1 /* 2 * Copyright (c) 2001, 2013, 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 * 15 * You should have received a copy of the GNU General Public License version 16 * 2 along with this work; if not, write to the Free Software Foundation, 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 20 * or visit www.oracle.com if you need additional information or have any 21 * questions. 22 * 23 */ 24 25 #ifndef SHARE_VM_GC_IMPLEMENTATION_G1_CONCURRENTMARK_HPP 26 #define SHARE_VM_GC_IMPLEMENTATION_G1_CONCURRENTMARK_HPP 27 28 #include "gc_implementation/g1/heapRegionSet.hpp" 29 #include "utilities/taskqueue.hpp" 30 31 class G1CollectedHeap; 32 class CMTask; 33 typedef GenericTaskQueue<oop, mtGC> CMTaskQueue; 34 typedef GenericTaskQueueSet<CMTaskQueue, mtGC> CMTaskQueueSet; 35 36 // Closure used by CM during concurrent reference discovery 37 // and reference processing (during remarking) to determine 38 // if a particular object is alive. It is primarily used 39 // to determine if referents of discovered reference objects 40 // are alive. An instance is also embedded into the 41 // reference processor as the _is_alive_non_header field 42 class G1CMIsAliveClosure: public BoolObjectClosure { 43 G1CollectedHeap* _g1; 44 public: 45 G1CMIsAliveClosure(G1CollectedHeap* g1) : _g1(g1) { } 46 47 bool do_object_b(oop obj); 48 }; 49 50 // A generic CM bit map. This is essentially a wrapper around the BitMap 51 // class, with one bit per (1<<_shifter) HeapWords. 52 53 class CMBitMapRO VALUE_OBJ_CLASS_SPEC { 54 protected: 55 HeapWord* _bmStartWord; // base address of range covered by map 56 size_t _bmWordSize; // map size (in #HeapWords covered) 57 const int _shifter; // map to char or bit 58 VirtualSpace _virtual_space; // underlying the bit map 59 BitMap _bm; // the bit map itself 60 61 public: 62 // constructor 63 CMBitMapRO(int shifter); 64 65 enum { do_yield = true }; 66 67 // inquiries 68 HeapWord* startWord() const { return _bmStartWord; } 69 size_t sizeInWords() const { return _bmWordSize; } 70 // the following is one past the last word in space 71 HeapWord* endWord() const { return _bmStartWord + _bmWordSize; } 72 73 // read marks 74 75 bool isMarked(HeapWord* addr) const { 76 assert(_bmStartWord <= addr && addr < (_bmStartWord + _bmWordSize), 77 "outside underlying space?"); 78 return _bm.at(heapWordToOffset(addr)); 79 } 80 81 // iteration 82 inline bool iterate(BitMapClosure* cl, MemRegion mr); 83 inline bool iterate(BitMapClosure* cl); 84 85 // Return the address corresponding to the next marked bit at or after 86 // "addr", and before "limit", if "limit" is non-NULL. If there is no 87 // such bit, returns "limit" if that is non-NULL, or else "endWord()". 88 HeapWord* getNextMarkedWordAddress(HeapWord* addr, 89 HeapWord* limit = NULL) const; 90 // Return the address corresponding to the next unmarked bit at or after 91 // "addr", and before "limit", if "limit" is non-NULL. If there is no 92 // such bit, returns "limit" if that is non-NULL, or else "endWord()". 93 HeapWord* getNextUnmarkedWordAddress(HeapWord* addr, 94 HeapWord* limit = NULL) const; 95 96 // conversion utilities 97 HeapWord* offsetToHeapWord(size_t offset) const { 98 return _bmStartWord + (offset << _shifter); 99 } 100 size_t heapWordToOffset(HeapWord* addr) const { 101 return pointer_delta(addr, _bmStartWord) >> _shifter; 102 } 103 int heapWordDiffToOffsetDiff(size_t diff) const; 104 105 // The argument addr should be the start address of a valid object 106 HeapWord* nextObject(HeapWord* addr) { 107 oop obj = (oop) addr; 108 HeapWord* res = addr + obj->size(); 109 assert(offsetToHeapWord(heapWordToOffset(res)) == res, "sanity"); 110 return res; 111 } 112 113 void print_on_error(outputStream* st, const char* prefix) const; 114 115 // debugging 116 NOT_PRODUCT(bool covers(ReservedSpace rs) const;) 117 }; 118 119 class CMBitMap : public CMBitMapRO { 120 121 public: 122 // constructor 123 CMBitMap(int shifter) : 124 CMBitMapRO(shifter) {} 125 126 // Allocates the back store for the marking bitmap 127 bool allocate(ReservedSpace heap_rs); 128 129 // write marks 130 void mark(HeapWord* addr) { 131 assert(_bmStartWord <= addr && addr < (_bmStartWord + _bmWordSize), 132 "outside underlying space?"); 133 _bm.set_bit(heapWordToOffset(addr)); 134 } 135 void clear(HeapWord* addr) { 136 assert(_bmStartWord <= addr && addr < (_bmStartWord + _bmWordSize), 137 "outside underlying space?"); 138 _bm.clear_bit(heapWordToOffset(addr)); 139 } 140 bool parMark(HeapWord* addr) { 141 assert(_bmStartWord <= addr && addr < (_bmStartWord + _bmWordSize), 142 "outside underlying space?"); 143 return _bm.par_set_bit(heapWordToOffset(addr)); 144 } 145 bool parClear(HeapWord* addr) { 146 assert(_bmStartWord <= addr && addr < (_bmStartWord + _bmWordSize), 147 "outside underlying space?"); 148 return _bm.par_clear_bit(heapWordToOffset(addr)); 149 } 150 void markRange(MemRegion mr); 151 void clearAll(); 152 void clearRange(MemRegion mr); 153 154 // Starting at the bit corresponding to "addr" (inclusive), find the next 155 // "1" bit, if any. This bit starts some run of consecutive "1"'s; find 156 // the end of this run (stopping at "end_addr"). Return the MemRegion 157 // covering from the start of the region corresponding to the first bit 158 // of the run to the end of the region corresponding to the last bit of 159 // the run. If there is no "1" bit at or after "addr", return an empty 160 // MemRegion. 