RFR (XS): CR 8006176: Switch to optimal identity hash code generator

David Holmes david.holmes at oracle.com
Mon Jan 14 14:39:49 PST 2013

On 14/01/2013 11:00 PM, Aleksey Shipilev wrote:
> On 01/14/2013 04:53 PM, David Holmes wrote:
>> This kind of switch can not be made lightly. It is not just a matter of
>> raw performance, we have to understand how the change might affect
>> existing applications. The default affects thousands of programs. A
>> particular program can be run with -XX:hashCode=5 if it suits.
> Agreed with the rationale; what should the appropriate test look like
> for this kind of change?

At a minimum you would need to verify the performance of all the major 
performance apps across all platforms. But changes like this go beyond 
basic benchmarking.

> Disagreed with the conclusion: on these grounds we might as well do not
> change the default behavior at all, frightened about all these
> applications relying on subtleties they were never guaranteed to be
> upheld. Having this in mind, I would say we should use most-performant
> version, and let users to fallback to -XX:hashCode=0 if they identify
> the problem with the new setting, which should be the red flag for their
> applications anyway.

This is a naive view. Changing a default can only be done after a very 
careful analysis and consideration of a wide range of factors. 
Applications become dependent on non-guaranteed behaviour all the time 
without realizing it.

BTW also see 7161302.


> -Aleksey.

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