8020860: cluster Hashtable/Vector field updates for better transactional memory behaviour

[David Holmes]

Mike,

On 26/03/2014 6:37 AM, Mike Duigou wrote:
> Hello all;
>
> Recently HotSpot gained additional support for transactional memory, <https://bugs.openjdk.java.net/browse/JDK-8031320>. This patch is a libraries followon to that change. RTM and other transactional memory implementations benefit from clustering writes towards the end of the transaction whenever possible. This change optimizes the behaviour of two collection classes, Hashtable and Vector by moving several field updates to cluster them together near the end of the transaction. Yes, we know, these are obsolete collections but these two classes were used as the basis for the original benchmarking and evaluation during the development of the transactional memory JVM features. Future changes providing similar optimizations to other collections will be pursued when it can be shown they offer value and don't add a cost to non TM performance (TM is not yet a mainstream feature).
>
> https://bugs.openjdk.java.net/browse/JDK-8020860
> http://cr.openjdk.java.net/~mduigou/JDK-8020860/0/webrev/
>
> It is not expected that this change will have any meaningful impact upon performance (either positive or negative) outside of TM-enabled configurations. The main change is to move existing field updates towards the end of the transaction and avoid conditionals between field updates.
>
> There is a slight behaviour change introduced in this changeset. Previously some methods updated the modcount unconditionally updated even when an ArrayIndexOutOfBoundsException was subsequently thrown for an invalid index and the Vector was not modified. With this change the modcount will only be updated if the Vector is actually changed. It is not expected that applications will have relied or should have relied on this behaviour.

I could live with that change in behaviour, but this change completely 
breaks the fail-fast semantics of the iterators in some cases! If you 
don't update modCount until after the change is complete, the iterator 
may access the updated state and not throw CME!.

I think this change is misguided.

David
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> Mike
>