[9] RFR (S) 8146436: Add -XX:+UseAggressiveHeapShrink option

Tom Benson tom.benson at oracle.com
Wed Feb 10 20:50:04 UTC 2016

Hi Chris,
I agree, it makes sense to move the _shrink_factor adjustments inside 
the conditional.
You may as well also add the missing word (...we don't want TO 
shrink...) in the comment while you're there.

I've heard from another GC team person that there might be more feedback 
on the name coming, after some discussion.  Not sure if it will 
constitute the 'landslide' I mentioned.  8^)

On 2/10/2016 3:39 PM, Chris Plummer wrote:
> Hi Tom,
>       if (!UseAggressiveHeapShrink) {
>         // If UseAggressiveHeapShrink is false (the default),
>         // we don't want shrink all the way back to initSize if people 
> call
>         // System.gc(), because some programs do that between "phases" 
> and then
>         // we'd just have to grow the heap up again for the next 
> phase.  So we
>         // damp the shrinking: 0% on the first call, 10% on the second 
> call, 40%
>         // on the third call, and 100% by the fourth call.  But if we 
> recompute
>         // size without shrinking, it goes back to 0%.
>         shrink_bytes = shrink_bytes / 100 * current_shrink_factor;
>       }
>       assert(shrink_bytes <= max_shrink_bytes, "invalid shrink size");
>       if (current_shrink_factor == 0) {
>         _shrink_factor = 10;
>       } else {
>         _shrink_factor = MIN2(current_shrink_factor * 4, (size_t) 100);
>       }
> I got rid of the changes at the start of the method, and added the 
> !UseAggressiveHeapShrink check and the comment, so the first 2 lines 
> above and the closing right brace are now the only change in the file, 
> other than the copyright date. If you want I could also move the 
> _shrink_factor adjustment into this block since the value of 
> _shrink_factor becomes irrelevant if UseAggressiveHeapShrink is true. 
> The assert should remain outside the block.
> cheers,
> Chris
> On 2/10/16 12:16 PM, Tom Benson wrote:
>> Hi Chris,
>> OK, that all sounds good.
>> >> I can change it, although that will mean filing a new CCC.
>> Ah, I'd forgotten about that.  Not worth it, unless there's a 
>> landslide of support for a different name.
>> Tnx,
>> Tom
>> On 2/10/2016 3:06 PM, Chris Plummer wrote:
>>> Hi Tom,
>>> Thanks for having a look. Comments inline below:
>>> On 2/10/16 11:27 AM, Tom Benson wrote:
>>>> Hi Chris,
>>>> My apologies if I missed the discussion somewhere, but is there a 
>>>> specific rationale for adding this that can be mentioned in the bug 
>>>> report?  I can imagine scenarios where it would be useful, but 
>>>> maybe the real need can be called out.
>>> In general, it is for customers that want to minimize the amount of 
>>> memory used by the java heap, and are willing to sacrifice some 
>>> performance (induce more frequent GCs) to save that memory. When 
>>> heap usage fluctuates greatly, the GC will tend to hold on to that 
>>> memory longer than needed due to the the current algorithm which 
>>> requires 4 full GCs before MaxHeapFreeRatio is fully honored. If 
>>> this is what you are looking for, I can add it to the CR.
>>>> I think it might be clearer if the new code in cardGeneration was 
>>>> moved down to where the values are used. IE, I would leave the 
>>>> inits of current_shrink_factor and _shrink_factor as they were at 
>>>> lines 190/191.   Then down at 270, just don't divide by the shrink 
>>>> factor if UseAggressiveHeapShrink is set, and the updates to shrink 
>>>> factor can be in the same conditional.  This has the advantage that 
>>>> you can fix the comment just above it to match this special case. 
>>>> Do you think that would work?
>>> Yes, that makes sense. I'll get started on it. I have a vacation 
>>> coming up shortly, so what I'll get a new webrev out soon, but 
>>> probably will need to wait until after my trip to do more thorough 
>>> testing and push the changes.
>>>> It looks like the ending "\" at line 3330 in globals.hpp isn't 
>>>> aligned, and the copyright in cardGeneration.cpp needs to be updated.
>>> Ok.
>>>> One other nit, which you can ignore unless someone comes forward to 
>>>> agree with me  8^)  , is that I'd prefer the name 
>>>> ShrinkHeapAggressively instead.   Maybe this was already debated 
>>>> elsewhere....
>>> The name choice hasn't really been discussed or questioned. It was 
>>> what was suggested to me, so I stuck with it (The initial work was 
>>> done by someone else. I'm just getting it integrated into 9). I can 
>>> change it, although that will mean filing a new CCC.
>>> thanks,
>>> Chris
>>>> Tom
>>>> On 2/4/2016 1:36 PM, Chris Plummer wrote:
>>>>> Hello,
>>>>> Please review the following for adding the -XX 
>>>>> UseAggressiveHeapShrink option. When turned on, it tells the GC to 
>>>>> reduce the heap size to the new target size immediately after a 
>>>>> full GC rather than doing it progressively over 4 GCs.
>>>>> Webrev: http://cr.openjdk.java.net/~cjplummer/8146436/webrev.02/
>>>>> Bug: https://bugs.openjdk.java.net/browse/JDK-8146436
>>>>> Testing:
>>>>>   -JPRT with '-testset hotspot'
>>>>>   -JPRT with '-testset hotspot -vmflags 
>>>>> "-XX:+UseAggressiveHeapShrink"'
>>>>>   -added new TestMaxMinHeapFreeRatioFlags.java test
>>>>> thanks,
>>>>> Chris

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