1 /*

     2  * Copyright (c) 2014, Oracle and/or its affiliates. All rights reserved.

     3  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.

     4  *

     5  * This code is free software; you can redistribute it and/or modify it

     6  * under the terms of the GNU General Public License version 2 only, as

     7  * published by the Free Software Foundation.

     8  *

     9  * This code is distributed in the hope that it will be useful, but WITHOUT

    10  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or

    11  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License

    12  * version 2 for more details (a copy is included in the LICENSE file that

    13  * accompanied this code).

    14  *

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    17  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.

    18  *

    19  * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA

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    24 

    25 #ifndef SHARE_VM_GC_IMPLEMENTATION_G1_G1STRINGDEDUP_HPP

    26 #define SHARE_VM_GC_IMPLEMENTATION_G1_G1STRINGDEDUP_HPP

    27 

    28 //

    29 // String Deduplication

    30 //

    31 // String deduplication aims to reduce the heap live-set by deduplicating identical

    32 // instances of String so that they share the same backing character array.

    33 //

    34 // The deduplication process is divided in two main parts, 1) finding the objects to

    35 // deduplicate, and 2) deduplicating those objects. The first part is done as part of

    36 // a normal GC cycle when objects are marked or evacuated. At this time a check is

    37 // applied on each object to check if it is a candidate for deduplication. If so, the

    38 // object is placed on the deduplication queue for later processing. The second part,

    39 // processing the objects on the deduplication queue, is a concurrent phase which

    40 // starts right after the stop-the-wold marking/evacuation phase. This phase is

    41 // executed by the deduplication thread, which pulls deduplication candidates of the

    42 // deduplication queue and tries to deduplicate them.

    43 //

    44 // A deduplication hashtable is used to keep track of all unique character arrays

    45 // used by String objects. When deduplicating, a lookup is made in this table to see

    46 // if there is already an identical character array somewhere on the heap. If so, the

    47 // String object is adjusted to point to that character array, releasing the reference

    48 // to the original array allowing it to eventually be garbage collected. If the lookup

    49 // fails the character array is instead inserted into the hashtable so that this array

    50 // can be shared at some point in the future.

    51 //

    52 // Candidate selection

    53 //

    54 // An object is considered a deduplication candidate if all of the following

    55 // statements are true:

    56 //

    57 // - The object is an instance of java.lang.String

    58 //

    59 // - The object is being evacuated from a young heap region

    60 //

    61 // - The object is being evacuated to a young/survivor heap region and the

    62 //   object's age is equal to the deduplication age threshold

    63 //

    64 //   or

    65 //

    66 //   The object is being evacuated to an old heap region and the object's age is

    67 //   less than the deduplication age threshold

    68 //

    69 // Once an string object has been promoted to an old region, or its age is higher

    70 // than the deduplication age threshold, is will never become a candidate again.

    71 // This approach avoids making the same object a candidate more than once.

    72 //

    73 // Interned strings are a bit special. They are explicitly deduplicated just before

    74 // being inserted into the StringTable (to avoid counteracting C2 optimizations done

    75 // on string literals), then they also become deduplication candidates if they reach

    76 // the deduplication age threshold or are evacuated to an old heap region. The second

    77 // attempt to deduplicate such strings will be in vain, but we have no fast way of

    78 // filtering them out. This has not shown to be a problem, as the number of interned

    79 // strings is usually dwarfed by the number of normal (non-interned) strings.

    80 //

    81 // For additional information on string deduplication, please see JEP 192,

    82 // http://openjdk.java.net/jeps/192

    83 //

    84 

    85 #include "memory/allocation.hpp"

    86 #include "oops/oop.hpp"

    87 

    88 class OopClosure;

    89 class BoolObjectClosure;

    90 class ThreadClosure;

    91 class outputStream;

    92 class G1StringDedupTable;

    93 

    94 //

    95 // Main interface for interacting with string deduplication.

    96 //

    97 class G1StringDedup : public AllStatic {

    98 private:

    99   // Single state for checking if both G1 and string deduplication is enabled.

