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

Mike Duigou mike.duigou at oracle.com
Tue Mar 5 22:46:48 UTC 2013

I have updated the webrev to remove the useAltHashing boolean.

http://cr.openjdk.java.net/~mduigou/JDK-8006593/5/webrev/

Mike

On Mar 5 2013, at 00:43 , Peter Levart wrote:

Hi Mike,

You could also get rid of boolean useAltHashing field in both HashMap and Hashtable. It saves 8 bytes in both HM and HT objects in 32bit addressing modes (64bit addressing modes are not affected due to different alignment). Like the following (a patch against your webrev): Index: jdk/src/share/classes/java/util/Hashtable.java =================================================================== --- jdk/src/share/classes/java/util/Hashtable.java (date 1362469665000) +++ jdk/src/share/classes/java/util/Hashtable.java (revision ) @@ -213,12 +213,6 @@ } /** - * If {@code true} then perform alternative hashing of String keys to reduce - * the incidence of collisions due to weak hash code calculation. - */ - transient boolean useAltHashing = false; - - /** * A randomizing value associated with this instance that is applied to * hash code of keys to make hash collisions harder to find. */ @@ -228,8 +222,8 @@ * Initialize the hashing mask value. */ final boolean initHashSeedAsNeeded(int capacity) { - boolean currentAltHashing = useAltHashing; - useAltHashing = sun.misc.VM.isBooted() && + boolean currentAltHashing = hashSeed != 0; + boolean useAltHashing = sun.misc.VM.isBooted() && (capacity >= Holder.ALTERNATIVEHASHINGTHRESHOLD); boolean switching = currentAltHashing ^ useAltHashing; if (switching) { Index: jdk/src/share/classes/java/util/HashMap.java =================================================================== --- jdk/src/share/classes/java/util/HashMap.java (date 1362472425000) +++ jdk/src/share/classes/java/util/HashMap.java (revision ) @@ -224,17 +224,11 @@ } /** - * If {@code true} then perform alternative hashing of String keys to reduce - * the incidence of collisions due to weak hash code calculation. - */ - transient boolean useAltHashing = false; - - /** * A randomizing value associated with this instance that is applied to - * hash code of keys to make hash collisions harder to find. Initialized via - * sun.misc.Unsafe when needed. + * hash code of keys to make hash collisions harder to find. + * Also used (when != 0) to indicate use of alternative String hashing. */ - transient int hashSeed = 0; + transient int hashSeed; /** * Constructs an empty HashMap with the specified initial @@ -319,8 +313,8 @@ * really need it. */ final boolean initHashSeedAsNeeded(int capacity) { - boolean currentAltHashing = useAltHashing; - useAltHashing = sun.misc.VM.isBooted() && + boolean currentAltHashing = hashSeed != 0; + boolean useAltHashing = sun.misc.VM.isBooted() && (capacity >= Holder.ALTERNATIVEHASHINGTHRESHOLD); boolean switching = currentAltHashing ^ useAltHashing; if (switching) { @@ -339,12 +333,9 @@ * in lower bits. Note: Null keys always map to hash 0, thus index 0. */ final int hash(Object k) { - int h = 0; - if (useAltHashing) { - if (k instanceof String) { + int h = hashSeed; + if (h != 0 && k instanceof String) { - return sun.misc.Hashing.stringHash32((String) k); + return sun.misc.Hashing.stringHash32((String) k); - } - h = hashSeed; } h ^= k.hashCode();

Regards, Peter On 03/05/2013 07:08 AM, Mike Duigou wrote: After looking more closely at the single read reference to hashSeed I decided to simplify things and make hashSeed non-final in both Hashtable and HashMap.

I've posted a refreshed webrev. http://cr.openjdk.java.net/~mduigou/JDK-8006593/4/webrev/ Mike On Mar 4 2013, at 14:25 , Peter Levart wrote:

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. c Once review is completed here this patch will be proposed to JDK7u-dev for integration into the next 7u performance/feature release. Mike

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