1 /*

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

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

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    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).

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    24 

    25 #ifndef SHARE_VM_GC_G1_G1STRINGDEDUP_HPP

    26 #define SHARE_VM_GC_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 class G1StringDedupUnlinkOrOopsDoClosure;

    94 class G1GCPhaseTimes;

    95 

    96 //

    97 // Main interface for interacting with string deduplication.

    98 //

    99 class G1StringDedup : public AllStatic {

   100 private:

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

   102   static bool _enabled;

   103 

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

   105   // candidate for string deduplication.

   106   static bool is_candidate_from_mark(oop obj);

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

   108 

   109 public:

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

   111   static bool is_enabled() {

   112     return _enabled;

   113   }

   114 

   115   // Initialize string deduplication.

   116   static void initialize();

   117 

   118   // Stop the deduplication thread.

   119   static void stop();

   120 

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

   122   // the deduplication queue.

   123   static void deduplicate(oop java_string);

   124 

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

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

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

   128   static void enqueue_from_mark(oop java_string);

   129   static void enqueue_from_evacuation(bool from_young, bool to_young,

   130                                       unsigned int queue, oop java_string);

   131 

   132   static void oops_do(OopClosure* keep_alive);

   133   static void parallel_unlink(G1StringDedupUnlinkOrOopsDoClosure* unlink, uint worker_id);

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

   135                                 bool allow_resize_and_rehash, G1GCPhaseTimes* phase_times = NULL);

   136 

   137   static void threads_do(ThreadClosure* tc);

   138   static void print_worker_threads_on(outputStream* st);

   139   static void verify();

   140 };

   141 

   142 //

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

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

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

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

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

   148 // queue or a table partition.

   149 //

   150 class G1StringDedupUnlinkOrOopsDoClosure : public StackObj {

   151 private:

   152   BoolObjectClosure*  _is_alive;

   153   OopClosure*         _keep_alive;

   154   G1StringDedupTable* _resized_table;

   155   G1StringDedupTable* _rehashed_table;

   156   size_t              _next_queue;

   157   size_t              _next_bucket;

   158 

   159 public:

   160   G1StringDedupUnlinkOrOopsDoClosure(BoolObjectClosure* is_alive,

   161                                      OopClosure* keep_alive,

   162                                      bool allow_resize_and_rehash);

   163   ~G1StringDedupUnlinkOrOopsDoClosure();

   164 

   165   bool is_resizing() {

   166     return _resized_table != NULL;

   167   }

   168 

   169   G1StringDedupTable* resized_table() {

   170     return _resized_table;

   171   }

   172 

   173   bool is_rehashing() {

   174     return _rehashed_table != NULL;

   175   }

   176 

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

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

   179   size_t claim_queue();

   180 

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

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

   183   // the claimed partition starts.

   184   size_t claim_table_partition(size_t partition_size);

   185 

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

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

   188   bool is_alive(oop o) {

   189     if (_is_alive != NULL) {

   190       return _is_alive->do_object_b(o);

   191     }

   192     return true;

   193   }

   194 

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

   196   // closure was provided.

   197   void keep_alive(oop* p) {

   198     if (_keep_alive != NULL) {

   199       _keep_alive->do_oop(p);

   200     }

   201   }

   202 };

   203 

   204 #endif // SHARE_VM_GC_G1_G1STRINGDEDUP_HPP