7158800: Improve storage of symbol tables

Summary: Use an alternate version of hashing algorithm for symbol string tables and after a certain bucket size to improve performance
Reviewed-by: pbk, kamg, dlong, kvn, fparain

src/share/vm/classfile/altHashing.cpp

+/*
+ * Copyright (c) 2012, Oracle and/or its affiliates. All rights reserved.
+ * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
+ *
+ * This code is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License version 2 only, as
+ * published by the Free Software Foundation.
+ *
+ * This code 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
+ * version 2 for more details (a copy is included in the LICENSE file that
+ * accompanied this code).
+ *
+ * You should have received a copy of the GNU General Public License version
+ * 2 along with this work; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
+ *
+ * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
+ * or visit www.oracle.com if you need additional information or have any
+ * questions.
+ *
+ */
+
+#include "precompiled.hpp"
+#include "classfile/altHashing.hpp"
+#include "classfile/symbolTable.hpp"
+#include "classfile/systemDictionary.hpp"
+#include "oops/markOop.hpp"
+#include "runtime/thread.hpp"
+
+// Get the hash code of the classes mirror if it exists, otherwise just
+// return a random number, which is one of the possible hash code used for
+// objects.  We don't want to call the synchronizer hash code to install
+// this value because it may safepoint.
+intptr_t object_hash(klassOop k) {
+  intptr_t hc = k->java_mirror()->mark()->hash();
+  return hc != markOopDesc::no_hash ? hc : os::random();
+}
+
+// Seed value used for each alternative hash calculated.
+jint AltHashing::compute_seed() {
+  jlong nanos = os::javaTimeNanos();
+  jlong now = os::javaTimeMillis();
+  jint SEED_MATERIAL[8] = {
+            (jint) object_hash(SystemDictionary::String_klass()),
+            (jint) object_hash(SystemDictionary::System_klass()),
+            (jint) os::random(),  // current thread isn't a java thread
+            (jint) (((julong)nanos) >> 32),
+            (jint) nanos,
+            (jint) (((julong)now) >> 32),
+            (jint) now,
+            (jint) (os::javaTimeNanos() >> 2)
+  };
+
+  return murmur3_32(SEED_MATERIAL, 8);
+}
+
+
+// Murmur3 hashing for Symbol
+jint AltHashing::murmur3_32(jint seed, const jbyte* data, int len) {
+  jint h1 = seed;
+  int count = len;
+  int offset = 0;
+
+  // body
+  while (count >= 4) {
+    jint k1 = (data[offset] & 0x0FF)
+        | (data[offset + 1] & 0x0FF) << 8
+        | (data[offset + 2] & 0x0FF) << 16
+        | data[offset + 3] << 24;
+
+    count -= 4;
+    offset += 4;
+
+    k1 *= 0xcc9e2d51;
+    k1 = Integer_rotateLeft(k1, 15);
+    k1 *= 0x1b873593;
+
+    h1 ^= k1;
+    h1 = Integer_rotateLeft(h1, 13);
+    h1 = h1 * 5 + 0xe6546b64;
+  }
+
+  // tail
+
+  if (count > 0) {
+    jint k1 = 0;
+
+    switch (count) {
+      case 3:
+        k1 ^= (data[offset + 2] & 0xff) << 16;
+      // fall through
+      case 2:
+        k1 ^= (data[offset + 1] & 0xff) << 8;
+      // fall through
+      case 1:
+        k1 ^= (data[offset] & 0xff);
+      // fall through
+      default:
+        k1 *= 0xcc9e2d51;
+        k1 = Integer_rotateLeft(k1, 15);
+        k1 *= 0x1b873593;
+        h1 ^= k1;
+    }
+  }
+
+  // finalization
+  h1 ^= len;
+
+  // finalization mix force all bits of a hash block to avalanche
+  h1 ^= ((unsigned int)h1) >> 16;
+  h1 *= 0x85ebca6b;
+  h1 ^= ((unsigned int)h1) >> 13;
+  h1 *= 0xc2b2ae35;
+  h1 ^= ((unsigned int)h1) >> 16;
+
+  return h1;
+}
+
+// Murmur3 hashing for Strings
+jint AltHashing::murmur3_32(jint seed, const jchar* data, int len) {
+  jint h1 = seed;
+
+  int off = 0;
+  int count = len;
+
+  // body
+  while (count >= 2) {
+    jchar d1 = data[off++] & 0xFFFF;
+    jchar d2 = data[off++];
+    jint k1 = (d1 | d2 << 16);
+
+    count -= 2;
+
+    k1 *= 0xcc9e2d51;
+    k1 = Integer_rotateLeft(k1, 15);
+    k1 *= 0x1b873593;
+
+    h1 ^= k1;
+    h1 = Integer_rotateLeft(h1, 13);
+    h1 = h1 * 5 + 0xe6546b64;
+  }
+
+  // tail
+
+  if (count > 0) {
+    int k1 = data[off];
+
+    k1 *= 0xcc9e2d51;
+    k1 = Integer_rotateLeft(k1, 15);
+    k1 *= 0x1b873593;
+    h1 ^= k1;
+  }
+
+  // finalization
+  h1 ^= len * 2; // (Character.SIZE / Byte.SIZE);
+
+  // finalization mix force all bits of a hash block to avalanche
+  h1 ^= ((unsigned int)h1) >> 16;
