On 03/04/2013 11:14 PM, Peter Levart wrote:
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?
Well, that's not what I wanted to ask. I wanted to ask if the first
access in source that happens after Unsafe.putIntVolatile() in same
thread is guaranteed to read the field and not re-use a cached value
ready in some register for example. 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
