On Wed, 4 Nov 2020 13:19:21 GMT, Coleen Phillimore <[email protected]> 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
