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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  20  * or visit www.oracle.com if you need additional information or have any
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  24 
  25 #ifndef SHARE_VM_MEMORY_GENERATION_HPP
  26 #define SHARE_VM_MEMORY_GENERATION_HPP
  27 
  28 #include "gc_implementation/shared/collectorCounters.hpp"
  29 #include "memory/allocation.hpp"
  30 #include "memory/memRegion.hpp"
  31 #include "memory/referenceProcessor.hpp"
  32 #include "memory/universe.hpp"
  33 #include "memory/watermark.hpp"
  34 #include "runtime/mutex.hpp"
  35 #include "runtime/perfData.hpp"
  36 #include "runtime/virtualspace.hpp"
  37 
  38 // A Generation models a heap area for similarly-aged objects.
  39 // It will contain one ore more spaces holding the actual objects.
  40 //
  41 // The Generation class hierarchy:
  42 //
  43 // Generation                      - abstract base class
  44 // - DefNewGeneration              - allocation area (copy collected)
  45 //   - ParNewGeneration            - a DefNewGeneration that is collected by
  46 //                                   several threads
  47 // - CardGeneration                 - abstract class adding offset array behavior
  48 //   - TenuredGeneration             - tenured (old object) space (markSweepCompact)
  49 //   - ConcurrentMarkSweepGeneration - Mostly Concurrent Mark Sweep Generation
  50 //                                       (Detlefs-Printezis refinement of
  51 //                                       Boehm-Demers-Schenker)
  52 //
  53 // The system configurations currently allowed are:
  54 //
  55 //   DefNewGeneration + TenuredGeneration
  56 //   DefNewGeneration + ConcurrentMarkSweepGeneration
  57 //
  58 //   ParNewGeneration + TenuredGeneration
  59 //   ParNewGeneration + ConcurrentMarkSweepGeneration
  60 //
  61 
  62 class DefNewGeneration;
  63 class GenerationSpec;
  64 class CompactibleSpace;
  65 class ContiguousSpace;
  66 class CompactPoint;
  67 class OopsInGenClosure;
  68 class OopClosure;
  69 class ScanClosure;
  70 class FastScanClosure;
  71 class GenCollectedHeap;
  72 class GenRemSet;
  73 class GCStats;
  74 
  75 // A "ScratchBlock" represents a block of memory in one generation usable by
  76 // another.  It represents "num_words" free words, starting at and including
  77 // the address of "this".
  78 struct ScratchBlock {
  79   ScratchBlock* next;
  80   size_t num_words;
  81   HeapWord scratch_space[1];  // Actually, of size "num_words-2" (assuming
  82                               // first two fields are word-sized.)
  83 };
  84 
  85 
  86 class Generation: public CHeapObj<mtGC> {
  87   friend class VMStructs;
  88  private:
  89   jlong _time_of_last_gc; // time when last gc on this generation happened (ms)
  90   MemRegion _prev_used_region; // for collectors that want to "remember" a value for
  91                                // used region at some specific point during collection.
  92 
  93  protected:
  94   // Minimum and maximum addresses for memory reserved (not necessarily
  95   // committed) for generation.
  96   // Used by card marking code. Must not overlap with address ranges of
  97   // other generations.
  98   MemRegion _reserved;
  99 
 100   // Memory area reserved for generation
 101   VirtualSpace _virtual_space;
 102 
 103   // Level in the generation hierarchy.
 104   int _level;
 105 
 106   // ("Weak") Reference processing support
 107   ReferenceProcessor* _ref_processor;
 108 
 109   // Performance Counters
 110   CollectorCounters* _gc_counters;
 111 
 112   // Statistics for garbage collection
 113   GCStats* _gc_stats;
 114 
 115   // Returns the next generation in the configuration, or else NULL if this
 116   // is the highest generation.
 117   Generation* next_gen() const;
 118 
 119   // Initialize the generation.
