RFR: 8212879: Make JVMTI TagMap table concurrent [v4]

[Coleen Phillimore]

On Tue, 3 Nov 2020 14:47:35 GMT, Kim Barrett <kbarrett at openjdk.org> wrote:

>> Coleen Phillimore has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains five commits:
>> 
>>  - Merge branch 'master' into jvmti-table
>>  - Merge branch 'master' into jvmti-table
>>  - More review comments from Stefan and ErikO
>>  - Code review comments from StefanK.
>>  - 8212879: Make JVMTI TagMap table not hash on oop address
>
> src/hotspot/share/prims/jvmtiTagMap.cpp line 3018:
> 
>> 3016:         }
>> 3017:         // Later GC code will relocate the oops, so defer rehashing until then.
>> 3018:         tag_map->_needs_rehashing = true;
> 
> This is wrong for some collectors. I think all collectors ought to be calling set_needs_rehashing in appropriate places, and it can't be be correctly piggybacked on the num-dead callback. (See discussion above for that function.)
> 
> For example, G1 remark pause does weak processing (including weak oopstorage) and will call the num-dead callback, but does not move objects, so does not require tagmap rehashing.
> 
> (I think CMS oldgen remark may have been similar, for what that's worth.)

Ok, so I'm going to need help to know where in all the different GCs to make this call.

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PR: https://git.openjdk.java.net/jdk/pull/967