+  h1 *= 0x85ebca6b;
+  h1 ^= ((unsigned int)h1) >> 13;
+  h1 *= 0xc2b2ae35;
+  h1 ^= ((unsigned int)h1) >> 16;
+
+  return h1;
+}
+
+// Hash used for the seed.
+jint AltHashing::murmur3_32(jint seed, const int* data, int len) {
+  jint h1 = seed;
+
+  int off = 0;
+  int end = len;
+
+  // body
+  while (off < end) {
+    jint k1 = data[off++];
+
+    k1 *= 0xcc9e2d51;
+    k1 = Integer_rotateLeft(k1, 15);
+    k1 *= 0x1b873593;
+
+    h1 ^= k1;
+    h1 = Integer_rotateLeft(h1, 13);
+    h1 = h1 * 5 + 0xe6546b64;
+  }
+
+  // tail (always empty, as body is always 32-bit chunks)
+
+  // finalization
+
+  h1 ^= len * 4; // (Integer.SIZE / Byte.SIZE);
+
+  // finalization mix force all bits of a hash block to avalanche
+  h1 ^= ((juint)h1) >> 16;
+  h1 *= 0x85ebca6b;
+  h1 ^= ((juint)h1) >> 13;
+  h1 *= 0xc2b2ae35;
+  h1 ^= ((juint)h1) >> 16;
+
+  return h1;
+}
+
+jint AltHashing::murmur3_32(const int* data, int len) {
+  return murmur3_32(0, data, len);
+}
+
+#ifndef PRODUCT
+// Overloaded versions for internal test.
+jint AltHashing::murmur3_32(const jbyte* data, int len) {
+  return murmur3_32(0, data, len);
+}
+
+jint AltHashing::murmur3_32(const jchar* data, int len) {
+  return murmur3_32(0, data, len);
+}
+
+// Internal test for alternate hashing.  Translated from JDK version
+// test/sun/misc/Hashing.java
+static const jbyte ONE_BYTE[] = { (jbyte) 0x80};
+static const jbyte TWO_BYTE[] = { (jbyte) 0x80, (jbyte) 0x81};
+static const jchar ONE_CHAR[] = { (jchar) 0x8180};
+static const jbyte THREE_BYTE[] = { (jbyte) 0x80, (jbyte) 0x81, (jbyte) 0x82};
+static const jbyte FOUR_BYTE[] = { (jbyte) 0x80, (jbyte) 0x81, (jbyte) 0x82, (jbyte) 0x83};
+static const jchar TWO_CHAR[] = { (jchar) 0x8180, (jchar) 0x8382};
+static const jint ONE_INT[] = { 0x83828180};
+static const jbyte SIX_BYTE[] = { (jbyte) 0x80, (jbyte) 0x81, (jbyte) 0x82, (jbyte) 0x83, (jbyte) 0x84, (jbyte) 0x85};
+static const jchar THREE_CHAR[] = { (jchar) 0x8180, (jchar) 0x8382, (jchar) 0x8584};
+static const jbyte EIGHT_BYTE[] = {
+  (jbyte) 0x80, (jbyte) 0x81, (jbyte) 0x82,
+  (jbyte) 0x83, (jbyte) 0x84, (jbyte) 0x85,
+  (jbyte) 0x86, (jbyte) 0x87};
+static const jchar FOUR_CHAR[] = {
+  (jchar) 0x8180, (jchar) 0x8382,
+  (jchar) 0x8584, (jchar) 0x8786};
+
+static const jint TWO_INT[] = { 0x83828180, 0x87868584};
+
+static const juint MURMUR3_32_X86_CHECK_VALUE = 0xB0F57EE3;
+
+void AltHashing::testMurmur3_32_ByteArray() {
+  // printf("testMurmur3_32_ByteArray\n");
+
+  jbyte* vector = new jbyte[256];
+  jbyte* hashes = new jbyte[4 * 256];
+
+  for (int i = 0; i < 256; i++) {
+    vector[i] = (jbyte) i;
+  }
+
+  // Hash subranges {}, {0}, {0,1}, {0,1,2}, ..., {0,...,255}
+  for (int i = 0; i < 256; i++) {
+    jint hash = murmur3_32(256 - i, vector, i);
+    hashes[i * 4] = (jbyte) hash;
+    hashes[i * 4 + 1] = (jbyte) (((juint)hash) >> 8);
+    hashes[i * 4 + 2] = (jbyte) (((juint)hash) >> 16);
+    hashes[i * 4 + 3] = (jbyte) (((juint)hash) >> 24);
+  }
+
+  // hash to get const result.
+  juint final_hash = murmur3_32(hashes, 4*256);
+
+  assert (MURMUR3_32_X86_CHECK_VALUE == final_hash,
+    err_msg(
+        "Calculated hash result not as expected. Expected %08X got %08X\n",
+        MURMUR3_32_X86_CHECK_VALUE,
+        final_hash));
+}
+
+void AltHashing::testEquivalentHashes() {
+  jint jbytes, jchars, ints;
+
+  // printf("testEquivalentHashes\n");
+
+  jbytes = murmur3_32(TWO_BYTE, 2);
+  jchars = murmur3_32(ONE_CHAR, 1);
+  assert (jbytes == jchars,
+    err_msg("Hashes did not match. b:%08x != c:%08x\n", jbytes, jchars));
+
+  jbytes = murmur3_32(FOUR_BYTE, 4);
+  jchars = murmur3_32(TWO_CHAR, 2);
+  ints = murmur3_32(ONE_INT, 1);
+  assert ((jbytes == jchars) && (jbytes == ints),
+    err_msg("Hashes did not match. b:%08x != c:%08x != i:%08x\n", jbytes, jchars, ints));
+
+  jbytes = murmur3_32(SIX_BYTE, 6);
+  jchars = murmur3_32(THREE_CHAR, 3);
+  assert (jbytes == jchars,
+    err_msg("Hashes did not match. b:%08x != c:%08x\n", jbytes, jchars));
+
+  jbytes = murmur3_32(EIGHT_BYTE, 8);
+  jchars = murmur3_32(FOUR_CHAR, 4);
+  ints = murmur3_32(TWO_INT, 2);
+  assert ((jbytes == jchars) && (jbytes == ints),
+    err_msg("Hashes did not match. b:%08x != c:%08x != i:%08x\n", jbytes, jchars, ints));
+}
+
+// Returns true if the alternate hashcode is correct
+void AltHashing::test_alt_hash() {
+  testMurmur3_32_ByteArray();
+  testEquivalentHashes();
+}
+#endif // PRODUCT

