RFR: 8251158: Implementation of JEP 387: Elastic Metaspace [v4]

[Thomas Stuefe]

> Hi all,
> 
> this is the continuation of the ongoing review for the JEP387 implementation (last rounds see [1] [2]). Sorry for the
> delay, had vacation then the entrance of Skara delayed things a bit.
> For the delta diff please see [3].
> 
> This is the first time I do a large PR after Skara, so if something is wrong please bear with me. I cannot answer all
> feedback individually in this PR body, but I incorporated almost all into the new revision.
> What changed since the last version:
> 
> - I renamed most metaspace files back to the original naming scheme or to something similar, hopefully capturing the
>   group consent.
> 
> - I changed the way allocation guards are checked if MetaspaceGuardAllocations is enabled. Before, I would test for
>   overwrites upon CLD destruction, but since that check was subject to VerifyMetaspaceInterval it only ran for every nth
>   class loader which made it rather pointless. Now I run it always.
> 
> - I also improved the printout on block corruption, and log block corruption unconditionally before asserting.
> 
> - I also fixed up and commented the death test which tests for allocation overwriters (test_allocationGuard.cpp)
> 
> Side note, I find the corruption check very useful but if you guys think it is too much I still can remove the feature.
> 
> - In ChunkManager::purge() I improved the comments after discussions with Leo.
> 
> - I fixed a bug with VerifyMetaspaceInterval: if set to 1 the "SOMETIMES" sections were supposed to fire always, but due
>   to a one-off error they only fired every second time. Now, if -XX:VerifyMetaspaceInterval=1, the checks really run
>   every time.
> 
> - Fixed indentation issues as Leo requested
> 
> - Rewrote the condition and the assert in VirtualSpaceList::allocate_root_chunk() as Leo requested
> 
> - I removed the "can_purge" logic from VirtualSpaceList. The list does not need to know. It just should iterate all nodes
>   and attempt purging, and if a node does not own its ReservedSpace, it refuses to be purged. That is simpler and more
>   flexible since it allows us to have list with purge-able and non-purge-able nodes.
> 
> - and various smaller fixes, mainly on request of Leo.
> 
> @lkorinth:
> 
>> VirtualSpaceNode.hpp
>>
>>102   // Start pointer of the area.
>>103   MetaWord* const _base;
>>
>>How does this differ from _rs._base? Really needed?
>>
>>105   // Size, in words, of the whole node
>>106   const size_t _word_size;
>>
>>Can we not calculate this from _rs.size()?
> 
> You are right, _base and _word_size are directly related to the underlying space. But I'd prefer to leave it the way it
> is. Mainly because ReservedSpace::_base and ::_size are nonconst and theoretically can change under me. It is highly
> improbable but I'd like to know. Note that VirtualSpaceNode::verify checks that.  Should we clean up ReservedSpace at
> some point and make those members const - as they should be - then I would rewrite this as you suggest.
> Thanks, again, for all your review work!
> 
> ------
> 
> 
> [1] https://mail.openjdk.java.net/pipermail/hotspot-runtime-dev/2020-August/041162.html
> [2] https://mail.openjdk.java.net/pipermail/hotspot-runtime-dev/2020-September/041628.html
> [3] https://github.com/openjdk/jdk/commit/731f795bc0c1c502dc6cac8f866ff45a15bdd02d

Thomas Stuefe has updated the pull request incrementally with one additional commit since the last revision:

  Make MetaspaceGuardAllocations a diagnostic flag (2)

-------------

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/336/files
  - new: https://git.openjdk.java.net/jdk/pull/336/files/0e414946..e64d8f02

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=336&range=03
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=336&range=02-03

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/336.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/336/head:pull/336

PR: https://git.openjdk.java.net/jdk/pull/336