RFR [9] 8151384: Examine sun.misc.ASCIICaseInsensitiveComparator

[Peter Levart]

Hi Chris,

On 03/09/2016 12:07 PM, Chris Hegarty wrote:
> Hi Peter,
>
> On 9 Mar 2016, at 08:42, Peter Levart <peter.levart at gmail.com> wrote:
>
>> Hi Chris,
>>
>> While Attribute.Name.equals() should be comparably fast, what about Attribute.Name.hashCode() ? Could you for example extend the AvailableCharsetsCompare benchmark with a a @Setup method that constructs (via Attributes.putValue(String, String)) a java.util.jar.Attributes instance with available charsets as keys, then a @Benchmark method that traverses the Attributes instance looking-up each available charset via the Attributes.getValue(String) ... Just to see the relative amount of regression that might be caused by constructing intermediate lowercase String(s)…
> Just for clarification, the change in java.util.jar.Attributes has nothing to do
> with the charsets. Attributes stores the individual attributes from the Jar
> Manifest. There are only a relatively small number of standard Attributes [1].
>
> When digging through old history to try to find out why java.util.jar.Attributes
> was ever using ASCIICaseInsensitiveComparator, it was not clear that
> performance was the motivation.

I guess looking-up a manifest attribute is not a performance critical 
operation, you are right.

> It seemed to be a knock on effect of the
> use of the comparator ( which was trying to avoid the continuous allocation
> cost to toLowerCase during comparisons ). I don’t think this code is particularly
> performance sensitive, and especially not hashCode.

The effects of equals() vs. hashCode() on the lookup performance 
(Attributes.getValue()) is approximately the same. As keys of HashMap 
are Name objects and each lookup constructs new Name object, there's 
typically one Name.hashCode() invocation and one Name.equals() 
invocation per lookup (hashCode caching doesn't help here).

>
> Maybe a benchmark with a number of standard Jar file attributes would make
> sense?

Right. Let me assist you with creating it. Stay tuned...

Regards, Peter

>
> -Chris.
>
> [1] http://docs.oracle.com/javase/8/docs/technotes/guides/jar/jar.html#Main_Attributes