SF.net SVN: jikesrvm:[15689] rvmroot/trunk

Revision: 15689
          http://jikesrvm.svn.sourceforge.net/jikesrvm/?rev=15689&view=rev
Author:   pizlo
Date:     2009-05-09 21:46:05 +0000 (Sat, 09 May 2009)

Log Message:
-----------
Fixed RC collectors by disabling biased locking for those collectors in which the mutator needs to be able to CAS on the status word.  This fix has the potential to be extended to add support for disabling biased locking at runtime.  According to my tests, this change adds no measurable overhead.

Modified Paths:
--------------
    rvmroot/trunk/MMTk/src/org/mmtk/plan/PlanConstraints.java
    rvmroot/trunk/MMTk/src/org/mmtk/plan/refcount/RCBaseConstraints.java
    rvmroot/trunk/rvm/src/org/jikesrvm/mm/mminterface/MemoryManagerConstants.java
    rvmroot/trunk/rvm/src/org/jikesrvm/scheduler/ThinLock.java

Modified: rvmroot/trunk/MMTk/src/org/mmtk/plan/PlanConstraints.java
===================================================================
--- rvmroot/trunk/MMTk/src/org/mmtk/plan/PlanConstraints.java 2009-05-08 22:14:51 UTC (rev 15688)
+++ rvmroot/trunk/MMTk/src/org/mmtk/plan/PlanConstraints.java 2009-05-09 21:46:05 UTC (rev 15689)
@@ -69,6 +69,10 @@
   /** @return The number of header words that are required. */
   public abstract int gcHeaderWords();
 
+  /** @return If the mutator needs to be able to CAS the header bits even
+   *          when the world is not stopped. */
+  public boolean mutatorNeedsToCASHeaderBits() { return false; }
+
   /** @return True if this plan contains GCspy. */
   public boolean withGCspy() { return false; }
 

Modified: rvmroot/trunk/MMTk/src/org/mmtk/plan/refcount/RCBaseConstraints.java
===================================================================
--- rvmroot/trunk/MMTk/src/org/mmtk/plan/refcount/RCBaseConstraints.java 2009-05-08 22:14:51 UTC (rev 15688)
+++ rvmroot/trunk/MMTk/src/org/mmtk/plan/refcount/RCBaseConstraints.java 2009-05-09 21:46:05 UTC (rev 15689)
@@ -33,4 +33,6 @@
   public boolean needsWriteBarrier() { return true; }
   @Override
   public int maxNonLOSDefaultAllocBytes() { return MAX_FREELIST_OBJECT_BYTES; }
+  @Override
+  public boolean mutatorNeedsToCASHeaderBits() { return true; }
 }

Modified: rvmroot/trunk/rvm/src/org/jikesrvm/mm/mminterface/MemoryManagerConstants.java
===================================================================
--- rvmroot/trunk/rvm/src/org/jikesrvm/mm/mminterface/MemoryManagerConstants.java 2009-05-08 22:14:51 UTC (rev 15688)
+++ rvmroot/trunk/rvm/src/org/jikesrvm/mm/mminterface/MemoryManagerConstants.java 2009-05-09 21:46:05 UTC (rev 15689)
@@ -27,6 +27,9 @@
   public static final int GC_HEADER_BITS = Selected.Constraints.get().gcHeaderBits();
   /** Number of additional bytes required in the header by the selected plan */
   public static final int GC_HEADER_BYTES = Selected.Constraints.get().gcHeaderWords() << LOG_BYTES_IN_WORD;
+  /** If the mutator needs to be able to CAS the header bits even when the world
+      is not stopped. */
+  public static final boolean MUTATOR_NEEDS_TO_CAS_HEADER_BITS = Selected.Constraints.get().mutatorNeedsToCASHeaderBits();
   /** True if the selected plan requires a read barrier on reference types */
   public static final boolean NEEDS_REFTYPE_READ_BARRIER = Selected.Constraints.get().needsReferenceTypeReadBarrier();
   /** True if the selected plan requires write barriers on putfield, arraystore or modifycheck */

Modified: rvmroot/trunk/rvm/src/org/jikesrvm/scheduler/ThinLock.java
===================================================================
--- rvmroot/trunk/rvm/src/org/jikesrvm/scheduler/ThinLock.java 2009-05-08 22:14:51 UTC (rev 15688)
+++ rvmroot/trunk/rvm/src/org/jikesrvm/scheduler/ThinLock.java 2009-05-09 21:46:05 UTC (rev 15689)
@@ -16,6 +16,7 @@
 import org.jikesrvm.Services;
 import org.jikesrvm.objectmodel.ThinLockConstants;
 import org.jikesrvm.runtime.Magic;
+import org.jikesrvm.mm.mminterface.MemoryManagerConstants;
 import org.vmmagic.pragma.Inline;
 import org.vmmagic.pragma.NoInline;
 import org.vmmagic.pragma.NoNullCheck;
@@ -30,6 +31,8 @@
 @Uninterruptible
 public final class ThinLock implements ThinLockConstants {
 
+  public static final boolean biasingAllowed=!MemoryManagerConstants.MUTATOR_NEEDS_TO_CAS_HEADER_BITS;
+
   @Inline
   @NoNullCheck
   @Unpreemptible
@@ -91,13 +94,25 @@
       if (stat.EQ(TL_STAT_BIASABLE)) {
         Word id = old.and(TL_THREAD_ID_MASK);
         if (id.isZero()) {
-          // lock is unbiased, bias it in our favor and grab it
-          if (Synchronization.tryCompareAndSwap(
-                o, lockOffset,
-                old,
-                old.or(threadId).toAddress().plus(TL_LOCK_COUNT_UNIT).toWord())) {
-            Magic.isync();
-            return;
+          if (biasingAllowed) {
+            // lock is unbiased, bias it in our favor and grab it
+            if (Synchronization.tryCompareAndSwap(
+                  o, lockOffset,
+                  old,
+                  old.or(threadId).toAddress().plus(TL_LOCK_COUNT_UNIT).toWord())) {
+              Magic.isync();
+              return;
+            }
+          } else {
+            // lock is unbiased but biasing is NOT allowed, so turn it into
+            // a thin lock
+            if (Synchronization.tryCompareAndSwap(
+                  o, lockOffset,
+                  old,
+                  old.or(threadId).or(TL_STAT_THIN))) {
+              Magic.isync();
+              return;
+            }
           }
         } else if (id.EQ(threadId)) {
           // lock is biased in our favor
@@ -477,6 +492,7 @@
   @NoNullCheck
   @Unpreemptible
   public static boolean lockHeader(Object o, Offset lockOffset) {
+    if (!biasingAllowed) return false;
     // what this should do:
     // 1) take advantage of the fact that if a lock is fat it can only go back to
     //    being thin, so concurrent modification of the lock word is allowed.
@@ -541,7 +557,8 @@
   @Inline
   @Uninterruptible
   public static boolean allowHeaderCAS(Object o, Offset lockOffset) {
-    return Magic.getWordAtOffset(o,lockOffset).and(TL_STAT_MASK).NE(TL_STAT_BIASABLE);
+    return !biasingAllowed ||
+      Magic.getWordAtOffset(o,lockOffset).and(TL_STAT_MASK).NE(TL_STAT_BIASABLE);
   }
 
   ////////////////////////////////////////////////////////////////


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