On Wed, 4 Nov 2020 08:56:54 GMT, Kim Barrett <[email protected]> wrote:
>> Coleen Phillimore has updated the pull request incrementally with one
>> additional commit since the last revision:
>>
>> Code review comments from Kim and Albert.
>
> 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.
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PR: https://git.openjdk.java.net/jdk/pull/967
