8015237: Parallelize string table scanning during strong root processing

Summary: Parallelize the scanning of the intern string table by having each GC worker claim a given number of buckets. Changes were also reviewed by Per Liden <per.liden@oracle.com>.
Reviewed-by: tschatzl, stefank, twisti

src/share/vm/classfile/symbolTable.cpp

@@ -598,6 +598,8 @@ StringTable* StringTable::_the_table = NULL;
 
 bool StringTable::_needs_rehashing = false;
 
+volatile int StringTable::_parallel_claimed_idx = 0;
+
 // Pick hashing algorithm
 unsigned int StringTable::hash_string(const jchar* s, int len) {
   return use_alternate_hashcode() ? AltHashing::murmur3_32(seed(), s, len) :
@@ -761,8 +763,18 @@ void StringTable::unlink_or_oops_do(BoolObjectClosure* is_alive, OopClosure* f)
   }
 }
 
-void StringTable::oops_do(OopClosure* f) {
-  for (int i = 0; i < the_table()->table_size(); ++i) {
+void StringTable::buckets_do(OopClosure* f, int start_idx, int end_idx) {
+  const int limit = the_table()->table_size();
+
+  assert(0 <= start_idx && start_idx <= limit,
+         err_msg("start_idx (" INT32_FORMAT ") oob?", start_idx));
+  assert(0 <= end_idx && end_idx <= limit,
+         err_msg("end_idx (" INT32_FORMAT ") oob?", end_idx));
+  assert(start_idx <= end_idx,
+         err_msg("Ordering: start_idx=" INT32_FORMAT", end_idx=" INT32_FORMAT,
+                 start_idx, end_idx));
+
+  for (int i = start_idx; i < end_idx; i += 1) {
     HashtableEntry<oop, mtSymbol>* entry = the_table()->bucket(i);
     while (entry != NULL) {
       assert(!entry->is_shared(), "CDS not used for the StringTable");
@@ -774,6 +786,27 @@ void StringTable::oops_do(OopClosure* f) {
   }
 }
 
+void StringTable::oops_do(OopClosure* f) {
+  buckets_do(f, 0, the_table()->table_size());
+}
+
+void StringTable::possibly_parallel_oops_do(OopClosure* f) {
+  const int ClaimChunkSize = 32;
+  const int limit = the_table()->table_size();
+
+  for (;;) {
+    // Grab next set of buckets to scan
+    int start_idx = Atomic::add(ClaimChunkSize, &_parallel_claimed_idx) - ClaimChunkSize;
+    if (start_idx >= limit) {
+      // End of table
+      break;
+    }
+
+    int end_idx = MIN2(limit, start_idx + ClaimChunkSize);
+    buckets_do(f, start_idx, end_idx);
+  }
+}
+
 void StringTable::verify() {
   for (int i = 0; i < the_table()->table_size(); ++i) {
     HashtableEntry<oop, mtSymbol>* p = the_table()->bucket(i);

src/share/vm/classfile/symbolTable.hpp

 /*
- * Copyright (c) 1997, 2012, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 1997, 2013, 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
@@ -246,12 +246,19 @@ private:
   // Set if one bucket is out of balance due to hash algorithm deficiency
   static bool _needs_rehashing;
 
+  // Claimed high water mark for parallel chunked scanning
+  static volatile int _parallel_claimed_idx;
+
   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);
 
   oop lookup(int index, jchar* chars, int length, unsigned int hashValue);
 
+  // Apply the give oop closure to the entries to the buckets
+  // in the range [start_idx, end_idx).
+  static void buckets_do(OopClosure* f, int start_idx, int end_idx);
+
   StringTable() : Hashtable<oop, mtSymbol>((int)StringTableSize,
                               sizeof (HashtableEntry<oop, mtSymbol>)) {}
 
@@ -277,9 +284,12 @@ public:
     unlink_or_oops_do(cl, NULL);
   }
 
-  // Invoke "f->do_oop" on the locations of all oops in the table.
+  // Serially invoke "f->do_oop" on the locations of all oops in the table.
   static void oops_do(OopClosure* f);
 
+  // Possibly parallel version of the above
+  static void possibly_parallel_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.
@@ -315,5 +325,8 @@ public:
   // Rehash the symbol table if it gets out of balance
   static void rehash_table();
   static bool needs_rehashing() { return _needs_rehashing; }
+
+  // Parallel chunked scanning
+  static void clear_parallel_claimed_index() { _parallel_claimed_idx = 0; }
 };
 #endif // SHARE_VM_CLASSFILE_SYMBOLTABLE_HPP

src/share/vm/memory/sharedHeap.cpp

 /*
- * Copyright (c) 2000, 2012, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2000, 2013, 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
@@ -47,7 +47,6 @@ enum SH_process_strong_roots_tasks {
   SH_PS_SystemDictionary_oops_do,
   SH_PS_ClassLoaderDataGraph_oops_do,
   SH_PS_jvmti_oops_do,
-  SH_PS_StringTable_oops_do,
   SH_PS_CodeCache_oops_do,
   // Leave this one last.
   SH_PS_NumElements
@@ -127,6 +126,8 @@ SharedHeap::StrongRootsScope::StrongRootsScope(SharedHeap* outer, bool activate)
 {
   if (_active) {
     outer->change_strong_roots_parity();
+    // Zero the claimed high water mark in the StringTable
+    StringTable::clear_parallel_claimed_index();
   }
 }
 
@@ -154,14 +155,16 @@ void SharedHeap::process_strong_roots(bool activate_scope,
   // Global (strong) JNI handles
   if (!_process_strong_tasks->is_task_claimed(SH_PS_JNIHandles_oops_do))
     JNIHandles::oops_do(roots);
+
   // All threads execute this; the individual threads are task groups.
   CLDToOopClosure roots_from_clds(roots);
   CLDToOopClosure* roots_from_clds_p = (is_scavenging ? NULL : &roots_from_clds);
-  if (ParallelGCThreads > 0) {
-    Threads::possibly_parallel_oops_do(roots, roots_from_clds_p ,code_roots);
+  if (CollectedHeap::use_parallel_gc_threads()) {
+    Threads::possibly_parallel_oops_do(roots, roots_from_clds_p, code_roots);
   } else {
     Threads::oops_do(roots, roots_from_clds_p, code_roots);
   }
+
   if (!_process_strong_tasks-> is_task_claimed(SH_PS_ObjectSynchronizer_oops_do))
     ObjectSynchronizer::oops_do(roots);
   if (!_process_strong_tasks->is_task_claimed(SH_PS_FlatProfiler_oops_do))
@@ -189,8 +192,12 @@ void SharedHeap::process_strong_roots(bool activate_scope,
     }
   }
 
-  if (!_process_strong_tasks->is_task_claimed(SH_PS_StringTable_oops_do)) {
+  // All threads execute the following. A specific chunk of buckets
+  // from the StringTable are the individual tasks.
   if (so & SO_Strings) {
+    if (CollectedHeap::use_parallel_gc_threads()) {
+      StringTable::possibly_parallel_oops_do(roots);
+    } else {
       StringTable::oops_do(roots);
     }
   }