src/share/vm/classfile/altHashing.hpp

+/*
+ * Copyright (c) 2012, Oracle and/or its affiliates. All rights reserved.
+ * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
+ *
+ * This code is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License version 2 only, as
+ * published by the Free Software Foundation.
+ *
+ * This code 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
+ * version 2 for more details (a copy is included in the LICENSE file that
+ * accompanied this code).
+ *
+ * You should have received a copy of the GNU General Public License version
+ * 2 along with this work; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
+ *
+ * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
+ * or visit www.oracle.com if you need additional information or have any
+ * questions.
+ *
+ */
+
+#ifndef SHARE_VM_CLASSFILE_ALTHASHING_HPP
+#define SHARE_VM_CLASSFILE_ALTHASHING_HPP
+
+#include "prims/jni.h"
+#include "classfile/symbolTable.hpp"
+
+/**
+ * Hashing utilities.
+ *
+ * Implementation of Murmur3 hashing.
+ * This code was translated from src/share/classes/sun/misc/Hashing.java
+ * code in the JDK.
+ */
+
+class AltHashing : AllStatic {
+
+  // utility function copied from java/lang/Integer
+  static jint Integer_rotateLeft(jint i, int distance) {
+    return (i << distance) | (((juint)i) >> (32-distance));
+  }
+  static jint murmur3_32(const int* data, int len);
+  static jint murmur3_32(jint seed, const int* data, int len);
+
+#ifndef PRODUCT
+  // Hashing functions used for internal testing
+  static jint murmur3_32(const jbyte* data, int len);
+  static jint murmur3_32(const jchar* data, int len);
+  static void testMurmur3_32_ByteArray();
+  static void testEquivalentHashes();
+#endif // PRODUCT
+
+ public:
+  static jint compute_seed();
+  static jint murmur3_32(jint seed, const jbyte* data, int len);
+  static jint murmur3_32(jint seed, const jchar* data, int len);
+  NOT_PRODUCT(static void test_alt_hash();)
+};
+#endif // SHARE_VM_CLASSFILE_ALTHASHING_HPP

src/share/vm/classfile/javaClasses.cpp

@@ -23,6 +23,7 @@
  */
 
 #include "precompiled.hpp"
+#include "classfile/altHashing.hpp"
 #include "classfile/javaClasses.hpp"
 #include "classfile/symbolTable.hpp"
 #include "classfile/vmSymbols.hpp"
@@ -347,13 +348,26 @@ jchar* java_lang_String::as_unicode_string(oop java_string, int& length) {
   return result;
 }
 
-unsigned int java_lang_String::hash_string(oop java_string) {
+unsigned int java_lang_String::to_hash(oop java_string) {
+  int          length = java_lang_String::length(java_string);
+  // Zero length string will hash to zero with String.toHash() function.
+  if (length == 0) return 0;
+
   typeArrayOop value  = java_lang_String::value(java_string);
   int          offset = java_lang_String::offset(java_string);
+  return java_lang_String::to_hash(value->char_at_addr(offset), length);
+}
+
+unsigned int java_lang_String::hash_string(oop java_string) {
   int          length = java_lang_String::length(java_string);
+  // Zero length string doesn't hash necessarily hash to zero.
+  if (length == 0) {
+    return StringTable::hash_string(NULL, 0);
+  }
 
-  if (length == 0) return 0;
-  return hash_string(value->char_at_addr(offset), length);
+  typeArrayOop value  = java_lang_String::value(java_string);
+  int          offset = java_lang_String::offset(java_string);
+  return StringTable::hash_string(value->char_at_addr(offset), length);
 }
 
 Symbol* java_lang_String::as_symbol(Handle java_string, TRAPS) {

src/share/vm/classfile/javaClasses.hpp

@@ -158,20 +158,16 @@ class java_lang_String : AllStatic {
   static jchar* as_unicode_string(oop java_string, int& length);
 
   // Compute the hash value for a java.lang.String object which would
-  // contain the characters passed in. This hash value is used for at
-  // least two purposes.
+  // contain the characters passed in.
   //
-  // (a) As the hash value used by the StringTable for bucket selection
-  //     and comparison (stored in the HashtableEntry structures).  This
-  //     is used in the String.intern() method.
+  // As the hash value used by the String object itself, in
+  // String.hashCode().  This value is normally calculated in Java code
+  // in the String.hashCode method(), but is precomputed for String
+  // objects in the shared archive file.
+  // hash P(31) from Kernighan & Ritchie
   //
-  // (b) As the hash value used by the String object itself, in
-  //     String.hashCode().  This value is normally calculate in Java code
-  //     in the String.hashCode method(), but is precomputed for String
-  //     objects in the shared archive file.
-  //
-  //     For this reason, THIS ALGORITHM MUST MATCH String.hashCode().
-  static unsigned int hash_string(jchar* s, int len) {
+  // For this reason, THIS ALGORITHM MUST MATCH String.toHash().
+  template <typename T> static unsigned int to_hash(T* s, int len) {
     unsigned int h = 0;
     while (len-- > 0) {
       h = 31*h + (unsigned int) *s;
@@ -179,6 +175,10 @@ class java_lang_String : AllStatic {
     }
     return h;
   }
+  static unsigned int to_hash(oop java_string);
+
+  // This is the string hash code used by the StringTable, which may be
+  // the same as String.toHash or an alternate hash code.
   static unsigned int hash_string(oop java_string);
 
   static bool equals(oop java_string, jchar* chars, int len);

src/share/vm/classfile/symbolTable.cpp

@@ -23,6 +23,7 @@
  */
 
 #include "precompiled.hpp"
+#include "classfile/altHashing.hpp"
 #include "classfile/javaClasses.hpp"
 #include "classfile/symbolTable.hpp"
 #include "classfile/systemDictionary.hpp"
@@ -34,12 +35,15 @@
 #include "oops/oop.inline2.hpp"
 #include "runtime/mutexLocker.hpp"
 #include "utilities/hashtable.inline.hpp"
+#include "utilities/numberSeq.hpp"
 