161 MemRegion getAndClearMarkedRegion(HeapWord* addr, HeapWord* end_addr); 162 }; 163 164 // Represents a marking stack used by ConcurrentMarking in the G1 collector. 165 class CMMarkStack VALUE_OBJ_CLASS_SPEC { 166 VirtualSpace _virtual_space; // Underlying backing store for actual stack 167 ConcurrentMark* _cm; 168 oop* _base; // bottom of stack 169 jint _index; // one more than last occupied index 170 jint _capacity; // max #elements 171 jint _saved_index; // value of _index saved at start of GC 172 NOT_PRODUCT(jint _max_depth;) // max depth plumbed during run 173 174 bool _overflow; 175 bool _should_expand; 176 DEBUG_ONLY(bool _drain_in_progress;) 177 DEBUG_ONLY(bool _drain_in_progress_yields;) 178 179 public: 180 CMMarkStack(ConcurrentMark* cm); 181 ~CMMarkStack(); 182 183 #ifndef PRODUCT 184 jint max_depth() const { 185 return _max_depth; 186 } 187 #endif 188 189 bool allocate(size_t capacity); 190 191 oop pop() { 192 if (!isEmpty()) { 193 return _base[--_index] ; 194 } 195 return NULL; 196 } 197 198 // If overflow happens, don't do the push, and record the overflow. 199 // *Requires* that "ptr" is already marked. 200 void push(oop ptr) { 201 if (isFull()) { 202 // Record overflow. 203 _overflow = true; 204 return; 205 } else { 206 _base[_index++] = ptr; 207 NOT_PRODUCT(_max_depth = MAX2(_max_depth, _index)); 208 } 209 } 210 // Non-block impl. Note: concurrency is allowed only with other 211 // "par_push" operations, not with "pop" or "drain". We would need 212 // parallel versions of them if such concurrency was desired. 213 void par_push(oop ptr); 214 215 // Pushes the first "n" elements of "ptr_arr" on the stack. 216 // Non-block impl. Note: concurrency is allowed only with other 217 // "par_adjoin_arr" or "push" operations, not with "pop" or "drain". 218 void par_adjoin_arr(oop* ptr_arr, int n); 219 220 // Pushes the first "n" elements of "ptr_arr" on the stack. 221 // Locking impl: concurrency is allowed only with 222 // "par_push_arr" and/or "par_pop_arr" operations, which use the same 223 // locking strategy. 224 void par_push_arr(oop* ptr_arr, int n); 225 226 // If returns false, the array was empty. Otherwise, removes up to "max" 227 // elements from the stack, and transfers them to "ptr_arr" in an 228 // unspecified order. The actual number transferred is given in "n" ("n 229 // == 0" is deliberately redundant with the return value.) Locking impl: 230 // concurrency is allowed only with "par_push_arr" and/or "par_pop_arr" 231 // operations, which use the same locking strategy. 232 bool par_pop_arr(oop* ptr_arr, int max, int* n); 233 234 // Drain the mark stack, applying the given closure to all fields of 235 // objects on the stack. (That is, continue until the stack is empty, 236 // even if closure applications add entries to the stack.) The "bm" 237 // argument, if non-null, may be used to verify that only marked objects 238 // are on the mark stack. If "yield_after" is "true", then the 239 // concurrent marker performing the drain offers to yield after 240 // processing each object. If a yield occurs, stops the drain operation 241 // and returns false. Otherwise, returns true. 242 template<class OopClosureClass> 243 bool drain(OopClosureClass* cl, CMBitMap* bm, bool yield_after = false); 244 245 bool isEmpty() { return _index == 0; } 246 bool isFull() { return _index == _capacity; } 247 int maxElems() { return _capacity; } 248 249 bool overflow() { return _overflow; } 250 void clear_overflow() { _overflow = false; } 251 252 bool should_expand() const { return _should_expand; } 253 void set_should_expand(); 254 255 // Expand the stack, typically in response to an overflow condition 256 void expand(); 257 258 int size() { return _index; } 259 260 void setEmpty() { _index = 0; clear_overflow(); } 261 262 // Record the current index. 263 void note_start_of_gc(); 264 265 // Make sure that we have not added any entries to the stack during GC. 266 void note_end_of_gc(); 267 268 // iterate over the oops in the mark stack, up to the bound recorded via 269 // the call above. 270 void oops_do(OopClosure* f); 271 }; 272 273 class ForceOverflowSettings VALUE_OBJ_CLASS_SPEC { 274 private: 275 #ifndef PRODUCT 276 uintx _num_remaining; 277 bool _force; 278 #endif // !defined(PRODUCT) 279 280 public: 281 void init() PRODUCT_RETURN; 282 void update() PRODUCT_RETURN; 283 bool should_force() PRODUCT_RETURN_( return false; ); 284 }; 285 286 // this will enable a variety of different statistics per GC task 287 #define _MARKING_STATS_ 0 288 // this will enable the higher verbose levels 289 #define _MARKING_VERBOSE_ 0 290 291 #if _MARKING_STATS_ 292 #define statsOnly(statement) \ 293 do { \ 294 statement ; \ 295 } while (0) 296 #else // _MARKING_STATS_ 297 #define statsOnly(statement) \ 298 do { \ 299 } while (0) 300 #endif // _MARKING_STATS_ 301 302 typedef enum { 303 no_verbose = 0, // verbose turned off 304 stats_verbose, // only prints stats at the end of marking 