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

[Aleksey Shipilev]

Hi,

This is a simple issue reported by one of our customers, relying heavily
on System.identityHashCode(...) in their code. They figured out the
global state in current identity hashcode generator penalizes
scalability. Their workaround is to switch -XX:hashCode=5 to the
thread-local Marsaglia's XorShift generator.

The motivation is really simple when the targeted microbenchmark is
considered: asking System.identityHashCode(new Object()) continuously in
multiple threads.

On 2x2 Intel i5-2520M, Linux x86_64, JDK 7u12-ea:

1 thread:
-XX:hashCode=0: 11.3 +- 0.9 ops/usec
-XX:hashCode=1: 12.3 +- 0.3 ops/usec
-XX:hashCode=2: 12.8 +- 0.2 ops/usec
-XX:hashCode=3: 11.7 +- 0.1 ops/usec
-XX:hashCode=4: 12.5 +- 0.1 ops/usec
-XX:hashCode=5: 12.1 +- 0.2 ops/usec

4 threads:
-XX:hashCode=0: 19.3 +- 0.1 ops/usec
-XX:hashCode=1: 25.5 +- 0.3 ops/usec
-XX:hashCode=2: 26.8 +- 0.1 ops/usec
-XX:hashCode=3: 20.8 +- 0.1 ops/usec
-XX:hashCode=4: 25.1 +- 0.5 ops/usec
-XX:hashCode=5: 24.9 +- 0.2 ops/usec

So, even on this small machine, the scalability improvement is very
telling 19.3 -> 24.9 ops/usec.

On 2x8x2 SandyBridge box, JDK8 HEAD:

1 thread:
-XX:hashCode=0: 16.9 +- 0.5 ops/usec
-XX:hashCode=1: 19.2 +- 0.1 ops/usec
-XX:hashCode=2: 18.9 +- 0.3 ops/usec
-XX:hashCode=3: 18.9 +- 0.4 ops/usec
-XX:hashCode=4: 19.0 +- 0.1 ops/usec
-XX:hashCode=5: 18.5 +- 0.3 ops/usec

32 threads:
-XX:hashCode=0: 10.7 +- 0.1 ops/usec
-XX:hashCode=1: 175.2 +- 4.9 ops/usec
-XX:hashCode=2: 184.8 +- 3.7 ops/usec
-XX:hashCode=3: 14.2 +- 0.1 ops/usec
-XX:hashCode=4: 160.0 +- 2.6 ops/usec
-XX:hashCode=5: 176.6 +- 6.0 ops/usec

Note whooping scalability increase when dealing with non-shared generators.

Randomicity is another concern, and our internal tests show that
thread-local PRNG (mode 5) is as good as the global PRNG (mode 0), you
can see that here:
 http://cr.openjdk.java.net/~shade/8006176/randomicity/

It turns out that modes 0, 2, and 5 are random enough; and mode 5 is the
most scalable one. Hence, we'd like to switch to mode 5, here's the webrev:
 http://cr.openjdk.java.net/~shade/8006176/webrev.00/

Thanks,
-Aleksey.