RFR: 8212879: Make JVMTI TagMap table concurrent [v5]

[Coleen Phillimore]

On Wed, 4 Nov 2020 13:19:21 GMT, Coleen Phillimore <coleenp at openjdk.org> wrote:

>> src/hotspot/share/prims/jvmtiTagMapTable.hpp line 36:
>> 
>>> 34: class JvmtiTagMapEntryClosure;
>>> 35: 
>>> 36: class JvmtiTagMapEntry : public HashtableEntry<WeakHandle, mtServiceability> {
>> 
>> By using utilities/hashtable this buys into having to use HashtableEntry, which includes the _hash member, even though that value is trivially computed from the key (since we're using address-based hashing here). This costs an additional 8 bytes (_LP64) per entry (a 25% increase) compared to the old JvmtiTagHashmapEntry.  (I think it doesn't currently make a difference on !_LP64 because of poorly chosen layout in the old code, but fixing that would make the difference 33%).
>> 
>> It seems like it should not have been hard to replace the oop _object member in the old code with a WeakHandle while otherwise maintaining the Entry interface, allowing much of the rest of the code to remain the same or similar and not incurring this additional space cost.
>
> Yes, there is 64/32 bits extra per hashtable entry with the standard hashtable implementation.  It wouldn't have been hard to replace the oop object, but using shared code was a goal of this change.

So looking at the concurrent hashtable, the entries can be created without saving the hashcode.  I was going to use that at first but didn't want to cut/paste the boilerplate to do so and the jvmti tag map hashtable is always accessed with a lock.  This could be a future RFE if necessary and would also serve to eliminate another ad-hoc hashtable.

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