Hi,
here is a revision of Daniel's patch, from Daniel, that revises a bug
with InheritableThreadLocals not calling childValue. This bug was found
through the JSR 166 TCK, which now passes its thread local tests.
Thanks,
Ian
Daniel Frampton wrote:
In order to tease out where the performance differences were coming
from, I implemented a map specialized for thread locals
(ThreadLocalMap.java) and found that the performance is essentially the
same as the version Ian posted earlier that stores ThreadLocal data in a
per-thread direct indexed array.
Steve ran the numbers for first and third iteration performance (-1 and
-3).
http://cs.anu.edu.au/people/Steve.Blackburn/jikesrvm/vole-2007-08-22-Wed
-213259-1.txt
http://cs.anu.edu.au/people/Steve.Blackburn/jikesrvm/vole-2007-08-22-Wed
-213259-3.txt
All numbers are from JikesRVM and listed from left to right are the
original (WeakIdentityHashMap) the array version suggested by Ian, and
the ThreadLocalMap version.
The implementation is essentially a map of weak references to values
that are lazily cleaned up as the map is manipulated. I think this is a
good balance of performance and design, but I am open to suggestions. To
allow other VMs to benefit from this I have made all these changes
within classpath (patch attached).
The job still remains to improve the WeakIdentityHashMap so it does not
perform allocation as it searches the buckets in the fast case for a
simple get(), but I believe that optimizing the ThreadLocal case is
worthwhile regardless as in some benchmarks they are performance
critical.
Thanks & Regards,
Daniel Frampton.
--- java/lang/Thread.java 2006-12-11 10:06:55.000000000 +1100
+++ java/lang/Thread.java 2007-08-22 19:20:32.000000000 +1000
@@ -159,7 +159,7 @@
/** Thread local storage. Package accessible for use by
* InheritableThreadLocal.
*/
- WeakIdentityHashMap locals;
+ final ThreadLocalMap locals;
/** The uncaught exception handler. */
UncaughtExceptionHandler exceptionHandler;
@@ -367,6 +367,7 @@
this.name = name.toString();
this.runnable = target;
this.stacksize = size;
+ this.locals = new ThreadLocalMap();
synchronized (Thread.class)
{
@@ -398,6 +399,7 @@
*/
Thread(VMThread vmThread, String name, int priority, boolean daemon)
{
+ this.locals = new ThreadLocalMap();
this.vmThread = vmThread;
this.runnable = null;
if (name == null)
@@ -1063,21 +1065,15 @@
{
group.removeThread(this);
vmThread = null;
- locals = null;
+ locals.clear();
}
/**
* Returns the map used by ThreadLocal to store the thread local values.
*/
- static Map getThreadLocals()
+ static ThreadLocalMap getThreadLocals()
{
- Thread thread = currentThread();
- Map locals = thread.locals;
- if (locals == null)
- {
- locals = thread.locals = new WeakIdentityHashMap();
- }
- return locals;
+ return currentThread().locals;
}
/**
--- java/lang/ThreadLocal.java 2006-12-11 07:25:44.000000000 +1100
+++ java/lang/ThreadLocal.java 2007-08-22 18:45:24.000000000 +1000
@@ -37,9 +37,6 @@
package java.lang;
-import java.util.Map;
-
-
/**
* ThreadLocal objects have a different state associated with every
* Thread that accesses them. Every access to the ThreadLocal object
@@ -93,13 +90,31 @@
* user. Do not expose this to the public. Package visible for use by
* InheritableThreadLocal
*/
- static final Object sentinel = new Object();
+ static final Object notFound = new Object();
+
+ /**
+ * The base for the computation of the next hash for a thread local.
+ */
+ private static int nextHashBase = 1;
+
+ /**
+ * Allocate a new hash.
+ */
+ private synchronized int computeNextHash() {
+ return nextHashBase++ * 6709;
+ }
+
+ /**
+ * Hash code computed for ThreadLocalMap
+ */
+ final int fastHash;
/**
* Creates a ThreadLocal object without associating any value to it yet.
