RFR: 8006593: Performance and compatibility improvements to hash based Map implementations

[Peter Levart]

Hi mike,

I doubt (haven't tried it really with your code) that hashSeed will be 
seen by code to be anything else but 0, since it is initialized to a 
constant value. For example, this code:

public class ModifyingFinalTest {
     static final Unsafe unsafe;
     static final long valueOffset;

     static {
         try {
             Field f = Unsafe.class.getDeclaredField("theUnsafe");
             f.setAccessible(true);
             unsafe = (Unsafe)f.get(null);
             valueOffset = 
unsafe.objectFieldOffset(ModifyingFinalTest.class.getDeclaredField("value"));
         }
         catch (IllegalAccessException | NoSuchFieldException e) {
             throw new Error(e);
         }
     }

     final int value = 0;

     void test() {
         unsafe.putIntVolatile(this, valueOffset, 1);
         printValue();
         unsafe.putIntVolatile(this, valueOffset, 2);
         printValue();
         unsafe.putIntVolatile(this, valueOffset, 3);
         printValue();
     }

     void printValue() {
         System.out.println(value);
     }

     public static void main(String[] args) {
         new ModifyingFinalTest().test();
     }
}


Prints:

0
0
0


It's a different thing, if the initialization is changed to:

final int value = "".length();

But I don't know if each access in source is actually guaranteed to 
translate to a real read of field in this case either. Is 
Unsafe.putIntVolatile() making this happen somehow magically?

Regards, Peter


On 03/04/2013 09:21 PM, Mike Duigou wrote:
> Hello all;
>
> The alternative hashing implementation introduced in 7u6 added an unfortunate bottleneck to the initialization of HashMap and Hashtable instances. This patch avoids the performance bottleneck of using a shared Random instance by using a ThreadLocalRandom instead.
>
> Along with this change are some additional performance improvements to further reduce the overhead of the alternative hashing feature and generally improve the performance of Hashtable or HashMap.
>
> http://cr.openjdk.java.net/~mduigou/JDK-8006593/3/webrev/
>
> Once review is completed here this patch will be proposed to JDK7u-dev for integration into the next 7u performance/feature release.
>
> Mike
>