RFR: 8251158: Implementation of JEP 387: Elastic Metaspace

Thomas Stuefe stuefe at openjdk.java.net
Thu Sep 24 13:09:36 UTC 2020

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.


> 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


Commit messages:
 - Remove trailing whitespaces
 - JEP387 review feedback round 2
 - Review version 2 (2020-09-04)

Changes: https://git.openjdk.java.net/jdk/pull/336/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=336&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8251158
  Stats: 23038 lines in 170 files changed: 14916 ins; 6912 del; 1210 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

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