305 low_verbose, // low verbose, mostly per region and per major event 306 medium_verbose, // a bit more detailed than low 307 high_verbose // per object verbose 308 } CMVerboseLevel; 309 310 class YoungList; 311 312 // Root Regions are regions that are not empty at the beginning of a 313 // marking cycle and which we might collect during an evacuation pause 314 // while the cycle is active. Given that, during evacuation pauses, we 315 // do not copy objects that are explicitly marked, what we have to do 316 // for the root regions is to scan them and mark all objects reachable 317 // from them. According to the SATB assumptions, we only need to visit 318 // each object once during marking. So, as long as we finish this scan 319 // before the next evacuation pause, we can copy the objects from the 320 // root regions without having to mark them or do anything else to them. 321 // 322 // Currently, we only support root region scanning once (at the start 323 // of the marking cycle) and the root regions are all the survivor 324 // regions populated during the initial-mark pause. 325 class CMRootRegions VALUE_OBJ_CLASS_SPEC { 326 private: 327 YoungList* _young_list; 328 ConcurrentMark* _cm; 329 330 volatile bool _scan_in_progress; 331 volatile bool _should_abort; 332 HeapRegion* volatile _next_survivor; 333 334 public: 335 CMRootRegions(); 336 // We actually do most of the initialization in this method. 337 void init(G1CollectedHeap* g1h, ConcurrentMark* cm); 338 339 // Reset the claiming / scanning of the root regions. 340 void prepare_for_scan(); 341 342 // Forces get_next() to return NULL so that the iteration aborts early. 343 void abort() { _should_abort = true; } 344 345 // Return true if the CM thread are actively scanning root regions, 346 // false otherwise. 347 bool scan_in_progress() { return _scan_in_progress; } 348 349 // Claim the next root region to scan atomically, or return NULL if 350 // all have been claimed. 351 HeapRegion* claim_next(); 352 353 // Flag that we're done with root region scanning and notify anyone 354 // who's waiting on it. If aborted is false, assume that all regions 355 // have been claimed. 356 void scan_finished(); 357 358 // If CM threads are still scanning root regions, wait until they 359 // are done. Return true if we had to wait, false otherwise. 360 bool wait_until_scan_finished(); 361 }; 362 363 class ConcurrentMarkThread; 364 365 class ConcurrentMark: public CHeapObj<mtGC> { 366 friend class CMMarkStack; 367 friend class ConcurrentMarkThread; 368 friend class CMTask; 369 friend class CMBitMapClosure; 370 friend class CMGlobalObjectClosure; 371 friend class CMRemarkTask; 372 friend class CMConcurrentMarkingTask; 373 friend class G1ParNoteEndTask; 374 friend class CalcLiveObjectsClosure; 375 friend class G1CMRefProcTaskProxy; 376 friend class G1CMRefProcTaskExecutor; 377 friend class G1CMKeepAliveAndDrainClosure; 378 friend class G1CMDrainMarkingStackClosure; 379 380 protected: 381 ConcurrentMarkThread* _cmThread; // The thread doing the work 382 G1CollectedHeap* _g1h; // The heap 383 uint _parallel_marking_threads; // The number of marking 384 // threads we're using 385 uint _max_parallel_marking_threads; // Max number of marking 386 // threads we'll ever use 387 double _sleep_factor; // How much we have to sleep, with 388 // respect to the work we just did, to 389 // meet the marking overhead goal 390 double _marking_task_overhead; // Marking target overhead for 391 // a single task 392 393 // Same as the two above, but for the cleanup task 394 double _cleanup_sleep_factor; 395 double _cleanup_task_overhead; 396 397 FreeRegionList _cleanup_list; 398 399 // Concurrent marking support structures 400 CMBitMap _markBitMap1; 401 CMBitMap _markBitMap2; 402 CMBitMapRO* _prevMarkBitMap; // Completed mark bitmap 403 CMBitMap* _nextMarkBitMap; // Under-construction mark bitmap 404 405 BitMap _region_bm; 406 BitMap _card_bm; 407 408 // Heap bounds 409 HeapWord* _heap_start; 410 HeapWord* _heap_end; 411 412 // Root region tracking and claiming 413 CMRootRegions _root_regions; 414 415 // For gray objects 416 CMMarkStack _markStack; // Grey objects behind global finger 417 HeapWord* volatile _finger; // The global finger, region aligned, 418 // always points to the end of the 419 // last claimed region 420 421 // Marking tasks 422 uint _max_worker_id;// Maximum worker id 423 uint _active_tasks; // Task num currently active 424 CMTask** _tasks; // Task queue array (max_worker_id len) 425 CMTaskQueueSet* _task_queues; // Task queue set 426 ParallelTaskTerminator _terminator; // For termination 427 428 // Two sync barriers that are used to synchronize tasks when an 429 // overflow occurs. The algorithm is the following. All tasks enter 430 // the first one to ensure that they have all stopped manipulating 431 // the global data structures. After they exit it, they re-initialize 432 // their data structures and task 0 re-initializes the global data 433 // structures. Then, they enter the second sync barrier. This 434 // ensure, that no task starts doing work before all data 435 // structures (local and global) have been re-initialized. When they 436 // exit it, they are free to start working again. 