*/
public ThreadLocal()
{
+ fastHash = computeNextHash();
}
/**
@@ -125,16 +140,16 @@
*/
public T get()
{
- Map<ThreadLocal<T>,T> map = (Map<ThreadLocal<T>,T>)
Thread.getThreadLocals();
+ ThreadLocalMap map = Thread.getThreadLocals();
// Note that we don't have to synchronize, as only this thread will
// ever modify the map.
- T value = map.get(this);
- if (value == null)
+ T value = (T) map.get(this);
+ if (value == notFound)
{
value = initialValue();
- map.put(this, (T) (value == null ? sentinel : value));
+ map.set(this, value);
}
- return value == (T) sentinel ? null : value;
+ return value;
}
/**
@@ -147,10 +162,10 @@
*/
public void set(T value)
{
- Map map = Thread.getThreadLocals();
+ ThreadLocalMap map = Thread.getThreadLocals();
// Note that we don't have to synchronize, as only this thread will
// ever modify the map.
- map.put(this, value == null ? sentinel : value);
+ map.set(this, value);
}
/**
@@ -160,7 +175,7 @@
*/
public void remove()
{
- Map map = Thread.getThreadLocals();
+ ThreadLocalMap map = Thread.getThreadLocals();
map.remove(this);
}
}
--- java/lang/InheritableThreadLocal.java 2006-12-11 07:25:44.000000000
+1100
+++ java/lang/InheritableThreadLocal.java 2007-08-22 19:05:57.000000000
+1000
@@ -37,10 +37,6 @@
package java.lang;
-import gnu.java.util.WeakIdentityHashMap;
-
-import java.util.Iterator;
-
/**
* A ThreadLocal whose value is inherited by child Threads. The value of the
* InheritableThreadLocal associated with the (parent) Thread is copied to
@@ -97,24 +93,6 @@
{
// The currentThread is the parent of the new thread.
Thread parentThread = Thread.currentThread();
- if (parentThread.locals != null)
- {
- Iterator keys = parentThread.locals.keySet().iterator();
- while (keys.hasNext())
- {
- Object key = keys.next();
- if (key instanceof InheritableThreadLocal)
- {
- InheritableThreadLocal local = (InheritableThreadLocal)key;
- Object parentValue = parentThread.locals.get(key);
- Object childValue = local.childValue(parentValue == sentinel
- ? null : parentValue);
- if (childThread.locals == null)
- childThread.locals = new WeakIdentityHashMap();
- childThread.locals.put(key, (childValue == null
- ? sentinel : childValue));
- }
- }
- }
+ childThread.locals.inherit(parentThread.locals);
}
}
--- java/lang/ThreadLocalMap.java 2007-04-15 21:49:35.000000000 +1000
+++ java/lang/ThreadLocalMap.java 2007-08-22 19:31:21.000000000 +1000
@@ -0,0 +1,325 @@
+/* ThreadLocal -- a variable with a unique value per thread
+ Copyright (C) 2000, 2002, 2003, 2004, 2005, 2006 Free Software Foundation,
Inc.
+
+This file is part of GNU Classpath.
+
+GNU Classpath is free software; you can redistribute it and/or modify
+it under the terms of the GNU General Public License as published by
+the Free Software Foundation; either version 2, or (at your option)
+any later version.
+
+GNU Classpath is distributed in the hope that it will be useful, but
+WITHOUT ANY WARRANTY; without even the implied warranty of
+MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
+General Public License for more details.
+
+You should have received a copy of the GNU General Public License
+along with GNU Classpath; see the file COPYING. If not, write to the
+Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
+02110-1301 USA.
+
+Linking this library statically or dynamically with other modules is
+making a combined work based on this library. Thus, the terms and
+conditions of the GNU General Public License cover the whole
+combination.
