Review request (S) JDK-8004241 NPG: Metaspace occupies more memory than specified by -XX:MaxMetaspaceSize option

[Mikael Gerdin]

Jon,

On 2013-03-07 21:57, Jon Masamitsu wrote:
>
>
> On 03/07/13 00:54, Mikael Gerdin wrote:
>> Hi
>>
>>
>> When deciding when to reserve more metaspace memory we erroneously
>> looked only at the "capacity" of the metaspace insted of the reserved
>> space (which is what we ask this function when expanding).
>
> Using MetaspaceAux::reserved_in_bytes() means that we
> could return false here
>
> 1105   if (!FLAG_IS_DEFAULT(MaxMetaspaceSize)&&
> 1106       MetaspaceAux::reserved_in_bytes()>= MaxMetaspaceSize) {
> 1107     return false;
> 1108   }
>
> when most of the space reserved in one or two  VirtualSpace's is unused.
> With the current value of parameters, that could almost be 512kb.

Yes.

I guess the question is:
Should MaxMetaspaceSize limit the amount of virtual address space 
reserved for metaspaces or should it limit the amount of committed pages?

To be consistent with how the Java heap is handled my opinion is that we 
should limit the amount of reserved memory.

I any case the current version is incorrect since 
MetaspaceGC::should_expand was queried to determine if we should reserve 
more virtual address space or not and the size check was against the 
amount of committed memory.

/Mikael

>
> Jon
>
>>
>> Additionally, we didn't check MaxMetaspaceSize against the sum of
>> reserved(Class) + reserved(NonClass) which caused us to use more than
>> MaxMetaspaceSize even when it was set.
>>
>> Bug:
>> http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=8004241
>> (not yet available at the time of writing this mail)
>>
>> Webrev:
>> http://cr.openjdk.java.net/~mgerdin/8004241/webrev.0
>>
>> Testing:
>> JPRT with -XX:MaxMetaspaceSize set for all tests to exercise the code
>> path.