437 WorkGangBarrierSync _first_overflow_barrier_sync; 438 WorkGangBarrierSync _second_overflow_barrier_sync; 439 440 // This is set by any task, when an overflow on the global data 441 // structures is detected 442 volatile bool _has_overflown; 443 // True: marking is concurrent, false: we're in remark 444 volatile bool _concurrent; 445 // Set at the end of a Full GC so that marking aborts 446 volatile bool _has_aborted; 447 448 // Used when remark aborts due to an overflow to indicate that 449 // another concurrent marking phase should start 450 volatile bool _restart_for_overflow; 451 452 // This is true from the very start of concurrent marking until the 453 // point when all the tasks complete their work. It is really used 454 // to determine the points between the end of concurrent marking and 455 // time of remark. 456 volatile bool _concurrent_marking_in_progress; 457 458 // Verbose level 459 CMVerboseLevel _verbose_level; 460 461 // All of these times are in ms 462 NumberSeq _init_times; 463 NumberSeq _remark_times; 464 NumberSeq _remark_mark_times; 465 NumberSeq _remark_weak_ref_times; 466 NumberSeq _cleanup_times; 467 double _total_counting_time; 468 double _total_rs_scrub_time; 469 470 double* _accum_task_vtime; // Accumulated task vtime 471 472 FlexibleWorkGang* _parallel_workers; 473 474 ForceOverflowSettings _force_overflow_conc; 475 ForceOverflowSettings _force_overflow_stw; 476 477 void weakRefsWork(bool clear_all_soft_refs); 478 479 void swapMarkBitMaps(); 480 481 // It resets the global marking data structures, as well as the 482 // task local ones; should be called during initial mark. 483 void reset(); 484 485 // Resets all the marking data structures. Called when we have to restart 486 // marking or when marking completes (via set_non_marking_state below). 487 void reset_marking_state(bool clear_overflow = true); 488 489 // We do this after we're done with marking so that the marking data 490 // structures are initialized to a sensible and predictable state. 491 void set_non_marking_state(); 492 493 // Called to indicate how many threads are currently active. 494 void set_concurrency(uint active_tasks); 495 496 // It should be called to indicate which phase we're in (concurrent 497 // mark or remark) and how many threads are currently active. 498 void set_concurrency_and_phase(uint active_tasks, bool concurrent); 499 500 // Prints all gathered CM-related statistics 501 void print_stats(); 502 503 bool cleanup_list_is_empty() { 504 return _cleanup_list.is_empty(); 505 } 506 507 // Accessor methods 508 uint parallel_marking_threads() const { return _parallel_marking_threads; } 509 uint max_parallel_marking_threads() const { return _max_parallel_marking_threads;} 510 double sleep_factor() { return _sleep_factor; } 511 double marking_task_overhead() { return _marking_task_overhead;} 512 double cleanup_sleep_factor() { return _cleanup_sleep_factor; } 513 double cleanup_task_overhead() { return _cleanup_task_overhead;} 514 515 bool use_parallel_marking_threads() const { 516 assert(parallel_marking_threads() <= 517 max_parallel_marking_threads(), "sanity"); 518 assert((_parallel_workers == NULL && parallel_marking_threads() == 0) || 519 parallel_marking_threads() > 0, 520 "parallel workers not set up correctly"); 521 return _parallel_workers != NULL; 522 } 523 524 HeapWord* finger() { return _finger; } 525 bool concurrent() { return _concurrent; } 526 uint active_tasks() { return _active_tasks; } 527 ParallelTaskTerminator* terminator() { return &_terminator; } 528 529 // It claims the next available region to be scanned by a marking 530 // task/thread. It might return NULL if the next region is empty or 531 // we have run out of regions. In the latter case, out_of_regions() 532 // determines whether we've really run out of regions or the task 533 // should call claim_region() again. This might seem a bit 534 // awkward. Originally, the code was written so that claim_region() 535 // either successfully returned with a non-empty region or there 536 // were no more regions to be claimed. The problem with this was 537 // that, in certain circumstances, it iterated over large chunks of 538 // the heap finding only empty regions and, while it was working, it 539 // was preventing the calling task to call its regular clock 540 // method. So, this way, each task will spend very little time in 541 // claim_region() and is allowed to call the regular clock method 542 // frequently. 543 HeapRegion* claim_region(uint worker_id); 544 545 // It determines whether we've run out of regions to scan. Note that 546 // the finger can point past the heap end in case the heap was expanded 547 // to satisfy an allocation without doing a GC. This is fine, because all 548 // objects in those regions will be considered live anyway because of 549 // SATB guarantees (i.e. their TAMS will be equal to bottom). 