 // --------------------------------------------------------------------------
 
 SymbolTable* SymbolTable::_the_table = NULL;
 // Static arena for symbols that are not deallocated
 Arena* SymbolTable::_arena = NULL;
+bool SymbolTable::_needs_rehashing = false;
+jint SymbolTable::_seed = 0;
 
 Symbol* SymbolTable::allocate_symbol(const u1* name, int len, bool c_heap, TRAPS) {
   // Don't allow symbols to be created which cannot fit in a Symbol*.
@@ -121,12 +125,41 @@ void SymbolTable::unlink() {
   }
 }
 
+unsigned int SymbolTable::new_hash(Symbol* sym) {
+  ResourceMark rm;
+  // Use alternate hashing algorithm on this symbol.
+  return AltHashing::murmur3_32(seed(), (const jbyte*)sym->as_C_string(), sym->utf8_length());
+}
+
+// Create a new table and using alternate hash code, populate the new table
+// with the existing strings.   Set flag to use the alternate hash code afterwards.
+void SymbolTable::rehash_table() {
+  assert(SafepointSynchronize::is_at_safepoint(), "must be at safepoint");
+  assert(!DumpSharedSpaces, "this should never happen with -Xshare:dump");
+  // Create a new symbol table
+  SymbolTable* new_table = new SymbolTable();
+
+  // Initialize the global seed for hashing.
+  _seed = AltHashing::compute_seed();
+  assert(seed() != 0, "shouldn't be zero");
+
+  the_table()->move_to(new_table);
+
+  // Delete the table and buckets (entries are reused in new table).
+  delete _the_table;
+  // Don't check if we need rehashing until the table gets unbalanced again.
+  // Then rehash with a new global seed.
+  _needs_rehashing = false;
+  _the_table = new_table;
+}
 
 // Lookup a symbol in a bucket.
 
 Symbol* SymbolTable::lookup(int index, const char* name,
                               int len, unsigned int hash) {
+  int count = 0;
   for (HashtableEntry<Symbol*>* e = bucket(index); e != NULL; e = e->next()) {
+    count++;  // count all entries in this bucket, not just ones with same hash
     if (e->hash() == hash) {
       Symbol* sym = e->literal();
       if (sym->equals(name, len)) {
@@ -136,9 +169,21 @@ Symbol* SymbolTable::lookup(int index, const char* name,
       }
     }
   }
+  // If the bucket size is too deep check if this hash code is insufficient.
+  if (count >= BasicHashtable::rehash_count && !needs_rehashing()) {
+    _needs_rehashing = check_rehash_table(count);
+  }
   return NULL;
 }
 
+// Pick hashing algorithm, but return value already given if not using a new
+// hash algorithm.
+unsigned int SymbolTable::hash_symbol(const char* s, int len, unsigned int hashValue) {
+  return use_alternate_hashcode() ?
+           AltHashing::murmur3_32(seed(), (const jbyte*)s, len) :
+           (hashValue != 0 ? hashValue : java_lang_String::to_hash(s, len));
+}
+
 
 // We take care not to be blocking while holding the
 // SymbolTable_lock. Otherwise, the system might deadlock, since the
@@ -287,13 +332,17 @@ Symbol* SymbolTable::new_permanent_symbol(const char* name, TRAPS) {
 }
 
 Symbol* SymbolTable::basic_add(int index, u1 *name, int len,
-                               unsigned int hashValue, bool c_heap, TRAPS) {
+                               unsigned int hashValue_arg, bool c_heap, TRAPS) {
   assert(!Universe::heap()->is_in_reserved(name) || GC_locker::is_active(),
          "proposed name of symbol must be stable");
 
   // Grab SymbolTable_lock first.
   MutexLocker ml(SymbolTable_lock, THREAD);
 
+  // Check if the symbol table has been rehashed, if so, need to recalculate
+  // the hash value.
+  unsigned int hashValue = hash_symbol((const char*)name, len, hashValue_arg);
+
   // Since look-up was done lock-free, we need to check if another
   // thread beat us in the race to insert the symbol.
   Symbol* test = lookup(index, (char*)name, len, hashValue);
@@ -332,10 +381,13 @@ bool SymbolTable::basic_add(Handle class_loader, constantPoolHandle cp,
   MutexLocker ml(SymbolTable_lock, THREAD);
 
   for (int i=0; i<names_count; i++) {
+    // Check if the symbol table has been rehashed, if so, need to recalculate
+    // the hash value.
+    unsigned int hashValue = hash_symbol(names[i], lengths[i], hashValues[i]);
     // Since look-up was done lock-free, we need to check if another
     // thread beat us in the race to insert the symbol.
-    int index = hash_to_index(hashValues[i]);
-    Symbol* test = lookup(index, names[i], lengths[i], hashValues[i]);
+    int index = hash_to_index(hashValue);
+    Symbol* test = lookup(index, names[i], lengths[i], hashValue);
     if (test != NULL) {
       // A race occurred and another thread introduced the symbol, this one
       // will be dropped and collected. Use test instead.
@@ -347,7 +399,7 @@ bool SymbolTable::basic_add(Handle class_loader, constantPoolHandle cp,
       bool c_heap = class_loader() != NULL;
       Symbol* sym = allocate_symbol((const u1*)names[i], lengths[i], c_heap, CHECK_(false));
       assert(sym->equals(names[i], lengths[i]), "symbol must be properly initialized");  // why wouldn't it be???
-      HashtableEntry<Symbol*>* entry = new_entry(hashValues[i], sym);
+      HashtableEntry<Symbol*>* entry = new_entry(hashValue, sym);
       add_entry(index, entry);
       cp->symbol_at_put(cp_indices[i], sym);
     }
@@ -370,6 +422,24 @@ void SymbolTable::verify() {
   }
 }
 