 120   Generation(ReservedSpace rs, size_t initial_byte_size, int level);
 121 
 122   // Apply "cl->do_oop" to (the address of) (exactly) all the ref fields in
 123   // "sp" that point into younger generations.
 124   // The iteration is only over objects allocated at the start of the
 125   // iterations; objects allocated as a result of applying the closure are
 126   // not included.
 127   void younger_refs_in_space_iterate(Space* sp, OopsInGenClosure* cl);
 128 
 129  public:
 130   // The set of possible generation kinds.
 131   enum Name {
 132     DefNew,
 133     ParNew,
 134     MarkSweepCompact,
 135     ConcurrentMarkSweep,
 136     Other
 137   };
 138 
 139   enum SomePublicConstants {
 140     // Generations are GenGrain-aligned and have size that are multiples of
 141     // GenGrain.
 142     // Note: on ARM we add 1 bit for card_table_base to be properly aligned
 143     // (we expect its low byte to be zero - see implementation of post_barrier)
 144     LogOfGenGrain = 16 ARM_ONLY(+1),
 145     GenGrain = 1 << LogOfGenGrain
 146   };
 147 
 148   // allocate and initialize ("weak") refs processing support
 149   virtual void ref_processor_init();
 150   void set_ref_processor(ReferenceProcessor* rp) {
 151     assert(_ref_processor == NULL, "clobbering existing _ref_processor");
 152     _ref_processor = rp;
 153   }
 154 
 155   virtual Generation::Name kind() { return Generation::Other; }
 156   GenerationSpec* spec();
 157 
 158   // This properly belongs in the collector, but for now this
 159   // will do.
 160   virtual bool refs_discovery_is_atomic() const { return true;  }
 161   virtual bool refs_discovery_is_mt()     const { return false; }
 162 
 163   // Space enquiries (results in bytes)
 164   virtual size_t capacity() const = 0;  // The maximum number of object bytes the
 165                                         // generation can currently hold.
 166   virtual size_t used() const = 0;      // The number of used bytes in the gen.
 167   virtual size_t free() const = 0;      // The number of free bytes in the gen.
 168 
 169   // Support for java.lang.Runtime.maxMemory(); see CollectedHeap.
 170   // Returns the total number of bytes  available in a generation
 171   // for the allocation of objects.
 172   virtual size_t max_capacity() const;
 173 
 174   // If this is a young generation, the maximum number of bytes that can be
 175   // allocated in this generation before a GC is triggered.
 176   virtual size_t capacity_before_gc() const { return 0; }
 177 
 178   // The largest number of contiguous free bytes in the generation,
 179   // including expansion  (Assumes called at a safepoint.)
 180   virtual size_t contiguous_available() const = 0;
 181   // The largest number of contiguous free bytes in this or any higher generation.
 182   virtual size_t max_contiguous_available() const;
 183 
 184   // Returns true if promotions of the specified amount are
 185   // likely to succeed without a promotion failure.
 186   // Promotion of the full amount is not guaranteed but
 187   // might be attempted in the worst case.
 188   virtual bool promotion_attempt_is_safe(size_t max_promotion_in_bytes) const;
 189 
 190   // For a non-young generation, this interface can be used to inform a
 191   // generation that a promotion attempt into that generation failed.
 192   // Typically used to enable diagnostic output for post-mortem analysis,
 193   // but other uses of the interface are not ruled out.
 194   virtual void promotion_failure_occurred() { /* does nothing */ }
 195 
 196   // Return an estimate of the maximum allocation that could be performed
 197   // in the generation without triggering any collection or expansion
 198   // activity.  It is "unsafe" because no locks are taken; the result
 199   // should be treated as an approximation, not a guarantee, for use in
 200   // heuristic resizing decisions.
 201   virtual size_t unsafe_max_alloc_nogc() const = 0;
 202 
 203   // Returns true if this generation cannot be expanded further
 204   // without a GC. Override as appropriate.
