RFR: 8006593: Performance and compatibility improvements to hash based Map implementations (original) (raw)

Peter Levart peter.levart at gmail.com
Mon Mar 4 22:25:36 UTC 2013

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

More information about the core-libs-dev mailing list