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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  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).
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  17  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
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  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 class G1GCPhaseTimes;
  94 
  95 //
  96 // Main interface for interacting with string deduplication.
  97 //
  98 class G1StringDedup : public AllStatic {
  99 private:
 100   // Single state for checking if both G1 and string deduplication is enabled.
 101   static bool _enabled;
 102 
 103   // Candidate selection policies, returns true if the given object is
 104   // candidate for string deduplication.
 105   static bool is_candidate_from_mark(oop obj);
 106   static bool is_candidate_from_evacuation(bool from_young, bool to_young, oop obj);
 107 
 108 public:
 109   // Returns true if both G1 and string deduplication is enabled.
 110   static bool is_enabled() {
 111     return _enabled;
 112   }
 113 
 114   // Initialize string deduplication.
 115   static void initialize();
 116 
 117   // Stop the deduplication thread.
 118   static void stop();
 119 
 120   // Immediately deduplicates the given String object, bypassing the
 121   // the deduplication queue.
 122   static void deduplicate(oop java_string);
 123 
 124   // Enqueues a deduplication candidate for later processing by the deduplication
 125   // thread. Before enqueuing, these functions apply the appropriate candidate
 126   // selection policy to filters out non-candidates.
 127   static void enqueue_from_mark(oop java_string);
 128   static void enqueue_from_evacuation(bool from_young, bool to_young,
 129                                       unsigned int queue, oop java_string);
 130 
 131   static void oops_do(OopClosure* keep_alive);
 132   static void unlink(BoolObjectClosure* is_alive);
 133   static void unlink_or_oops_do(BoolObjectClosure* is_alive, OopClosure* keep_alive,
 134                                 bool allow_resize_and_rehash, G1GCPhaseTimes* phase_times = NULL);
 135 
 136   static void threads_do(ThreadClosure* tc);
 137   static void print_worker_threads_on(outputStream* st);
 138   static void verify();
 139 };
 140 
 141 //
 142 // This closure encapsulates the state and the closures needed when scanning
 143 // the deduplication queue and table during the unlink_or_oops_do() operation.
 144 // A single instance of this closure is created and then shared by all worker
 145 // threads participating in the scan. The _next_queue and _next_bucket fields
 146 // provide a simple mechanism for GC workers to claim exclusive access to a
 147 // queue or a table partition.
 148 //
 149 class G1StringDedupUnlinkOrOopsDoClosure : public StackObj {
 150 private:
 151   BoolObjectClosure*  _is_alive;
 152   OopClosure*         _keep_alive;
 153   G1StringDedupTable* _resized_table;
 154   G1StringDedupTable* _rehashed_table;
 155   size_t              _next_queue;
 156   size_t              _next_bucket;
 157 
 158 public:
 159   G1StringDedupUnlinkOrOopsDoClosure(BoolObjectClosure* is_alive,
 160                                      OopClosure* keep_alive,
 161                                      bool allow_resize_and_rehash);
 162   ~G1StringDedupUnlinkOrOopsDoClosure();
 163 
 164   bool is_resizing() {
 165     return _resized_table != NULL;
 166   }
 167 
 168   G1StringDedupTable* resized_table() {
 169     return _resized_table;
 170   }
 171 
 172   bool is_rehashing() {
 173     return _rehashed_table != NULL;
 174   }
 175 
 176   // Atomically claims the next available queue for exclusive access by
 177   // the current thread. Returns the queue number of the claimed queue.
 178   size_t claim_queue() {
 179     return (size_t)Atomic::add_ptr(1, &_next_queue) - 1;
 180   }
 181 
 182   // Atomically claims the next available table partition for exclusive
 183   // access by the current thread. Returns the table bucket number where
 184   // the claimed partition starts.
 185   size_t claim_table_partition(size_t partition_size) {
 186     return (size_t)Atomic::add_ptr(partition_size, &_next_bucket) - partition_size;
 187   }
 188 
 189   // Applies and returns the result from the is_alive closure, or
 190   // returns true if no such closure was provided.
 191   bool is_alive(oop o) {
 192     if (_is_alive != NULL) {
 193       return _is_alive->do_object_b(o);
 194     }
 195     return true;
 196   }
 197 
 198   // Applies the keep_alive closure, or does nothing if no such
 199   // closure was provided.
 200   void keep_alive(oop* p) {
 201     if (_keep_alive != NULL) {
 202       _keep_alive->do_oop(p);
 203     }
 204   }
 205 };
 206 
 207 #endif // SHARE_VM_GC_IMPLEMENTATION_G1_G1STRINGDEDUP_HPP