 205   virtual bool is_maximal_no_gc() const {
 206     return _virtual_space.uncommitted_size() == 0;
 207   }
 208 
 209   MemRegion reserved() const { return _reserved; }
 210 
 211   // Returns a region guaranteed to contain all the objects in the
 212   // generation.
 213   virtual MemRegion used_region() const { return _reserved; }
 214 
 215   MemRegion prev_used_region() const { return _prev_used_region; }
 216   virtual void  save_used_region()   { _prev_used_region = used_region(); }
 217 
 218   // Returns "TRUE" iff "p" points into the committed areas in the generation.
 219   // For some kinds of generations, this may be an expensive operation.
 220   // To avoid performance problems stemming from its inadvertent use in
 221   // product jvm's, we restrict its use to assertion checking or
 222   // verification only.
 223   virtual bool is_in(const void* p) const;
 224 
 225   /* Returns "TRUE" iff "p" points into the reserved area of the generation. */
 226   bool is_in_reserved(const void* p) const {
 227     return _reserved.contains(p);
 228   }
 229 
 230   // Check that the generation kind is DefNewGeneration or a sub
 231   // class of DefNewGeneration and return a DefNewGeneration*
 232   DefNewGeneration*  as_DefNewGeneration();
 233 
 234   // If some space in the generation contains the given "addr", return a
 235   // pointer to that space, else return "NULL".
 236   virtual Space* space_containing(const void* addr) const;
 237 
 238   // Iteration - do not use for time critical operations
 239   virtual void space_iterate(SpaceClosure* blk, bool usedOnly = false) = 0;
 240 
 241   // Returns the first space, if any, in the generation that can participate
 242   // in compaction, or else "NULL".
 243   virtual CompactibleSpace* first_compaction_space() const = 0;
 244 
 245   // Returns "true" iff this generation should be used to allocate an
 246   // object of the given size.  Young generations might
 247   // wish to exclude very large objects, for example, since, if allocated
 248   // often, they would greatly increase the frequency of young-gen
 249   // collection.
 250   virtual bool should_allocate(size_t word_size, bool is_tlab) {
 251     bool result = false;
 252     size_t overflow_limit = (size_t)1 << (BitsPerSize_t - LogHeapWordSize);
 253     if (!is_tlab || supports_tlab_allocation()) {
 254       result = (word_size > 0) && (word_size < overflow_limit);
 255     }
 256     return result;
 257   }
 258 
 259   // Allocate and returns a block of the requested size, or returns "NULL".
 260   // Assumes the caller has done any necessary locking.
 261   virtual HeapWord* allocate(size_t word_size, bool is_tlab) = 0;
 262 
 263   // Like "allocate", but performs any necessary locking internally.
 264   virtual HeapWord* par_allocate(size_t word_size, bool is_tlab) = 0;
 265 
 266   // Some generation may offer a region for shared, contiguous allocation,
 267   // via inlined code (by exporting the address of the top and end fields
 268   // defining the extent of the contiguous allocation region.)
 269 
 270   // This function returns "true" iff the heap supports this kind of
 271   // allocation.  (More precisely, this means the style of allocation that
 272   // increments *top_addr()" with a CAS.) (Default is "no".)
 273   // A generation that supports this allocation style must use lock-free
 274   // allocation for *all* allocation, since there are times when lock free
 275   // allocation will be concurrent with plain "allocate" calls.
 276   virtual bool supports_inline_contig_alloc() const { return false; }
 277 
 278   // These functions return the addresses of the fields that define the
 279   // boundaries of the contiguous allocation area.  (These fields should be
 280   // physically near to one another.)