+void SymbolTable::dump(outputStream* st) {
+  NumberSeq summary;
+  for (int i = 0; i < the_table()->table_size(); ++i) {
+    int count = 0;
+    for (HashtableEntry<Symbol*>* e = the_table()->bucket(i);
+       e != NULL; e = e->next()) {
+      count++;
+    }
+    summary.add((double)count);
+  }
+  st->print_cr("SymbolTable statistics:");
+  st->print_cr("Number of buckets       : %7d", summary.num());
+  st->print_cr("Average bucket size     : %7.0f", summary.avg());
+  st->print_cr("Variance of bucket size : %7.0f", summary.variance());
+  st->print_cr("Std. dev. of bucket size: %7.0f", summary.sd());
+  st->print_cr("Maximum bucket size     : %7.0f", summary.maximum());
+}
+
 
 //---------------------------------------------------------------------------
 // Non-product code
@@ -468,7 +538,6 @@ void SymbolTable::print() {
     }
   }
 }
-
 #endif // PRODUCT
 
 // --------------------------------------------------------------------------
@@ -514,21 +583,36 @@ class StableMemoryChecker : public StackObj {
 // --------------------------------------------------------------------------
 StringTable* StringTable::_the_table = NULL;
 
+bool StringTable::_needs_rehashing = false;
+jint StringTable::_seed = 0;
+
+// Pick hashing algorithm
+unsigned int StringTable::hash_string(const jchar* s, int len, unsigned int hashValue) {
+  return use_alternate_hashcode() ? AltHashing::murmur3_32(seed(), s, len) :
+            (hashValue != 0 ? hashValue : java_lang_String::to_hash(s, len));
+}
+
 oop StringTable::lookup(int index, jchar* name,
                         int len, unsigned int hash) {
+  int count = 0;
   for (HashtableEntry<oop>* l = bucket(index); l != NULL; l = l->next()) {
+    count++;
     if (l->hash() == hash) {
       if (java_lang_String::equals(l->literal(), name, len)) {
         return l->literal();
       }
     }
   }
+  // If the bucket size is too deep check if this hash code is insufficient.
+  if (count >= BasicHashtable::rehash_count && !needs_rehashing()) {
+    _needs_rehashing = check_rehash_table(count);
+  }
   return NULL;
 }
 
 
 oop StringTable::basic_add(int index, Handle string_or_null, jchar* name,
-                           int len, unsigned int hashValue, TRAPS) {
+                           int len, unsigned int hashValue_arg, TRAPS) {
   debug_only(StableMemoryChecker smc(name, len * sizeof(name[0])));
   assert(!Universe::heap()->is_in_reserved(name) || GC_locker::is_active(),
          "proposed name of symbol must be stable");
@@ -547,6 +631,10 @@ oop StringTable::basic_add(int index, Handle string_or_null, jchar* name,
   assert(java_lang_String::equals(string(), name, len),
          "string must be properly initialized");
 
+  // Check if the symbol table has been rehashed, if so, need to recalculate
+  // the hash value before second lookup.
+  unsigned int hashValue = hash_string(name, len, hashValue_arg);
+
   // Since look-up was done lock-free, we need to check if another
   // thread beat us in the race to insert the symbol.
 
@@ -566,7 +654,7 @@ oop StringTable::lookup(Symbol* symbol) {
   ResourceMark rm;
   int length;
   jchar* chars = symbol->as_unicode(length);
-  unsigned int hashValue = java_lang_String::hash_string(chars, length);
+  unsigned int hashValue = hash_string(chars, length);
   int index = the_table()->hash_to_index(hashValue);
   return the_table()->lookup(index, chars, length, hashValue);
 }
@@ -574,7 +662,7 @@ oop StringTable::lookup(Symbol* symbol) {
 
 oop StringTable::intern(Handle string_or_null, jchar* name,
                         int len, TRAPS) {
-  unsigned int hashValue = java_lang_String::hash_string(name, len);
+  unsigned int hashValue = hash_string(name, len);
   int index = the_table()->hash_to_index(hashValue);
   oop string = the_table()->lookup(index, name, len, hashValue);
 
@@ -675,3 +763,52 @@ void StringTable::verify() {
     }
   }
 }
+
+void StringTable::dump(outputStream* st) {
+  NumberSeq summary;
+  for (int i = 0; i < the_table()->table_size(); ++i) {
+    HashtableEntry<oop>* p = the_table()->bucket(i);
+    int count = 0;
+    for ( ; p != NULL; p = p->next()) {
+      count++;
+    }
+    summary.add((double)count);
+  }
+  st->print_cr("StringTable statistics:");
+  st->print_cr("Number of buckets       : %7d", summary.num());
+  st->print_cr("Average bucket size     : %7.0f", summary.avg());
+  st->print_cr("Variance of bucket size : %7.0f", summary.variance());
+  st->print_cr("Std. dev. of bucket size: %7.0f", summary.sd());
+  st->print_cr("Maximum bucket size     : %7.0f", summary.maximum());
+}
+
+
+unsigned int StringTable::new_hash(oop string) {
+  ResourceMark rm;
+  int length;
+  jchar* chars = java_lang_String::as_unicode_string(string, length);
+  // Use alternate hashing algorithm on the string
+  return AltHashing::murmur3_32(seed(), chars, length);
+}
+
+// Create a new table and using alternate hash code, populate the new table
+// with the existing strings.   Set flag to use the alternate hash code afterwards.
+void StringTable::rehash_table() {
+  assert(SafepointSynchronize::is_at_safepoint(), "must be at safepoint");
+  assert(!DumpSharedSpaces, "this should never happen with -Xshare:dump");
+  StringTable* new_table = new StringTable();
+
+  // Initialize new global seed for hashing.
+  _seed = AltHashing::compute_seed();
+  assert(seed() != 0, "shouldn't be zero");
+
+  // Rehash the table
+  the_table()->move_to(new_table);
+
+  // Delete the table and buckets (entries are reused in new table).
+  delete _the_table;
+  // Don't check if we need rehashing until the table gets unbalanced again.
+  // Then rehash with a new global seed.
+  _needs_rehashing = false;
+  _the_table = new_table;
+}