550 bool out_of_regions() { return _finger >= _heap_end; } 551 552 // Returns the task with the given id 553 CMTask* task(int id) { 554 assert(0 <= id && id < (int) _active_tasks, 555 "task id not within active bounds"); 556 return _tasks[id]; 557 } 558 559 // Returns the task queue with the given id 560 CMTaskQueue* task_queue(int id) { 561 assert(0 <= id && id < (int) _active_tasks, 562 "task queue id not within active bounds"); 563 return (CMTaskQueue*) _task_queues->queue(id); 564 } 565 566 // Returns the task queue set 567 CMTaskQueueSet* task_queues() { return _task_queues; } 568 569 // Access / manipulation of the overflow flag which is set to 570 // indicate that the global stack has overflown 571 bool has_overflown() { return _has_overflown; } 572 void set_has_overflown() { _has_overflown = true; } 573 void clear_has_overflown() { _has_overflown = false; } 574 bool restart_for_overflow() { return _restart_for_overflow; } 575 576 // Methods to enter the two overflow sync barriers 577 void enter_first_sync_barrier(uint worker_id); 578 void enter_second_sync_barrier(uint worker_id); 579 580 ForceOverflowSettings* force_overflow_conc() { 581 return &_force_overflow_conc; 582 } 583 584 ForceOverflowSettings* force_overflow_stw() { 585 return &_force_overflow_stw; 586 } 587 588 ForceOverflowSettings* force_overflow() { 589 if (concurrent()) { 590 return force_overflow_conc(); 591 } else { 592 return force_overflow_stw(); 593 } 594 } 595 596 // Live Data Counting data structures... 597 // These data structures are initialized at the start of 598 // marking. They are written to while marking is active. 599 // They are aggregated during remark; the aggregated values 600 // are then used to populate the _region_bm, _card_bm, and 601 // the total live bytes, which are then subsequently updated 602 // during cleanup. 603 604 // An array of bitmaps (one bit map per task). Each bitmap 605 // is used to record the cards spanned by the live objects 606 // marked by that task/worker. 607 BitMap* _count_card_bitmaps; 608 609 // Used to record the number of marked live bytes 610 // (for each region, by worker thread). 611 size_t** _count_marked_bytes; 612 613 // Card index of the bottom of the G1 heap. Used for biasing indices into 614 // the card bitmaps. 615 intptr_t _heap_bottom_card_num; 616 617 // Set to true when initialization is complete 618 bool _completed_initialization; 619 620 public: 621 // Manipulation of the global mark stack. 622 // Notice that the first mark_stack_push is CAS-based, whereas the 623 // two below are Mutex-based. This is OK since the first one is only 624 // called during evacuation pauses and doesn't compete with the 625 // other two (which are called by the marking tasks during 626 // concurrent marking or remark). 627 bool mark_stack_push(oop p) { 628 _markStack.par_push(p); 629 if (_markStack.overflow()) { 630 set_has_overflown(); 631 return false; 632 } 633 return true; 634 } 635 bool mark_stack_push(oop* arr, int n) { 636 _markStack.par_push_arr(arr, n); 637 if (_markStack.overflow()) { 638 set_has_overflown(); 639 return false; 640 } 641 return true; 642 } 643 void mark_stack_pop(oop* arr, int max, int* n) { 644 _markStack.par_pop_arr(arr, max, n); 645 } 646 size_t mark_stack_size() { return _markStack.size(); } 647 size_t partial_mark_stack_size_target() { return _markStack.maxElems()/3; } 648 bool mark_stack_overflow() { return _markStack.overflow(); } 649 bool mark_stack_empty() { return _markStack.isEmpty(); } 650 651 CMRootRegions* root_regions() { return &_root_regions; } 652 653 bool concurrent_marking_in_progress() { 654 return _concurrent_marking_in_progress; 655 } 656 void set_concurrent_marking_in_progress() { 657 _concurrent_marking_in_progress = true; 658 } 659 void clear_concurrent_marking_in_progress() { 660 _concurrent_marking_in_progress = false; 661 } 662 663 void update_accum_task_vtime(int i, double vtime) { 664 _accum_task_vtime[i] += vtime; 665 } 666 667 double all_task_accum_vtime() { 668 double ret = 0.0; 669 for (uint i = 0; i < _max_worker_id; ++i) 670 ret += _accum_task_vtime[i]; 671 return ret; 672 } 673 674 // Attempts to steal an object from the task queues of other tasks 675 bool try_stealing(uint worker_id, int* hash_seed, oop& obj) { 676 return _task_queues->steal(worker_id, hash_seed, obj); 677 } 678 679 ConcurrentMark(G1CollectedHeap* g1h, ReservedSpace heap_rs); 680 ~ConcurrentMark(); 681 682 ConcurrentMarkThread* cmThread() { return _cmThread; } 683 684 CMBitMapRO* prevMarkBitMap() const { return _prevMarkBitMap; } 685 CMBitMap* nextMarkBitMap() const { return _nextMarkBitMap; } 686 687 // Returns the number of GC threads to be used in a concurrent 688 // phase based on the number of GC threads being used in a STW 689 // phase. 690 uint scale_parallel_threads(uint n_par_threads); 691 692 // Calculates the number of GC threads to be used in a concurrent phase. 693 uint calc_parallel_marking_threads(); 694 695 // The following three are interaction between CM and 696 // G1CollectedHeap 697 698 // This notifies CM that a root during initial-mark needs to be 699 // grayed. It is MT-safe. word_size is the size of the object in 700 // words. It is passed explicitly as sometimes we cannot calculate 701 // it from the given object because it might be in an inconsistent 702 // state (e.g., in to-space and being copied). So the caller is 703 // responsible for dealing with this issue (e.g., get the size from 704 // the from-space image when the to-space image might be 705 // inconsistent) and always passing the size. hr is the region that 706 // contains the object and it's passed optionally from callers who 707 // might already have it (no point in recalculating it). 