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

Remi Forax forax at univ-mlv.fr
Tue Mar 25 22:25:01 UTC 2014

On 03/25/2014 09:37 PM, 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.
> Mike

Hi Mike,
I don't like the principle behind this patch.
Your changing the behavior because it's better with the current state of 
the implementation of the RTM.
First, as Martin said, you need a very good reason to do that and it's a 
kind of black magic change, when the RTM code will change you will have 
to update the JDK code too.

It's better that the VM cluster the write itself by pushing the writes 
at the end of every code path if RTM is enabled
(I don't say that this is easy to implement that but it will benefit to 
more code).


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