 281   virtual HeapWord** top_addr() const { return NULL; }
 282   virtual HeapWord** end_addr() const { return NULL; }
 283 
 284   // Thread-local allocation buffers
 285   virtual bool supports_tlab_allocation() const { return false; }
 286   virtual size_t tlab_capacity() const {
 287     guarantee(false, "Generation doesn't support thread local allocation buffers");
 288     return 0;
 289   }
 290   virtual size_t tlab_used() const {
 291     guarantee(false, "Generation doesn't support thread local allocation buffers");
 292     return 0;
 293   }
 294   virtual size_t unsafe_max_tlab_alloc() const {
 295     guarantee(false, "Generation doesn't support thread local allocation buffers");
 296     return 0;
 297   }
 298 
 299   // "obj" is the address of an object in a younger generation.  Allocate space
 300   // for "obj" in the current (or some higher) generation, and copy "obj" into
 301   // the newly allocated space, if possible, returning the result (or NULL if
 302   // the allocation failed).
 303   //
 304   // The "obj_size" argument is just obj->size(), passed along so the caller can
 305   // avoid repeating the virtual call to retrieve it.
 306   virtual oop promote(oop obj, size_t obj_size);
 307 
 308   // Thread "thread_num" (0 <= i < ParalleGCThreads) wants to promote
 309   // object "obj", whose original mark word was "m", and whose size is
 310   // "word_sz".  If possible, allocate space for "obj", copy obj into it
 311   // (taking care to copy "m" into the mark word when done, since the mark
 312   // word of "obj" may have been overwritten with a forwarding pointer, and
 313   // also taking care to copy the klass pointer *last*.  Returns the new
 314   // object if successful, or else NULL.
 315   virtual oop par_promote(int thread_num,
 316                           oop obj, markOop m, size_t word_sz);
 317 
 318   // Informs the current generation that all par_promote_alloc's in the
 319   // collection have been completed; any supporting data structures can be
 320   // reset.  Default is to do nothing.
 321   virtual void par_promote_alloc_done(int thread_num) {}
 322 
 323   // Informs the current generation that all oop_since_save_marks_iterates
 324   // performed by "thread_num" in the current collection, if any, have been
 325   // completed; any supporting data structures can be reset.  Default is to
 326   // do nothing.
 327   virtual void par_oop_since_save_marks_iterate_done(int thread_num) {}
 328 
 329   // This generation will collect all younger generations
 330   // during a full collection.
 331   virtual bool full_collects_younger_generations() const { return false; }
 332 
 333   // This generation does in-place marking, meaning that mark words
 334   // are mutated during the marking phase and presumably reinitialized
 335   // to a canonical value after the GC. This is currently used by the
 336   // biased locking implementation to determine whether additional
 337   // work is required during the GC prologue and epilogue.
 338   virtual bool performs_in_place_marking() const { return true; }
 339 
 340   // Returns "true" iff collect() should subsequently be called on this
 341   // this generation. See comment below.
 342   // This is a generic implementation which can be overridden.
 343   //
 344   // Note: in the current (1.4) implementation, when genCollectedHeap's
 345   // incremental_collection_will_fail flag is set, all allocations are
 346   // slow path (the only fast-path place to allocate is DefNew, which
 347   // will be full if the flag is set).
 348   // Thus, older generations which collect younger generations should
 349   // test this flag and collect if it is set.
 350   virtual bool should_collect(bool   full,
 351                               size_t word_size,
 352                               bool   is_tlab) {
 353     return (full || should_allocate(word_size, is_tlab));
 354   }
 355 
 356   // Returns true if the collection is likely to be safely
 357   // completed. Even if this method returns true, a collection
 358   // may not be guaranteed to succeed, and the system should be
 359   // able to safely unwind and recover from that failure, albeit
 360   // at some additional cost.
 361   virtual bool collection_attempt_is_safe() {
 362     guarantee(false, "Are you sure you want to call this method?");
 363     return true;
 364   }
 365 
 366   // Perform a garbage collection.
 367   // If full is true attempt a full garbage collection of this generation.
 368   // Otherwise, attempting to (at least) free enough space to support an
 369   // allocation of the given "word_size".