src/share/vm/classfile/symbolTable.hpp

@@ -40,6 +40,7 @@
 //  - symbolTableEntrys are allocated in blocks to reduce the space overhead.
 
 class BoolObjectClosure;
+class outputStream;
 
 
 // Class to hold a newly created or referenced Symbol* temporarily in scope.
@@ -78,6 +79,10 @@ private:
   // The symbol table
   static SymbolTable* _the_table;
 
+  // Set if one bucket is out of balance due to hash algorithm deficiency
+  static bool _needs_rehashing;
+  static jint _seed;
+
   // For statistics
   static int symbols_removed;
   static int symbols_counted;
@@ -119,6 +124,11 @@ private:
   static Arena* arena() { return _arena; }  // called for statistics
 
   static void initialize_symbols(int arena_alloc_size = 0);
+
+  static bool use_alternate_hashcode()  { return _seed != 0; }
+  static jint seed()                    { return _seed; }
+
+  unsigned int new_hash(Symbol* sym);
 public:
   enum {
     symbol_alloc_batch_size = 8,
@@ -146,6 +156,8 @@ public:
     initialize_symbols();
   }
 
+  static unsigned int hash_symbol(const char* s, int len, unsigned int hashValue = 0);
+
   static Symbol* lookup(const char* name, int len, TRAPS);
   // lookup only, won't add. Also calculate hash.
   static Symbol* lookup_only(const char* name, int len, unsigned int& hash);
@@ -208,6 +220,7 @@ public:
 
   // Debugging
   static void verify();
+  static void dump(outputStream* st);
 
   // Sharing
   static void copy_buckets(char** top, char*end) {
@@ -219,8 +232,13 @@ public:
   static void reverse(void* boundary = NULL) {
     the_table()->Hashtable<Symbol*>::reverse(boundary);
   }
+
+  // Rehash the symbol table if it gets out of balance
+  static void rehash_table();
+  static bool needs_rehashing()         { return _needs_rehashing; }
 };
 
+
 class StringTable : public Hashtable<oop> {
   friend class VMStructs;
 
@@ -228,6 +246,10 @@ private:
   // The string table
   static StringTable* _the_table;
 
+  // Set if one bucket is out of balance due to hash algorithm deficiency
+  static bool _needs_rehashing;
+  static jint _seed;
+
   static oop intern(Handle string_or_null, jchar* chars, int length, TRAPS);
   oop basic_add(int index, Handle string_or_null, jchar* name, int len,
                 unsigned int hashValue, TRAPS);
@@ -241,6 +263,10 @@ private:
     : Hashtable<oop>((int)StringTableSize, sizeof (HashtableEntry<oop>), t,
                      number_of_entries) {}
 
+  static bool use_alternate_hashcode()  { return _seed != 0; }
+  static jint seed()                    { return _seed; }
+
+  unsigned int new_hash(oop s);
 public:
   // The string table
   static StringTable* the_table() { return _the_table; }
@@ -265,6 +291,14 @@ public:
   // Invoke "f->do_oop" on the locations of all oops in the table.
   static void oops_do(OopClosure* f);
 
+  // Hashing algorithm, used as the hash value used by the
+  //     StringTable for bucket selection and comparison (stored in the
+  //     HashtableEntry structures).  This is used in the String.intern() method.
+  static unsigned int hash_string(const jchar* s, int len, unsigned int hashValue = 0);
+
+  // Internal test.
+  static void test_alt_hash() PRODUCT_RETURN;
+
   // Probing
   static oop lookup(Symbol* symbol);
 
@@ -275,6 +309,7 @@ public:
 
   // Debugging
   static void verify();
+  static void dump(outputStream* st);
 
   // Sharing
   static void copy_buckets(char** top, char*end) {
@@ -286,6 +321,9 @@ public:
   static void reverse() {
     the_table()->Hashtable<oop>::reverse();
   }
-};
 
+  // Rehash the symbol table if it gets out of balance
+  static void rehash_table();
+  static bool needs_rehashing() { return _needs_rehashing; }
+};
 #endif // SHARE_VM_CLASSFILE_SYMBOLTABLE_HPP

src/share/vm/memory/dump.cpp

@@ -62,8 +62,8 @@ public:
 // written later, increasing the likelihood that the shared page contain
 // the hash can be shared.
 //
-// NOTE THAT the algorithm in StringTable::hash_string() MUST MATCH the
-// algorithm in java.lang.String.hashCode().
+// NOTE THAT we have to call java_lang_String::to_hash() to match the
+// algorithm in java.lang.String.toHash().
 
