[jdk21u-dev] RFR: 8293850: need a largest_committed metric for each category of NMT's output

[María Arias de Reyna]

On Fri, 5 Apr 2024 08:58:38 GMT, Severin Gehwolf <sgehwolf at openjdk.org> wrote:

>> This is a backport for https://bugs.openjdk.org/browse/JDK-8293850 (need a largest_committed metric for each category of NMT's output) needed in preparation to backport https://bugs.openjdk.org/browse/JDK-8320061 ( [nmt] Multiple issues with peak accounting )
>> 
>> This backport adds peak usage of the memory to the NMT output to be able to detect spikes.
>> 
>> Example of usage before the fix:
>> 
>> 
>> $ jcmd 160812 VM.native_memory detail | grep "mmap: reserved="
>>                             (mmap: reserved=8388608KB, committed=1277952KB) 
>>                             (mmap: reserved=528392KB, committed=55348KB) 
>>                             (mmap: reserved=197140KB, committed=58260KB) 
>>                             (mmap: reserved=40KB, committed=36KB) 
>>                             (mmap: reserved=8KB, committed=8KB) 
>>                             (mmap: reserved=262144KB, committed=244864KB) 
>>                             (mmap: reserved=1048576KB, committed=34816KB) 
>> 
>> 
>> After the fix (I just started the app, I wasn't really expecting any peak at this point):
>> 
>> 
>> $ jcmd 161688 VM.native_memory detail | grep "mmap: reserved="
>>                             (mmap: reserved=8388608KB, committed=507904KB, largest_committed=0KB) 
>>                             (mmap: reserved=1048576KB, committed=31232KB, largest_committed=0KB) 
>>                             (mmap: reserved=528392KB, committed=32836KB, largest_committed=0KB) 
>>                             (mmap: reserved=197140KB, committed=43220KB, largest_committed=0KB) 
>>                             (mmap: reserved=40KB, committed=36KB, largest_committed=0KB) 
>>                             (mmap: reserved=8KB, committed=8KB, largest_committed=0KB) 
>>                             (mmap: reserved=262144KB, committed=216640KB, largest_committed=0KB)
>
> src/hotspot/share/services/memReporter.cpp line 59:
> 
>> 57:   if (peak != 0) {
>> 58:     output()->print(", largest_committed=" SIZE_FORMAT "%s", amount_in_current_scale(peak), scale);
>> 59:   }
> 
> This seems odd. Not a problem of the backport, but of the original change. For any value lower than `scale` we'll get `0` as a result if I'm reading this right.
> 
> Your example output showed:
> 
> (mmap: reserved=8388608KB, committed=507904KB, largest_committed=0KB) 
> 
> 
> Yet, it shouldn't be there (due to `peak != 0`). We should get this fixed before we backport.

Ok, I'll investigate

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PR Review Comment: https://git.openjdk.org/jdk21u-dev/pull/440#discussion_r1553261238