708 inline void grayRoot(oop obj, size_t word_size, 709 uint worker_id, HeapRegion* hr = NULL); 710 711 // It iterates over the heap and for each object it comes across it 712 // will dump the contents of its reference fields, as well as 713 // liveness information for the object and its referents. The dump 714 // will be written to a file with the following name: 715 // G1PrintReachableBaseFile + "." + str. 716 // vo decides whether the prev (vo == UsePrevMarking), the next 717 // (vo == UseNextMarking) marking information, or the mark word 718 // (vo == UseMarkWord) will be used to determine the liveness of 719 // each object / referent. 720 // If all is true, all objects in the heap will be dumped, otherwise 721 // only the live ones. In the dump the following symbols / breviations 722 // are used: 723 // M : an explicitly live object (its bitmap bit is set) 724 // > : an implicitly live object (over tams) 725 // O : an object outside the G1 heap (typically: in the perm gen) 726 // NOT : a reference field whose referent is not live 727 // AND MARKED : indicates that an object is both explicitly and 728 // implicitly live (it should be one or the other, not both) 729 void print_reachable(const char* str, 730 VerifyOption vo, bool all) PRODUCT_RETURN; 731 732 // Clear the next marking bitmap (will be called concurrently). 733 void clearNextBitmap(); 734 735 // These two do the work that needs to be done before and after the 736 // initial root checkpoint. Since this checkpoint can be done at two 737 // different points (i.e. an explicit pause or piggy-backed on a 738 // young collection), then it's nice to be able to easily share the 739 // pre/post code. It might be the case that we can put everything in 740 // the post method. TP 741 void checkpointRootsInitialPre(); 742 void checkpointRootsInitialPost(); 743 744 // Scan all the root regions and mark everything reachable from 745 // them. 746 void scanRootRegions(); 747 748 // Scan a single root region and mark everything reachable from it. 749 void scanRootRegion(HeapRegion* hr, uint worker_id); 750 751 // Do concurrent phase of marking, to a tentative transitive closure. 752 void markFromRoots(); 753 754 void checkpointRootsFinal(bool clear_all_soft_refs); 755 void checkpointRootsFinalWork(); 756 void cleanup(); 757 void completeCleanup(); 758 759 // Mark in the previous bitmap. NB: this is usually read-only, so use 760 // this carefully! 761 inline void markPrev(oop p); 762 763 // Clears marks for all objects in the given range, for the prev, 764 // next, or both bitmaps. NB: the previous bitmap is usually 765 // read-only, so use this carefully! 766 void clearRangePrevBitmap(MemRegion mr); 767 void clearRangeNextBitmap(MemRegion mr); 768 void clearRangeBothBitmaps(MemRegion mr); 769 770 // Notify data structures that a GC has started. 771 void note_start_of_gc() { 772 _markStack.note_start_of_gc(); 773 } 774 775 // Notify data structures that a GC is finished. 776 void note_end_of_gc() { 777 _markStack.note_end_of_gc(); 778 } 779 780 // Verify that there are no CSet oops on the stacks (taskqueues / 781 // global mark stack), enqueued SATB buffers, per-thread SATB 782 // buffers, and fingers (global / per-task). The boolean parameters 783 // decide which of the above data structures to verify. If marking 784 // is not in progress, it's a no-op. 785 void verify_no_cset_oops(bool verify_stacks, 786 bool verify_enqueued_buffers, 787 bool verify_thread_buffers, 788 bool verify_fingers) PRODUCT_RETURN; 789 790 // It is called at the end of an evacuation pause during marking so 791 // that CM is notified of where the new end of the heap is. It 792 // doesn't do anything if concurrent_marking_in_progress() is false, 793 // unless the force parameter is true. 794 void update_g1_committed(bool force = false); 795 796 bool isMarked(oop p) const { 797 assert(p != NULL && p->is_oop(), "expected an oop"); 798 HeapWord* addr = (HeapWord*)p; 799 assert(addr >= _nextMarkBitMap->startWord() || 800 addr < _nextMarkBitMap->endWord(), "in a region"); 801 802 return _nextMarkBitMap->isMarked(addr); 803 } 804 805 inline bool not_yet_marked(oop p) const; 806 807 // XXX Debug code 808 bool containing_card_is_marked(void* p); 809 bool containing_cards_are_marked(void* start, void* last); 810 811 bool isPrevMarked(oop p) const { 812 assert(p != NULL && p->is_oop(), "expected an oop"); 813 HeapWord* addr = (HeapWord*)p; 814 assert(addr >= _prevMarkBitMap->startWord() || 815 addr < _prevMarkBitMap->endWord(), "in a region"); 816 817 return _prevMarkBitMap->isMarked(addr); 818 } 819 820 inline bool do_yield_check(uint worker_i = 0); 821 822 // Called to abort the marking cycle after a Full GC takes place. 823 void abort(); 824 825 bool has_aborted() { return _has_aborted; } 826 void set_has_aborted() { _has_aborted = true; } 827 void clear_has_aborted() { _has_aborted = false; } 828 829 // This prints the global/local fingers. It is used for debugging. 