   100   static bool _enabled;

   101 

   102   // Candidate selection policies, returns true if the given object is

   103   // candidate for string deduplication.

   104   static bool is_candidate_from_mark(oop obj);

   105   static bool is_candidate_from_evacuation(bool from_young, bool to_young, oop obj);

   106 

   107 public:

   108   // Returns true if both G1 and string deduplication is enabled.

   109   static bool is_enabled() {

   110     return _enabled;

   111   }

   112 

   113   static void initialize();

   114 

   115   // Immediately deduplicates the given String object, bypassing the

   116   // the deduplication queue.

   117   static void deduplicate(oop java_string);

   118 

   119   // Enqueues a deduplication candidate for later processing by the deduplication

   120   // thread. Before enqueuing, these functions apply the appropriate candidate

   121   // selection policy to filters out non-candidates.

   122   static void enqueue_from_mark(oop java_string);

   123   static void enqueue_from_evacuation(bool from_young, bool to_young,

   124                                       unsigned int queue, oop java_string);

   125 

   126   static void oops_do(OopClosure* keep_alive);

   127   static void unlink(BoolObjectClosure* is_alive);

   128   static void unlink_or_oops_do(BoolObjectClosure* is_alive, OopClosure* keep_alive,

   129                                 bool allow_resize_and_rehash = true);

   130 

   131   static void threads_do(ThreadClosure* tc);

   132   static void print_worker_threads_on(outputStream* st);

   133   static void verify();

   134 };

   135 

   136 //

   137 // This closure encapsulates the state and the closures needed when scanning

   138 // the deduplication queue and table during the unlink_or_oops_do() operation.

   139 // A single instance of this closure is created and then shared by all worker

   140 // threads participating in the scan. The _next_queue and _next_bucket fields

   141 // provide a simple mechanism for GC workers to claim exclusive access to a

   142 // queue or a table partition.

   143 //

   144 class G1StringDedupUnlinkOrOopsDoClosure : public StackObj {

   145 private:

   146   BoolObjectClosure*  _is_alive;

   147   OopClosure*         _keep_alive;

   148   G1StringDedupTable* _resized_table;

   149   G1StringDedupTable* _rehashed_table;

   150   size_t              _next_queue;

   151   size_t              _next_bucket;

   152 

   153 public:

   154   G1StringDedupUnlinkOrOopsDoClosure(BoolObjectClosure* is_alive,

   155                                      OopClosure* keep_alive,

   156                                      bool allow_resize_and_rehash);

   157   ~G1StringDedupUnlinkOrOopsDoClosure();

   158 

   159   bool is_resizing() {

   160     return _resized_table != NULL;

   161   }

   162 

   163   G1StringDedupTable* resized_table() {

   164     return _resized_table;

   165   }

   166 

   167   bool is_rehashing() {

   168     return _rehashed_table != NULL;

   169   }

   170 

   171   // Atomically claims the next available queue for exclusive access by

   172   // the current thread. Returns the queue number of the claimed queue.

   173   size_t claim_queue() {

   174     return (size_t)Atomic::add_ptr(1, &_next_queue) - 1;

   175   }

   176 

   177   // Atomically claims the next available table partition for exclusive

   178   // access by the current thread. Returns the table bucket number where

   179   // the claimed partition starts.

   180   size_t claim_table_partition(size_t partition_size) {

   181     return (size_t)Atomic::add_ptr(partition_size, &_next_bucket) - partition_size;

   182   }

   183 

   184   // Applies and returns the result from the is_alive closure, or

   185   // returns true if no such closure was provided.

   186   bool is_alive(oop o) {

   187     if (_is_alive != NULL) {

   188       return _is_alive->do_object_b(o);

   189     }

   190     return true;

   191   }

   192 

   193   // Applies the keep_alive closure, or does nothing if no such

   194   // closure was provided.

   195   void keep_alive(oop* p) {

   196     if (_keep_alive != NULL) {

   197       _keep_alive->do_oop(p);

   198     }

   199   }

   200 };

   201 

   202 #endif // SHARE_VM_GC_IMPLEMENTATION_G1_G1STRINGDEDUP_HPP