RFR(M): 8218049: Survivor MemoryMXBean used() size granularity is region based

[sangheon.kim at oracle.com]

Hi Kim,

On 4/4/19 1:45 PM, Kim Barrett wrote:
>> On Apr 4, 2019, at 2:35 PM, sangheon.kim at oracle.com wrote:
>>
>> Hi Kim and Thomas,
>>
>> On 4/4/19 11:08 AM, Kim Barrett wrote:
>>>> For this change I suggest going back to use the subtract_until_zero()
>>>> method (whether we use the racy region counts or the actual "used
>>>> bytes" makes not difference with respect to the problem) and
>>>> investigate improvements in a separate CR.
>>> That seems like a reasonable plan.
>> For other reviews: Kim, Thomas and I had separate discussion mostly suggestions from Kim to fix the inconsistency issue but we concluded to revert to use subtract_until_zero().
>>
>> Here's the updated webrev which includes:
>> 1) Remove newly added barriers and use subtract_until_zero():
>>      I filed JDK-8221994 for further investigation.
>> 2) Change indent at G1SurvivorRegions (from Kim)
>>
>> http://cr.openjdk.java.net/~sangheki/8218049/webrev.3
>> http://cr.openjdk.java.net/~sangheki/8218049/webrev.3_inc/
>>
>> Thanks,
>> Sangheon
> ------------------------------------------------------------------------------
> src/hotspot/share/gc/g1/g1MonitoringSupport.cpp
>   236   assert(_overall_used >= _eden_space_used + _survivor_space_used,
>   237          "Overall used(" SIZE_FORMAT ") should be larger than or equal to sum(" SIZE_FORMAT ") of"
>   238          " eden(" SIZE_FORMAT ") and survivor(" SIZE_FORMAT ")",
>   239          _overall_used, _eden_space_used + _survivor_space_used,
>   240          _eden_space_used, _survivor_space_used);
>   241   _old_gen_used = subtract_up_to_zero(_overall_used, _eden_space_used + _survivor_space_used);
>
> That (new) assert can fail because of inconsistent values. That's
> pretty much the point of the careful subtraction.
>
> Also, I wouldn't add (back) subtract_up_to_zero for just this use.
> Instead, I suggest doing something like (with a comment about why)
>
>    _old_gen_used = _overall_used - MIN2(_overall_used, _eden_space_used + _survivor_space_used);
>
> ------------------------------------------------------------------------------
Looks better and updated as you suggested.
Sorry for not removing the assert!

http://cr.openjdk.java.net/~sangheki/8218049/webrev.4
http://cr.openjdk.java.net/~sangheki/8218049/webrev.4_inc/

Thanks,
Sangheon