Intentional heap bounds change?

[Stefan Johansson]

Thanks for filing the issue Stefan,

I took a short look at it and have updated the JBS entry.

Cheers,
Stefan

On 2021-08-12 13:16, Stefan Karlsson wrote:
> I logged a Bug for this:
> https://bugs.openjdk.java.net/browse/JDK-8272364
> 
> Thanks,
> StefanK
> 
> On 2021-08-11 13:04, Roman Kennke wrote:
>> Hi,
>>
>> thanks for the comments. I have an example that shows the 'problem':
>>
>> http://cr.openjdk.java.net/~rkennke/Example.java
>>
>> Reproduce it using:
>>
>> java -Xms1g -Xmx4g -XX:+UseParallelOldGC -XX:MinHeapFreeRatio=10 
>> -XX:MaxHeapFreeRatio=20 -XX:GCTimeRatio=4 
>> -XX:AdaptiveSizePolicyWeight=90 
>> -Xlog:gc*=info,gc=debug,gc+heap*=trace,gc+ergo*=trace:file=gc.log:time,uptimemillis,level,tags 
>> Example 300
>>
>> Check the heap log for heap size entries:
>>
>> grep "Pause Young" gc.log
>>
>> This yields the following:
>>
>> http://cr.openjdk.java.net/~rkennke/gc-heapsize.txt
>>
>> It shows that the heap is indeed shrinking well below -Xms, e.g. 
>> ~600MB instead of 1024MB here:
>>
>> GC(12) Pause Young (Allocation Failure) 559M->589M(592M) 4,797ms
>>
>> Thanks for your help!
>> Roman
>>
>>
>>> Hi,
>>>
>>> On 2021-08-05 16:21, Roman Kennke wrote:
>>>> Hello GC devs,
>>>>
>>>> I see a case where heap may shrink below -Xms. This may be 
>>>> intentional or not. -Xms is initial heap size after all, not 
>>>> necessarily minimum heap size. However, there is also documentation 
>>>> that -Xms *does* provide the lower bounds for heap sizing policy, 
>>>> e.g. [1]
>>>
>>> The documentation around this didn't reflect the actual 
>>> implementation in HotSpot. As a part of recent cleanup and 
>>> introduction of the -XX:InitialHeapSize flag, we updated the 
>>> documentation to reflect the way -Xms is actually working:
>>>
>>> https://docs.oracle.com/en/java/javase/16/docs/specs/man/java.html
>>> |
>>> ---
>>> -Xms| /size/
>>>
>>>     Sets the minimum and initial size (in bytes) of the heap. This value
>>>     must be a multiple of 1024 and greater than 1 MB. Append the letter
>>>     |k| or |K| to indicate kilobytes, |m| or |M| to indicate megabytes,
>>>     |g| or |G| to indicate gigabytes. The following examples show how to
>>>     set the size of allocated memory to 6 MB using various units:
>>>
>>>     |-Xms6291456 -Xms6144k -Xms6m|
>>>
>>>     Instead of the |-Xms| option to set both the minimum and initial
>>>     size of the heap, you can use |-XX:MinHeapSize| to set the minimum
>>>     size and |-XX:InitialHeapSize| to set the initial size."
>>>
>>> ---
>>>
>>>
>>> Concretely, the code that parses -Xms sets both MinHeapSize and 
>>> InitialHeapSize:
>>>
>>>
>>>      } else if (match_option(option, "-Xms", &tail)) {
>>>        julong size = 0;
>>>        // an initial heap size of 0 means automatically determine
>>>        ArgsRange errcode = parse_memory_size(tail, &size, 0);
>>> ...
>>>        if (FLAG_SET_CMDLINE(MinHeapSize, (size_t)size) != 
>>> JVMFlag::SUCCESS) {
>>>          return JNI_EINVAL;
>>>        }
>>>        if (FLAG_SET_CMDLINE(InitialHeapSize, (size_t)size) != 
>>> JVMFlag::SUCCESS) {
>>>          return JNI_EINVAL;
>>>        }
>>>
>>>
>>>>
>>>> I believe this is due to this change:
>>>>
>>>> https://urldefense.com/v3/__https://github.com/openjdk/jdk/commit/1ed5b22d6e48ffbebaa53cf79df1c5c790aa9d71*diff-74a766b0aa16c688981a4d7b20da1c93315ce72358cac7158e9b50f82c9773a3L567__;Iw!!ACWV5N9M2RV99hQ!Ydh0taE2rXnnONFDOoIBlbg_dh6YcXqA_2JPPqZUMWcQQvWFah7azM-toyRUxGwfAbPx$ 
>>>>
>>>>
>>>> I.e. instead of adjusting old-gen minimum size with young-gen 
>>>> minimum size, it is set to _gen_alignment.
>>>>
>>>> Is this intentional?
>>>
>>> Hopefully StefanJ will be able to answer this. The code has this 
>>> comment, so maybe it is intentional?
>>>
>>> // Minimum sizes of the generations may be different than
>>> // the initial sizes.  An inconsistency is permitted here
>>> // in the total size that can be specified explicitly by
>>> // command line specification of OldSize and NewSize and
>>> // also a command line specification of -Xms.  Issue a warning
>>> // but allow the values to pass.
>>>
>>> Do you have a command line that reproduces this problem?
>>>
>>> StefanK
>>>
>>>>
>>>> Thanks,
>>>> Roman
>>>>
>>>>
>>>> [1] 
>>>> https://docs.oracle.com/en/java/javase/11/gctuning/factors-affecting-garbage-collection-performance.html#GUID-B0BFEFCB-F045-4105-BFA4-C97DE81DAC5B 
>>>>
>>>>
>>>
>>
>