 370   virtual void collect(bool   full,
 371                        bool   clear_all_soft_refs,
 372                        size_t word_size,
 373                        bool   is_tlab) = 0;
 374 
 375   // Perform a heap collection, attempting to create (at least) enough
 376   // space to support an allocation of the given "word_size".  If
 377   // successful, perform the allocation and return the resulting
 378   // "oop" (initializing the allocated block). If the allocation is
 379   // still unsuccessful, return "NULL".
 380   virtual HeapWord* expand_and_allocate(size_t word_size,
 381                                         bool is_tlab,
 382                                         bool parallel = false) = 0;
 383 
 384   // Some generations may require some cleanup or preparation actions before
 385   // allowing a collection.  The default is to do nothing.
 386   virtual void gc_prologue(bool full) {};
 387 
 388   // Some generations may require some cleanup actions after a collection.
 389   // The default is to do nothing.
 390   virtual void gc_epilogue(bool full) {};
 391 
 392   // Save the high water marks for the used space in a generation.
 393   virtual void record_spaces_top() {};
 394 
 395   // Some generations may need to be "fixed-up" after some allocation
 396   // activity to make them parsable again. The default is to do nothing.
 397   virtual void ensure_parsability() {};
 398 
 399   // Time (in ms) when we were last collected or now if a collection is
 400   // in progress.
 401   virtual jlong time_of_last_gc(jlong now) {
 402     // Both _time_of_last_gc and now are set using a time source
 403     // that guarantees monotonically non-decreasing values provided
 404     // the underlying platform provides such a source. So we still
 405     // have to guard against non-monotonicity.
 406     NOT_PRODUCT(
 407       if (now < _time_of_last_gc) {
 408         warning("time warp: "INT64_FORMAT" to "INT64_FORMAT, (int64_t)_time_of_last_gc, (int64_t)now);
 409       }
 410     )
 411     return _time_of_last_gc;
 412   }
 413 
 414   virtual void update_time_of_last_gc(jlong now)  {
 415     _time_of_last_gc = now;
 416   }
 417 
 418   // Generations may keep statistics about collection.  This
 419   // method updates those statistics.  current_level is
 420   // the level of the collection that has most recently
 421   // occurred.  This allows the generation to decide what
 422   // statistics are valid to collect.  For example, the
 423   // generation can decide to gather the amount of promoted data
 424   // if the collection of the younger generations has completed.
 425   GCStats* gc_stats() const { return _gc_stats; }
 426   virtual void update_gc_stats(int current_level, bool full) {}
 427 
 428   // Mark sweep support phase2
 429   virtual void prepare_for_compaction(CompactPoint* cp);
 430   // Mark sweep support phase3
 431   virtual void adjust_pointers();
 432   // Mark sweep support phase4
 433   virtual void compact();
 434   virtual void post_compact() {ShouldNotReachHere();}
 435 
 436   // Support for CMS's rescan. In this general form we return a pointer
 437   // to an abstract object that can be used, based on specific previously
 438   // decided protocols, to exchange information between generations,
 439   // information that may be useful for speeding up certain types of
 440   // garbage collectors. A NULL value indicates to the client that
 441   // no data recording is expected by the provider. The data-recorder is
 442   // expected to be GC worker thread-local, with the worker index
 443   // indicated by "thr_num".
 444   virtual void* get_data_recorder(int thr_num) { return NULL; }
 445   virtual void sample_eden_chunk() {}
 446 
 447   // Some generations may require some cleanup actions before allowing
 448   // a verification.
 449   virtual void prepare_for_verify() {};
 450 
 451   // Accessing "marks".
 452 
 453   // This function gives a generation a chance to note a point between
 454   // collections.  For example, a contiguous generation might note the
 455   // beginning allocation point post-collection, which might allow some later
 456   // operations to be optimized.
 457   virtual void save_marks() {}
 458 
 459   // This function allows generations to initialize any "saved marks".  That
 460   // is, should only be called when the generation is empty.
 461   virtual void reset_saved_marks() {}
 462 
 463   // This function is "true" iff any no allocations have occurred in the
 464   // generation since the last call to "save_marks".