+
+As a special exception, the copyright holders of this library give you
+permission to link this library with independent modules to produce an
+executable, regardless of the license terms of these independent
+modules, and to copy and distribute the resulting executable under
+terms of your choice, provided that you also meet, for each linked
+independent module, the terms and conditions of the license of that
+module. An independent module is a module which is not derived from
+or based on this library. If you modify this library, you may extend
+this exception to your version of the library, but you are not
+obligated to do so. If you do not wish to do so, delete this
+exception statement from your version. */
+
+package java.lang;
+
+import java.lang.ref.WeakReference;
+
+/**
+ * ThreadLocalMap is the basic storage for the map of ThreadLocal instance
+ * to a thread's current value.
+ *
+ * Some applications really work out ThreadLocals, leading to this
+ * optimized implementation.
+ */
+final class ThreadLocalMap
+{
+ /**
+ * The log (base 2) of the initial size of the map
+ */
+ private static final int LOG_INITIAL_SIZE = 3;
+
+ /**
+ * The maximum occupancy rate (after which we grow)
+ */
+ private static final float MAX_OCCUPANCY = 0.7f;
+
+ /**
+ * The target occupancy rate.
+ */
+ private static final float TARGET_OCCUPANCY = 0.5f;
+
+ /**
+ * The deleted entry sentinel value.
+ */
+ private static final Entry deletedEntry = new Entry(null);
+
+ /**
+ * Constructor
+ */
+ ThreadLocalMap()
+ {
+ /* Dummy value to ensure fast path can be optimized */
+ entries = new Entry[1];
+ hashMask = 0;
+ count = 0;
+ }
+
+ /**
+ * The map entries
+ */
+ private Entry[] entries;
+
+ /**
+ * Used for start index computation
+ */
+ private int hashMask;
+
+ /**
+ * The number of entries currently in the map
+ */
+ private int count;
+
+ /**
+ * Create or grow the table to the specified size. The size must be a
+ * power of two for the efficient mask/hash computation.
+ *
+ * @param newSize The new table size.
+ */
+ private void newEntryArray(int newSize)
+ {
+ int mask = newSize - 1;
+ Entry[] oldEntries = this.entries;
+ this.entries = new Entry[newSize];
+ this.hashMask = mask;
+
+ /* Copy old entries to new table */
+ count = 0;
+ if (oldEntries != null)
+ {
+ for(Entry e: oldEntries)
+ {
+ if (e != null)
+ {
+ ThreadLocal<?> key = e.get();
+ if (e != deletedEntry && key != null)
+ {
+ for(int i = key.fastHash & mask;; i = (i + 1) & mask)
+ {
+ if (entries[i] == null)
+ {
+ entries[i] = e;
+ count++;
+ break;
+ }
+ }
+ }
+ }
+ }
+ }
+ }
+
+ /**
+ * We have run out of space in our locals. We use this as the
+ * trigger to attempt to find unused slots as ThreadLocals have
+ * died. If we recover any slots this way then we do not grow.
+ */
+ private void overflow()
+ {
+ /* First 'actual' use */
+ if (entries.length == 1)
+ {
+ newEntryArray(1 << LOG_INITIAL_SIZE);
+ return;
+ }
+
+ /* Attempt to recover unused slots */
+ int deleted = 0;
+ for(int i=0; i < entries.length; i++)
+ {
+ Entry e = entries[i];
+ if (e != null)
+ {
+ if (e == deletedEntry)
+ {
+ deleted++;
+ }
+ else if (e.get() == null)
+ {
+ entries[i] = deletedEntry;
+ deleted++;
+ }
+ }
+ }
+
+ if ((count-deleted) <= (TARGET_OCCUPANCY * entries.length))
+ {
+ /* We currently rehash by simple reallocating into a same-sized table.