830 NOT_PRODUCT(void print_finger();) 831 832 void print_summary_info(); 833 834 void print_worker_threads_on(outputStream* st) const; 835 836 void print_on_error(outputStream* st) const; 837 838 // The following indicate whether a given verbose level has been 839 // set. Notice that anything above stats is conditional to 840 // _MARKING_VERBOSE_ having been set to 1 841 bool verbose_stats() { 842 return _verbose_level >= stats_verbose; 843 } 844 bool verbose_low() { 845 return _MARKING_VERBOSE_ && _verbose_level >= low_verbose; 846 } 847 bool verbose_medium() { 848 return _MARKING_VERBOSE_ && _verbose_level >= medium_verbose; 849 } 850 bool verbose_high() { 851 return _MARKING_VERBOSE_ && _verbose_level >= high_verbose; 852 } 853 854 // Liveness counting 855 856 // Utility routine to set an exclusive range of cards on the given 857 // card liveness bitmap 858 inline void set_card_bitmap_range(BitMap* card_bm, 859 BitMap::idx_t start_idx, 860 BitMap::idx_t end_idx, 861 bool is_par); 862 863 // Returns the card number of the bottom of the G1 heap. 864 // Used in biasing indices into accounting card bitmaps. 865 intptr_t heap_bottom_card_num() const { 866 return _heap_bottom_card_num; 867 } 868 869 // Returns the card bitmap for a given task or worker id. 870 BitMap* count_card_bitmap_for(uint worker_id) { 871 assert(0 <= worker_id && worker_id < _max_worker_id, "oob"); 872 assert(_count_card_bitmaps != NULL, "uninitialized"); 873 BitMap* task_card_bm = &_count_card_bitmaps[worker_id]; 874 assert(task_card_bm->size() == _card_bm.size(), "size mismatch"); 875 return task_card_bm; 876 } 877 878 // Returns the array containing the marked bytes for each region, 879 // for the given worker or task id. 880 size_t* count_marked_bytes_array_for(uint worker_id) { 881 assert(0 <= worker_id && worker_id < _max_worker_id, "oob"); 882 assert(_count_marked_bytes != NULL, "uninitialized"); 883 size_t* marked_bytes_array = _count_marked_bytes[worker_id]; 884 assert(marked_bytes_array != NULL, "uninitialized"); 885 return marked_bytes_array; 886 } 887 888 // Returns the index in the liveness accounting card table bitmap 889 // for the given address 890 inline BitMap::idx_t card_bitmap_index_for(HeapWord* addr); 891 892 // Counts the size of the given memory region in the the given 893 // marked_bytes array slot for the given HeapRegion. 894 // Sets the bits in the given card bitmap that are associated with the 895 // cards that are spanned by the memory region. 896 inline void count_region(MemRegion mr, HeapRegion* hr, 897 size_t* marked_bytes_array, 898 BitMap* task_card_bm); 899 900 // Counts the given memory region in the task/worker counting 901 // data structures for the given worker id. 902 inline void count_region(MemRegion mr, HeapRegion* hr, uint worker_id); 903 904 // Counts the given memory region in the task/worker counting 905 // data structures for the given worker id. 906 inline void count_region(MemRegion mr, uint worker_id); 907 908 // Counts the given object in the given task/worker counting 909 // data structures. 910 inline void count_object(oop obj, HeapRegion* hr, 911 size_t* marked_bytes_array, 912 BitMap* task_card_bm); 913 914 // Counts the given object in the task/worker counting data 915 // structures for the given worker id. 916 inline void count_object(oop obj, HeapRegion* hr, uint worker_id); 917 918 // Attempts to mark the given object and, if successful, counts 919 // the object in the given task/worker counting structures. 920 inline bool par_mark_and_count(oop obj, HeapRegion* hr, 921 size_t* marked_bytes_array, 922 BitMap* task_card_bm); 923 924 // Attempts to mark the given object and, if successful, counts 925 // the object in the task/worker counting structures for the 926 // given worker id. 927 inline bool par_mark_and_count(oop obj, size_t word_size, 928 HeapRegion* hr, uint worker_id); 929 930 // Attempts to mark the given object and, if successful, counts 931 // the object in the task/worker counting structures for the 932 // given worker id. 933 inline bool par_mark_and_count(oop obj, HeapRegion* hr, uint worker_id); 934 935 // Similar to the above routine but we don't know the heap region that 936 // contains the object to be marked/counted, which this routine looks up. 937 inline bool par_mark_and_count(oop obj, uint worker_id); 938 939 // Similar to the above routine but there are times when we cannot 940 // safely calculate the size of obj due to races and we, therefore, 941 // pass the size in as a parameter. It is the caller's responsibility 942 // to ensure that the size passed in for obj is valid. 943 inline bool par_mark_and_count(oop obj, size_t word_size, uint worker_id); 944 945 // Unconditionally mark the given object, and unconditionally count 946 // the object in the counting structures for worker id 0. 947 // Should *not* be called from parallel code. 948 inline bool mark_and_count(oop obj, HeapRegion* hr); 949 950 // Similar to the above routine but we don't know the heap region that 951 // contains the object to be marked/counted, which this routine looks up. 952 // Should *not* be called from parallel code. 953 inline bool mark_and_count(oop obj); 954 955 // Returns true if initialization was successfully completed. 