking we're not doing much work on CMS these days.   We are 
still accepting patches
for improvements, of course.

Jon


>
> Kind regards,
> Kirk
>
> On Mar 10, 2015, at 7:07 AM, Jon Masamitsu <jon.masamitsu at oracle.com 
> <mailto:jon.masamitsu at oracle.com>> wrote:
>
>>
>> On 3/9/2015 10:39 PM, Jungwoo Ha wrote:
>>> We were investigating some features and figured out that 
>>> ParallelScavange and ParNew has different TenuringTreshold heuristics.
>>> We thought it would be nice to have a flag that can control the 
>>> adaptiveness, so that we can fix it on purpose to isolate other 
>>> performance factors.
>>
>> Do you use UsePSAdaptiveSurvivorSizePolicy to turn off the
>> survivor space resizing and tenuring threshold adaptiveness
>> for ParallelGC?
>>
>> Jon
>>
>>> --Jungwoo
>>>
>>> On Mon, Mar 9, 2015 at 11:27 AM, Jon Masamitsu 
>>> <jon.masamitsu at oracle.com <mailto:jon.masamitsu at oracle.com>> wrote:
>>>
>>>     Jungwoo,
>>>
>>>     On 03/06/2015 06:03 PM, Jungwoo Ha wrote:
>>>>     I think it is the same for all JDK{7,8,9}.
>>>>
>>>>     arguments.cpp disabled UseAdaptiveSizePolicy for CMS.
>>>>     parallel scavange initializes _tenuring_threshold with
>>>>       _tenuring_threshold = (UseAdaptiveSizePolicy) ?
>>>>     InitialTenuringThreshold : MaxTenuringThreshold;
>>>>     whereas CMS uses
>>>>       _tenuring_threshold = MaxTenuringThreshold;
>>>>
>>>>     adjust_desired_tenuring_threshold() is called inside
>>>>     ParNewGeneration::collect(), which calls
>>>>     compute_tenuring_threshold.
>>>>     It only uses MaxTenuringThreshold and TargetSurvivorRatio.
>>>>     I don't see any other flags here.
>>>
>>>     Yes, this is the intended behavior.
>>>
>>>     You're saying we're sometimes wasting survivor space?
>>>
>>>     Jon
>>>
>>>>
>>>>     On Fri, Mar 6, 2015 at 4:28 PM, Jon Masamitsu
>>>>     <jon.masamitsu at oracle.com <mailto:jon.masamitsu at oracle.com>> wrote:
>>>>
>>>>         This is in jdk9?
>>>>
>>>>         Jon
>>>>
>>>>
>>>>         On 3/6/2015 4:21 PM, Jungwoo Ha wrote:
>>>>
>>>>             On CMS when UseAdaptiveSizePolicy is off, still
>>>>             tenuring_threshold is being changed.
>>>>             Is this an intended behavior?
>>>>             (yes. I know it is not "size" related heuristics)
>>>>
>>>>             It would be nice to have a flag that can disable
>>>>             adjusting tenuring threshold,
>>>>             or at least set minimum threshold. When the default
>>>>             heuristics doesn't work,
>>>>             there are nothing much we can do other than changing
>>>>             the Max.
>>>>
>>>>             --Jungwoo
>>>>
>>>>
>>>>
>>>
>>>
>>
>

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