+ * An alternative would be to implement a clever hashing algorithm but
+ * as this happens infrequently this seems preferred */
+ newEntryArray(entries.length);
+ return;
+ }
+
+ /* Double the size */
+ newEntryArray(entries.length << 1);
+ }
+
+ /**
+ * This is the class that is used to refer to a thread local weakly.
+ *
+ * As we want to minimize indirections we extend WeakReference.
+ */
+ static final class Entry extends WeakReference<ThreadLocal<?>> {
+ /**
+ * The value stored in this slot
+ */
+ Object value;
+
+ /**
+ * Constructor
+ */
+ Entry(ThreadLocal<?> threadLocal) {
+ super(threadLocal);
+ }
+ }
+
+ /**
+ * Gets the value associated with the ThreadLocal object for the currently
+ * executing Thread. If this is the first time the current thread has called
+ * get(), and it has not already called set(), the sentinel value is
returned.
+ *
+ * @return the value of the variable in this thread, or sentinel if not
present.
+ */
+ public Object get(ThreadLocal<?> key)
+ {
+ int mask = this.hashMask;
+ for(int i = key.fastHash & mask;; i = (i + 1) & mask) {
+ Entry e = entries[i];
+ if (e != null) {
+ if (e.get() == key) {
+ return e.value;
+ }
+ } else {
+ return ThreadLocal.notFound;
+ }
+ }
+ }
+
+ /**
+ * Sets the value associated with the ThreadLocal object for the currently
+ * executing Thread. This overrides any existing value associated with the
+ * current Thread and prevents <code>initialValue()</code> from being
+ * called if this is the first access to this ThreadLocal in this Thread.
+ *
+ * @param value the value to set this thread's view of the variable to
+ */
+ public void set(ThreadLocal<?> key, Object value)
+ {
+ /* Overflow ? */
+ if ((count+1) >= (MAX_OCCUPANCY * entries.length))
+ {
+ overflow();
+ }
+
+ /* Set the entry */
+ int mask = this.hashMask;
+ for(int i = key.fastHash & mask;; i = (i + 1) & mask)
+ {
+ Entry e = entries[i];
+ if (e == null || e == deletedEntry)
+ {
+ /* Create entry */
+ if (e == null) count++;
+ entries[i] = e = new Entry(key);
+ e.value = value;
+ return;
+ }
+ else
+ {
+ ThreadLocal<?> entryKey = e.get();
+ if (entryKey == null)
+ {
+ entries[i] = deletedEntry;
+ }
+ else if (entryKey == key)
+ {
+ /* Update entry */
+ e.value = value;
+ return;
+ }
+ }
+ }
+ }
+
+ /**
+ * Removes the value associated with the ThreadLocal object for the
+ * currently executing Thread.
+ * @since 1.5
+ */
+ public void remove(ThreadLocal<?> key)
+ {
+ int mask = this.hashMask;
+ for(int i = key.fastHash & mask;; i = (i + 1) & mask)
+ {
+ Entry e = entries[i];
+ if (e != null)
+ {
+ ThreadLocal<?> entryKey = e.get();
+ if (entryKey != key)
+ {
+ if (entryKey == null) {
+ entries[i] = deletedEntry;
+ }
+ continue;
+ }
+ else
+ {
+ /* Remove from the table */
+ entries[i] = deletedEntry;
+ }
+ }
+ return;
+ }
+ }
+
+ /**
+ * Clear out the map. Done once during thread death.
+ */
+ void clear() {
+ entries = null;
+ }
+
+ /**
+ * Inherit all the InheritableThreadLocal instances from the given parent.
+ *
+ * @param parentMap The map to inherit from.
+ */
+ public void inherit(ThreadLocalMap parentMap) {
+ for(Entry e: parentMap.entries)
+ {
+ if (e != null && e != deletedEntry)
+ {
+ ThreadLocal<?> key = e.get();
+ if (key instanceof InheritableThreadLocal)
+ {
+ set(key, ((InheritableThreadLocal)key).childValue(e.value));
+ }
+ }
+ }
+ }
+}
