jdk8-mips64-public/hotspot: src/share/vm/gc_implementation/g1/g1StringDedup.hpp@595c0f60d50d (annotated)

src/share/vm/gc_implementation/g1/g1StringDedup.hpp@595c0f60d50d (annotated)

src/share/vm/gc_implementation/g1/g1StringDedup.hpp

Tue, 18 Mar 2014 19:07:22 +0100

author: pliden
date: Tue, 18 Mar 2014 19:07:22 +0100
changeset 6413: 595c0f60d50d
child 6690: 1772223a25a2
permissions: -rw-r--r--

8029075: String deduplication in G1
Summary: Implementation of JEP 192, http://openjdk.java.net/jeps/192
Reviewed-by: brutisso, tschatzl, coleenp

 /*
  * Copyright (c) 2014, 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_GC_IMPLEMENTATION_G1_G1STRINGDEDUP_HPP
 #define SHARE_VM_GC_IMPLEMENTATION_G1_G1STRINGDEDUP_HPP
 //
 // String Deduplication
 //
 // String deduplication aims to reduce the heap live-set by deduplicating identical
 // instances of String so that they share the same backing character array.
 //
 // The deduplication process is divided in two main parts, 1) finding the objects to
 // deduplicate, and 2) deduplicating those objects. The first part is done as part of
 // a normal GC cycle when objects are marked or evacuated. At this time a check is
 // applied on each object to check if it is a candidate for deduplication. If so, the
 // object is placed on the deduplication queue for later processing. The second part,
 // processing the objects on the deduplication queue, is a concurrent phase which
 // starts right after the stop-the-wold marking/evacuation phase. This phase is
 // executed by the deduplication thread, which pulls deduplication candidates of the
 // deduplication queue and tries to deduplicate them.
 //
 // A deduplication hashtable is used to keep track of all unique character arrays
 // used by String objects. When deduplicating, a lookup is made in this table to see
 // if there is already an identical character array somewhere on the heap. If so, the
 // String object is adjusted to point to that character array, releasing the reference
 // to the original array allowing it to eventually be garbage collected. If the lookup
 // fails the character array is instead inserted into the hashtable so that this array
 // can be shared at some point in the future.
 //
 // Candidate selection
 //
 // An object is considered a deduplication candidate if all of the following
 // statements are true:
 //
 // - The object is an instance of java.lang.String
 //
 // - The object is being evacuated from a young heap region
 //
 // - The object is being evacuated to a young/survivor heap region and the
 //   object's age is equal to the deduplication age threshold
 //
 //   or
 //
 //   The object is being evacuated to an old heap region and the object's age is
 //   less than the deduplication age threshold
 //
 // Once an string object has been promoted to an old region, or its age is higher
 // than the deduplication age threshold, is will never become a candidate again.
 // This approach avoids making the same object a candidate more than once.
 //
 // Interned strings are a bit special. They are explicitly deduplicated just before
 // being inserted into the StringTable (to avoid counteracting C2 optimizations done
 // on string literals), then they also become deduplication candidates if they reach
 // the deduplication age threshold or are evacuated to an old heap region. The second
 // attempt to deduplicate such strings will be in vain, but we have no fast way of
 // filtering them out. This has not shown to be a problem, as the number of interned
 // strings is usually dwarfed by the number of normal (non-interned) strings.
 //
 // For additional information on string deduplication, please see JEP 192,
 // http://openjdk.java.net/jeps/192
 //
 #include "memory/allocation.hpp"
 #include "oops/oop.hpp"
 class OopClosure;
 class BoolObjectClosure;
 class ThreadClosure;
 class outputStream;
 class G1StringDedupTable;
 //
 // Main interface for interacting with string deduplication.
 //
 class G1StringDedup : public AllStatic {
 private:
   // Single state for checking if both G1 and string deduplication is enabled.
   static bool _enabled;
   // Candidate selection policies, returns true if the given object is
   // candidate for string deduplication.
   static bool is_candidate_from_mark(oop obj);
   static bool is_candidate_from_evacuation(bool from_young, bool to_young, oop obj);
 public:
   // Returns true if both G1 and string deduplication is enabled.
   static bool is_enabled() {
     return _enabled;
   }
   static void initialize();
   // Immediately deduplicates the given String object, bypassing the
   // the deduplication queue.
   static void deduplicate(oop java_string);
   // Enqueues a deduplication candidate for later processing by the deduplication
   // thread. Before enqueuing, these functions apply the appropriate candidate
   // selection policy to filters out non-candidates.
   static void enqueue_from_mark(oop java_string);
   static void enqueue_from_evacuation(bool from_young, bool to_young,
                                       unsigned int queue, oop java_string);
   static void oops_do(OopClosure* keep_alive);
   static void unlink(BoolObjectClosure* is_alive);
   static void unlink_or_oops_do(BoolObjectClosure* is_alive, OopClosure* keep_alive,
                                 bool allow_resize_and_rehash = true);
   static void threads_do(ThreadClosure* tc);
   static void print_worker_threads_on(outputStream* st);
   static void verify();
 };
 //
 // This closure encapsulates the state and the closures needed when scanning
 // the deduplication queue and table during the unlink_or_oops_do() operation.
 // A single instance of this closure is created and then shared by all worker
 // threads participating in the scan. The _next_queue and _next_bucket fields
 // provide a simple mechanism for GC workers to claim exclusive access to a
 // queue or a table partition.
 //
 class G1StringDedupUnlinkOrOopsDoClosure : public StackObj {
 private:
   BoolObjectClosure*  _is_alive;
   OopClosure*         _keep_alive;
   G1StringDedupTable* _resized_table;
   G1StringDedupTable* _rehashed_table;
   size_t              _next_queue;
   size_t              _next_bucket;
 public:
   G1StringDedupUnlinkOrOopsDoClosure(BoolObjectClosure* is_alive,
                                      OopClosure* keep_alive,
                                      bool allow_resize_and_rehash);
   ~G1StringDedupUnlinkOrOopsDoClosure();
   bool is_resizing() {
     return _resized_table != NULL;
   }
   G1StringDedupTable* resized_table() {
     return _resized_table;
   }
   bool is_rehashing() {
     return _rehashed_table != NULL;
   }
   // Atomically claims the next available queue for exclusive access by
   // the current thread. Returns the queue number of the claimed queue.
   size_t claim_queue() {
     return (size_t)Atomic::add_ptr(1, &_next_queue) - 1;
   }
   // Atomically claims the next available table partition for exclusive
   // access by the current thread. Returns the table bucket number where
   // the claimed partition starts.
   size_t claim_table_partition(size_t partition_size) {
     return (size_t)Atomic::add_ptr(partition_size, &_next_bucket) - partition_size;
   }
   // Applies and returns the result from the is_alive closure, or
   // returns true if no such closure was provided.
   bool is_alive(oop o) {
     if (_is_alive != NULL) {
       return _is_alive->do_object_b(o);
     }
     return true;
   }
   // Applies the keep_alive closure, or does nothing if no such
   // closure was provided.
   void keep_alive(oop* p) {
     if (_keep_alive != NULL) {
       _keep_alive->do_oop(p);
     }
   }
 };
 #endif // SHARE_VM_GC_IMPLEMENTATION_G1_G1STRINGDEDUP_HPP

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src/share/vm/gc_implementation/g1/g1StringDedup.hpp@595c0f60d50d (annotated)

src/share/vm/gc_implementation/g1/g1StringDedup.hpp