 class StringHashCodeClosure: public OopClosure {
 private:
@@ -80,7 +80,7 @@ public:
       oop obj = *p;
       if (obj->klass() == SystemDictionary::String_klass() &&
           java_lang_String::has_hash_field()) {
-        int hash = java_lang_String::hash_string(obj);
+        int hash = java_lang_String::to_hash(obj);
         obj->int_field_put(hash_offset, hash);
       }
     }

src/share/vm/prims/jni.cpp

@@ -23,6 +23,7 @@
  */
 
 #include "precompiled.hpp"
+#include "classfile/altHashing.hpp"
 #include "classfile/classLoader.hpp"
 #include "classfile/javaClasses.hpp"
 #include "classfile/symbolTable.hpp"
@@ -5053,6 +5054,7 @@ void execute_internal_vm_tests() {
     run_unit_test(arrayOopDesc::test_max_array_length());
     run_unit_test(CollectedHeap::test_is_in());
     run_unit_test(QuickSort::test_quick_sort());
+    run_unit_test(AltHashing::test_alt_hash());
     tty->print_cr("All internal VM tests passed");
   }
 }

src/share/vm/runtime/globals.hpp

@@ -2662,6 +2662,9 @@ class CommandLineFlags {
   product(bool, UseHeavyMonitors, false,                                    \
           "use heavyweight instead of lightweight Java monitors")           \
                                                                             \
+  product(bool, PrintStringTableStatistics, false,                          \
+          "print statistics about the StringTable and SymbolTable")         \
+                                                                            \
   notproduct(bool, PrintSymbolTableSizeHistogram, false,                    \
           "print histogram of the symbol table")                            \
                                                                             \

src/share/vm/runtime/init.cpp

@@ -23,6 +23,7 @@
  */
 
 #include "precompiled.hpp"
+#include "classfile/symbolTable.hpp"
 #include "code/icBuffer.hpp"
 #include "gc_interface/collectedHeap.hpp"
 #include "interpreter/bytecodes.hpp"
@@ -157,6 +158,10 @@ void exit_globals() {
       // Print the collected safepoint statistics.
       SafepointSynchronize::print_stat_on_exit();
     }
+    if (PrintStringTableStatistics) {
+      SymbolTable::dump(tty);
+      StringTable::dump(tty);
+    }
     ostream_exit();
   }
 }

src/share/vm/runtime/safepoint.cpp

@@ -23,6 +23,7 @@
  */
 
 #include "precompiled.hpp"
+#include "classfile/symbolTable.hpp"
 #include "classfile/systemDictionary.hpp"
 #include "code/codeCache.hpp"
 #include "code/icBuffer.hpp"
@@ -526,8 +527,20 @@ void SafepointSynchronize::do_cleanup_tasks() {
     CompilationPolicy::policy()->do_safepoint_work();
   }
 
-  TraceTime t4("sweeping nmethods", TraceSafepointCleanupTime);
-  NMethodSweeper::scan_stacks();
+  {
+    TraceTime t4("sweeping nmethods", TraceSafepointCleanupTime);
+    NMethodSweeper::scan_stacks();
+  }
+
+  if (SymbolTable::needs_rehashing()) {
+    TraceTime t5("rehashing symbol table", TraceSafepointCleanupTime);
+    SymbolTable::rehash_table();
+  }
+
+  if (StringTable::needs_rehashing()) {
+    TraceTime t6("rehashing string table", TraceSafepointCleanupTime);
+    StringTable::rehash_table();
+  }
 
   // rotate log files?
   if (UseGCLogFileRotation) {

src/share/vm/utilities/hashtable.cpp

 /*
- * Copyright (c) 2003, 2011, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2003, 2012, Oracle and/or its affiliates. All rights reserved.
  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  *
  * This code is free software; you can redistribute it and/or modify it
@@ -86,6 +86,55 @@ template <class T> HashtableEntry<T>* Hashtable<T>::new_entry(unsigned int hashV
 }
 
 
+// Check to see if the hashtable is unbalanced.  The caller set a flag to
+// rehash at the next safepoint.  If this bucket is 60 times greater than the
+// expected average bucket length, it's an unbalanced hashtable.
+// This is somewhat an arbitrary heuristic but if one bucket gets to
+// rehash_count which is currently 100, there's probably something wrong.
+
+bool BasicHashtable::check_rehash_table(int count) {
+  assert(table_size() != 0, "underflow");
+  if (count > (((double)number_of_entries()/(double)table_size())*rehash_multiple)) {
+    // Set a flag for the next safepoint, which should be at some guaranteed
+    // safepoint interval.
+    return true;
+  }
+  return false;
+}
+
+// Create a new table and using alternate hash code, populate the new table
+// with the existing elements.   This can be used to change the hash code
+// and could in the future change the size of the table.
+
+template <class T> void Hashtable<T>::move_to(Hashtable<T>* new_table) {
+  int saved_entry_count = number_of_entries();
+
+  // Iterate through the table and create a new entry for the new table
+  for (int i = 0; i < new_table->table_size(); ++i) {
+    for (HashtableEntry<T>* p = bucket(i); p != NULL; ) {
+      HashtableEntry<T>* next = p->next();
+      T string = p->literal();
+      // Use alternate hashing algorithm on the symbol in the first table
+      unsigned int hashValue = new_hash(string);
+      // Get a new index relative to the new table (can also change size)
+      int index = new_table->hash_to_index(hashValue);
+      p->set_hash(hashValue);
+      unlink_entry(p);
+      new_table->add_entry(index, p);
+      p = next;
+    }
+  }
+  // give the new table the free list as well
+  new_table->copy_freelist(this);
+  assert(new_table->number_of_entries() == saved_entry_count, "lost entry on dictionary copy?");
+
+  // Destroy memory used by the buckets in the hashtable.  The memory
+  // for the elements has been used in a new table and is not
+  // destroyed.  The memory reuse will benefit resizing the SystemDictionary
+  // to avoid a memory allocation spike at safepoint.
+  free_buckets();
+}
+
 // Reverse the order of elements in the hash buckets.
 
 void BasicHashtable::reverse() {

src/share/vm/utilities/hashtable.hpp

@@ -159,8 +159,6 @@ public:
   // Reverse the order of elements in each of the buckets.
   void reverse();
 