 465   virtual bool no_allocs_since_save_marks() = 0;
 466 
 467   // Apply "cl->apply" to (the addresses of) all reference fields in objects
 468   // allocated in the current generation since the last call to "save_marks".
 469   // If more objects are allocated in this generation as a result of applying
 470   // the closure, iterates over reference fields in those objects as well.
 471   // Calls "save_marks" at the end of the iteration.
 472   // General signature...
 473   virtual void oop_since_save_marks_iterate_v(OopsInGenClosure* cl) = 0;
 474   // ...and specializations for de-virtualization.  (The general
 475   // implementation of the _nv versions call the virtual version.
 476   // Note that the _nv suffix is not really semantically necessary,
 477   // but it avoids some not-so-useful warnings on Solaris.)
 478 #define Generation_SINCE_SAVE_MARKS_DECL(OopClosureType, nv_suffix)             \
 479   virtual void oop_since_save_marks_iterate##nv_suffix(OopClosureType* cl) {    \
 480     oop_since_save_marks_iterate_v((OopsInGenClosure*)cl);                      \
 481   }
 482   SPECIALIZED_SINCE_SAVE_MARKS_CLOSURES(Generation_SINCE_SAVE_MARKS_DECL)
 483 
 484 #undef Generation_SINCE_SAVE_MARKS_DECL
 485 
 486   // The "requestor" generation is performing some garbage collection
 487   // action for which it would be useful to have scratch space.  If
 488   // the target is not the requestor, no gc actions will be required
 489   // of the target.  The requestor promises to allocate no more than
 490   // "max_alloc_words" in the target generation (via promotion say,
 491   // if the requestor is a young generation and the target is older).
 492   // If the target generation can provide any scratch space, it adds
 493   // it to "list", leaving "list" pointing to the head of the
 494   // augmented list.  The default is to offer no space.
 495   virtual void contribute_scratch(ScratchBlock*& list, Generation* requestor,
 496                                   size_t max_alloc_words) {}
 497 
 498   // Give each generation an opportunity to do clean up for any
 499   // contributed scratch.
 500   virtual void reset_scratch() {};
 501 
 502   // When an older generation has been collected, and perhaps resized,
 503   // this method will be invoked on all younger generations (from older to
 504   // younger), allowing them to resize themselves as appropriate.
 505   virtual void compute_new_size() = 0;
 506 
 507   // Printing
 508   virtual const char* name() const = 0;
 509   virtual const char* short_name() const = 0;
 510 
 511   int level() const { return _level; }
 512 
 513   // Reference Processing accessor
 514   ReferenceProcessor* const ref_processor() { return _ref_processor; }
 515 
 516   // Iteration.
 517 
 518   // Iterate over all the ref-containing fields of all objects in the
 519   // generation, calling "cl.do_oop" on each.
 520   virtual void oop_iterate(ExtendedOopClosure* cl);
 521 
 522   // Iterate over all objects in the generation, calling "cl.do_object" on
 523   // each.
 524   virtual void object_iterate(ObjectClosure* cl);
 525 
 526   // Iterate over all safe objects in the generation, calling "cl.do_object" on
 527   // each.  An object is safe if its references point to other objects in
 528   // the heap.  This defaults to object_iterate() unless overridden.
 529   virtual void safe_object_iterate(ObjectClosure* cl);
 530 
 531   // Apply "cl->do_oop" to (the address of) all and only all the ref fields
 532   // in the current generation that contain pointers to objects in younger
 533   // generations. Objects allocated since the last "save_marks" call are
 534   // excluded.
 535   virtual void younger_refs_iterate(OopsInGenClosure* cl) = 0;
 536 
 537   // Inform a generation that it longer contains references to objects
 538   // in any younger generation.    [e.g. Because younger gens are empty,
 539   // clear the card table.]
 540   virtual void clear_remembered_set() { }
 541 
 542   // Inform a generation that some of its objects have moved.  [e.g. The
 543   // generation's spaces were compacted, invalidating the card table.]
