RFR(S) 8186770: NMT: Report metadata information in NMT summary

[Zhengyu Gu]

Hi Andrew,

>> Class summary:
>>
>> -                     Class (reserved=1071790KB, committed=24750KB)
>>                              (classes #3078)
>>                              (malloc=686KB #7122)
>>                              (mmap: reserved=1071104KB, committed=24064KB)
>>                              (  Metadata:                               )
>>                              (    reserved=22528KB, committed=21504KB)
>>                              (    capacity=21327KB, used=20654KB)
>>                              (    free chunks=113KB)
>>                              (    available=0KB)
>>                              (  Class space:                            )
>>                              (    reserved=1048576KB, committed=2560KB)
>>                              (    capacity=2525KB, used=2268KB)
>>                              (    free chunks=0KB)
>>                              (    available=35KB)
> I think this change is to ship modulo a few small quibbles.
> 
> The four figures quoted here for each of the data and class metaspace
> regions don't quite add up i.e.
> 
>    used + free chunks + available = total_in_use =/= capacity
> 
>    2268 + 0 + 35    =  2303   =/=  2525
> 
>    20654 + 113 + 0  =  20767  =/=  21327
> 
I struggled to come out an intuitive way for representing the numbers. 
Hopefully, we can get it right through this review process.

Actually, the formula should be:
  committed = capacity + free chunks + available + waste


capacity :   amount of all in-used chunks.
used:        used amount out of capacity
free chunks: amount of free chunk memory
available:   memory that was committed, but has yet to slice into chunks.


> 
> As I understand it this is because of waste caused either by the need to
> insert block and chunk headers or by the inability to allocate objects
> out of small fragments at the end of in use chunks. Is that correct?
> 
chunk headers are counted in *used* memory.


> If so then would it not be clearer to account for this waste explicitly?
> e.g.
> 
>                              (  Metadata:                            )
>                              (    reserved=22528KB,   committed=21504KB)
>                              (    capacity=21327KB,   used=20654KB)
>                              (    free chunks=113KB,  available=0KB)
>                              (    waste = 560KB = 2.6%)

Make sense to report *waste*. How about
                               (  Metadata:                            )
                               (    reserved=22528KB,  committed=21504KB)
                               (    capacity=21327KB,   used=20654KB)
                               (    free chunks=113KB)
                               (    available=0KB)
                               (    waste = 560KB = 2.6%)

Thanks,

-Zhengyu

> n.b. the above figures are calculated as
> 
>    waste = capacity - total_in_use
>    waste% = waste / capacity
> 
> Also, whether or not waste space gets reported, I think the output would
> look cleaner if you were to report free and available space on one line.
> 
> I'm happ
> 
> regards,
> 
> 
> Andrew Dinn
> -----------
> Senior Principal Software Engineer
> Red Hat UK Ltd
> Registered in England and Wales under Company Registration No. 03798903
> Directors: Michael Cunningham, Michael ("Mike") O'Neill, Eric Shander
>