Question about VirtualSpace and MPSS support

[Erik Helin]

On 11/09/2016 04:00 PM, Thomas Stüfe wrote:
> Hi all,

Hey Thomas, sorry my somewhat late reply.

> please help me understand some details about the VirtualSpace class.
>
> Background: I am experimenting with putting humongous metaspace chunks at
> the end of the VirtualSpace (part of
> https://bugs.openjdk.java.net/browse/JDK-8166690). So I would like to have
> a VirtualSpace which allows me to allocate from both bottom and top.
> Therefore I am checking how difficult it would be to extend the
> VirtualSpace class to allow me to grow from both sides.
>
> Now I try to understand the three-region-logic implemented to support
> multiple page sizes:
>
> VirtualSpace has three regions with three different granularities. I think
> the intent is to support large pages for address ranges which are not
> necessarily large page aligned, so this leaves small-paged regions at top
> and/or bottom.
>
> Looking at VirtualSpace::expand_by() I think the idea is that there is
> always one continuous committed area spanning those three regions, right?
> (instead of, say, each of the regions being committed independently). So,
> just one high water mark, which is either in the low, middle or upper
> region. If it is in the middle region, low region should be completely
> committed, if it is in the upper region, low and middle regions should be
> completely committed? I think it must be this way, because from the outside
> I only should have one continuous committed address range, otherwise it
> would be difficult to use this VirtualSpace.
>
> But then, I have a function like VirtualSpace::actual_committed_size() - in
> addition to VirtualSpace::committed_size() - which counts the committed
> area for all three regions separately. Is there ever a case where comitted
> size and actual comitted size are different?

Yes, consider the case when you have small pages for the "lower" part of 
the Virtualspace. Due to how paging works in operating systems, you 
can't commit less memory than the size of a small page. So even if a 
user of a VirtualSpace instance wants to expand by a half small page 
size, the OS will still commit a whole small page (the OS can't work on 
a smaller granularity). If that has happened, then commited_size() will 
return half a small page size as its result and actual_commited_size 
will return one small page size as its result. Do you follow?

> Can it be that lower regions are partly comitted even if higher regions are already used?

No, that should never happen (if it does, then that is a bug). The 
following assert should trigger in that case (for fastdebug or debug 
builds):
http://hg.openjdk.java.net/jdk9/hs/hotspot/file/4d1cc698c0b6/src/share/vm/memory/virtualspace.cpp#l734

Thanks,
Erik

> Thank you!
>
> Kind Regards, Thomas
>