 544   virtual void invalidate_remembered_set() { }
 545 
 546   // Block abstraction.
 547 
 548   // Returns the address of the start of the "block" that contains the
 549   // address "addr".  We say "blocks" instead of "object" since some heaps
 550   // may not pack objects densely; a chunk may either be an object or a
 551   // non-object.
 552   virtual HeapWord* block_start(const void* addr) const;
 553 
 554   // Requires "addr" to be the start of a chunk, and returns its size.
 555   // "addr + size" is required to be the start of a new chunk, or the end
 556   // of the active area of the heap.
 557   virtual size_t block_size(const HeapWord* addr) const ;
 558 
 559   // Requires "addr" to be the start of a block, and returns "TRUE" iff
 560   // the block is an object.
 561   virtual bool block_is_obj(const HeapWord* addr) const;
 562 
 563 
 564   // PrintGC, PrintGCDetails support
 565   void print_heap_change(size_t prev_used) const;
 566 
 567   // PrintHeapAtGC support
 568   virtual void print() const;
 569   virtual void print_on(outputStream* st) const;
 570 
 571   virtual void verify() = 0;
 572 
 573   struct StatRecord {
 574     int invocations;
 575     elapsedTimer accumulated_time;
 576     StatRecord() :
 577       invocations(0),
 578       accumulated_time(elapsedTimer()) {}
 579   };
 580 private:
 581   StatRecord _stat_record;
 582 public:
 583   StatRecord* stat_record() { return &_stat_record; }
 584 
 585   virtual void print_summary_info();
 586   virtual void print_summary_info_on(outputStream* st);
 587 
 588   // Performance Counter support
 589   virtual void update_counters() = 0;
 590   virtual CollectorCounters* counters() { return _gc_counters; }
 591 };
 592 
 593 // Class CardGeneration is a generation that is covered by a card table,
 594 // and uses a card-size block-offset array to implement block_start.
 595 
 596 // class BlockOffsetArray;
 597 // class BlockOffsetArrayContigSpace;
 598 class BlockOffsetSharedArray;
 599 
 600 class CardGeneration: public Generation {
 601   friend class VMStructs;
 602  protected:
 603   // This is shared with other generations.
 604   GenRemSet* _rs;
 605   // This is local to this generation.
 606   BlockOffsetSharedArray* _bts;
 607 
 608   // current shrinking effect: this damps shrinking when the heap gets empty.
 609   size_t _shrink_factor;
 610 
 611   size_t _min_heap_delta_bytes;   // Minimum amount to expand.
 612 
 613   // Some statistics from before gc started.
 614   // These are gathered in the gc_prologue (and should_collect)
 615   // to control growing/shrinking policy in spite of promotions.
 616   size_t _capacity_at_prologue;
 617   size_t _used_at_prologue;
 618 
 619   CardGeneration(ReservedSpace rs, size_t initial_byte_size, int level,
 620                  GenRemSet* remset);
 621 
 622  public:
 623 
 624   // Attempt to expand the generation by "bytes".  Expand by at a
 625   // minimum "expand_bytes".  Return true if some amount (not
 626   // necessarily the full "bytes") was done.
 627   virtual bool expand(size_t bytes, size_t expand_bytes);
 628 
 629   // Shrink generation with specified size (returns false if unable to shrink)
 630   virtual void shrink(size_t bytes) = 0;
 631 
 632   virtual void compute_new_size();
 633 
 634   virtual void clear_remembered_set();
 635 
 636   virtual void invalidate_remembered_set();
 637 
 638   virtual void prepare_for_verify();
 639 
 640   // Grow generation with specified size (returns false if unable to grow)
 641   virtual bool grow_by(size_t bytes) = 0;
 642   // Grow generation to reserved size.
 643   virtual bool grow_to_reserved() = 0;
 644 };
 645 
 646 #endif // SHARE_VM_MEMORY_GENERATION_HPP