-  static unsigned int hash_symbol(const char* s, int len);
-
 private:
   // Instance variables
   int               _table_size;
@@ -179,6 +177,11 @@ protected:
   void verify_lookup_length(double load);
 #endif
 
+  enum {
+    rehash_count = 100,
+    rehash_multiple = 60
+  };
+
   void initialize(int table_size, int entry_size, int number_of_entries);
 
   // Accessor
@@ -193,6 +196,34 @@ protected:
   // Table entry management
   BasicHashtableEntry* new_entry(unsigned int hashValue);
 
+  // Check that the table is unbalanced
+  bool check_rehash_table(int count);
+
+  // Used when moving the entry to another table
+  // Clean up links, but do not add to free_list
+  void unlink_entry(BasicHashtableEntry* entry) {
+    entry->set_next(NULL);
+    --_number_of_entries;
+  }
+
+  // Move over freelist and free block for allocation
+  void copy_freelist(BasicHashtable* src) {
+    _free_list = src->_free_list;
+    src->_free_list = NULL;
+    _first_free_entry = src->_first_free_entry;
+    src->_first_free_entry = NULL;
+    _end_block = src->_end_block;
+    src->_end_block = NULL;
+  }
+
+  // Free the buckets in this hashtable
+  void free_buckets() {
+    if (NULL != _buckets) {
+      FREE_C_HEAP_ARRAY(HashtableBucket, _buckets);
+      _buckets = NULL;
+    }
+  }
+
 public:
   int table_size() { return _table_size; }
   void set_entry(int index, BasicHashtableEntry* entry);
@@ -249,6 +280,10 @@ protected:
   HashtableEntry<T>** bucket_addr(int i) {
     return (HashtableEntry<T>**)BasicHashtable::bucket_addr(i);
   }
+
+  // Function to move these elements into the new table.
+  void move_to(Hashtable<T>* new_table);
+  virtual unsigned int new_hash(T) { ShouldNotReachHere(); return 0; } // should be overridden
 };
 
 

src/share/vm/utilities/hashtable.inline.hpp

 /*
- * Copyright (c) 2003, 2011, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2003, 2012, Oracle and/or its affiliates. All rights reserved.
  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  *
  * This code is free software; you can redistribute it and/or modify it
@@ -30,27 +30,6 @@
 
 // Inline function definitions for hashtable.hpp.
 
-
-// --------------------------------------------------------------------------
-// Hash function
-
-// We originally used hashpjw, but hash P(31) gives just as good results
-// and is slighly faster. We would like a hash function that looks at every
-// character, since package names have large common prefixes, and also because
-// hash_or_fail does error checking while iterating.
-
-// hash P(31) from Kernighan & Ritchie
-
-inline unsigned int BasicHashtable::hash_symbol(const char* s, int len) {
-  unsigned int h = 0;
-  while (len-- > 0) {
-    h = 31*h + (unsigned) *s;
-    s++;
-  }
-  return h;
-}
-
-
 // --------------------------------------------------------------------------
 
 // Initialize a table.

test/runtime/7158800/InternTest.java

+/*
+ * Copyright (c) 2012, Oracle and/or its affiliates. All rights reserved.
+ * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
+ *
+ * This code is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License version 2 only, as
+ * published by the Free Software Foundation.
+ *
+ * This code 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
+ * version 2 for more details (a copy is included in the LICENSE file that
+ * accompanied this code).
+ *
+ * You should have received a copy of the GNU General Public License version
+ * 2 along with this work; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
+ *
+ * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
+ * or visit www.oracle.com if you need additional information or have any
+ * questions.
+ */
+
+/*
+ * @test
+ * @bug 7158800
+ * @run shell/timeout=400 Test7158800.sh
+ * @summary This test performs poorly if alternate hashing isn't used for
+ * string table.
+ * The timeout is handled by the shell file (which kills the process)
+ */
+import java.util.*;
+import java.io.*;
+
+public class InternTest {
+    public static void main (String args[]) throws Exception {
+        final String badStringsFilename = "badstrings.txt";
+
+        if (args.length == 0 || (!args[0].equals("bad") && !args[0].equals("normal"))) {
+            System.out.println("Usage:  java InternTest [normal|bad]");
+            System.exit(1);
+        }
+
+        FileInputStream fstream = new FileInputStream(badStringsFilename);
+        DataInputStream in = new DataInputStream(fstream);
+        BufferedReader br = new BufferedReader(new InputStreamReader(in));
+        String toIntern, toDiscard;
+        int count = 0;
+        long current = 0L;
+        long last = System.currentTimeMillis();
+
+        if (args[0].equals("bad")) {
+            while ((toIntern = br.readLine()) != null) {
+                toDiscard = new String((new Integer((int)(Math.random() * Integer.MAX_VALUE))).toString());
+                toIntern.intern();
+                count++;
+                if (count % 10000 == 0 && count != 0) {
+                    current = System.currentTimeMillis();
+                    System.out.println(new Date(current) + ": interned " + count + " 0-hash strings - last 10000 took " + ((float)(current - last))/1000 + "s (" + ((float)(current - last))/10000000 + "s per String)");
+                    last = current;
+                }
+            }
+        }
+        if (args[0].equals("normal")) {
+            while ((toDiscard = br.readLine()) != null) { // do the same read from the file to try and make the test fair
+                toIntern = new String((new Integer((int)(Math.random() * Integer.MAX_VALUE))).toString());
+                toIntern.intern();
+                count++;
+                if (count % 10000 == 0 && count != 0) {
+                    current = System.currentTimeMillis();
+                    System.out.println(new Date(current) + ": interned " + count + " normal strings - last 10000 took " + ((float)(current - last))/1000 + "s (" + ((float)(current - last))/10000000 + "s per String)");
+                    last = current;
+                }
+            }
+        }
+        in.close();
+    }
+}
+
+