956 bool completed_initialization() const { 957 return _completed_initialization; 958 } 959 960 protected: 961 // Clear all the per-task bitmaps and arrays used to store the 962 // counting data. 963 void clear_all_count_data(); 964 965 // Aggregates the counting data for each worker/task 966 // that was constructed while marking. Also sets 967 // the amount of marked bytes for each region and 968 // the top at concurrent mark count. 969 void aggregate_count_data(); 970 971 // Verification routine 972 void verify_count_data(); 973 }; 974 975 // A class representing a marking task. 976 class CMTask : public TerminatorTerminator { 977 private: 978 enum PrivateConstants { 979 // the regular clock call is called once the scanned words reaches 980 // this limit 981 words_scanned_period = 12*1024, 982 // the regular clock call is called once the number of visited 983 // references reaches this limit 984 refs_reached_period = 384, 985 // initial value for the hash seed, used in the work stealing code 986 init_hash_seed = 17, 987 // how many entries will be transferred between global stack and 988 // local queues 989 global_stack_transfer_size = 16 990 }; 991 992 uint _worker_id; 993 G1CollectedHeap* _g1h; 994 ConcurrentMark* _cm; 995 CMBitMap* _nextMarkBitMap; 996 // the task queue of this task 997 CMTaskQueue* _task_queue; 998 private: 999 // the task queue set---needed for stealing 1000 CMTaskQueueSet* _task_queues; 1001 // indicates whether the task has been claimed---this is only for 1002 // debugging purposes 1003 bool _claimed; 1004 1005 // number of calls to this task 1006 int _calls; 1007 1008 // when the virtual timer reaches this time, the marking step should 1009 // exit 1010 double _time_target_ms; 1011 // the start time of the current marking step 1012 double _start_time_ms; 1013 1014 // the oop closure used for iterations over oops 1015 G1CMOopClosure* _cm_oop_closure; 1016 1017 // the region this task is scanning, NULL if we're not scanning any 1018 HeapRegion* _curr_region; 1019 // the local finger of this task, NULL if we're not scanning a region 1020 HeapWord* _finger; 1021 // limit of the region this task is scanning, NULL if we're not scanning one 1022 HeapWord* _region_limit; 1023 1024 // the number of words this task has scanned 1025 size_t _words_scanned; 1026 // When _words_scanned reaches this limit, the regular clock is 1027 // called. Notice that this might be decreased under certain 1028 // circumstances (i.e. when we believe that we did an expensive 1029 // operation). 1030 size_t _words_scanned_limit; 1031 // the initial value of _words_scanned_limit (i.e. what it was 1032 // before it was decreased). 1033 size_t _real_words_scanned_limit; 1034 1035 // the number of references this task has visited 1036 size_t _refs_reached; 1037 // When _refs_reached reaches this limit, the regular clock is 1038 // called. Notice this this might be decreased under certain 1039 // circumstances (i.e. when we believe that we did an expensive 1040 // operation). 1041 size_t _refs_reached_limit; 1042 // the initial value of _refs_reached_limit (i.e. what it was before 1043 // it was decreased). 1044 size_t _real_refs_reached_limit; 1045 1046 // used by the work stealing stuff 1047 int _hash_seed; 1048 // if this is true, then the task has aborted for some reason 1049 bool _has_aborted; 1050 // set when the task aborts because it has met its time quota 1051 bool _has_timed_out; 1052 // true when we're draining SATB buffers; this avoids the task 1053 // aborting due to SATB buffers being available (as we're already 1054 // dealing with them) 1055 bool _draining_satb_buffers; 1056 1057 // number sequence of past step times 1058 NumberSeq _step_times_ms; 1059 // elapsed time of this task 1060 double _elapsed_time_ms; 1061 // termination time of this task 1062 double _termination_time_ms; 1063 // when this task got into the termination protocol 1064 double _termination_start_time_ms; 1065 1066 // true when the task is during a concurrent phase, false when it is 1067 // in the remark phase (so, in the latter case, we do not have to 1068 // check all the things that we have to check during the concurrent 1069 // phase, i.e. SATB buffer availability...) 1070 bool _concurrent; 1071 1072 TruncatedSeq _marking_step_diffs_ms; 1073 1074 // Counting data structures. Embedding the task's marked_bytes_array 1075 // and card bitmap into the actual task saves having to go through 1076 // the ConcurrentMark object. 1077 size_t* _marked_bytes_array; 1078 BitMap* _card_bm; 1079 1080 // LOTS of statistics related with this task 1081 #if _MARKING_STATS_ 1082 NumberSeq _all_clock_intervals_ms; 1083 double _interval_start_time_ms; 1084 1085 int _aborted; 1086 int _aborted_overflow; 1087 int _aborted_cm_aborted; 1088 int _aborted_yield; 1089 int _aborted_timed_out; 1090 int _aborted_satb; 1091 int _aborted_termination; 1092 1093 int _steal_attempts; 1094 int _steals; 1095 1096 int _clock_due_to_marking; 1097 int _clock_due_to_scanning; 1098 1099 int _local_pushes; 1100 int _local_pops; 1101 int _local_max_size; 1102 int _objs_scanned; 1103 1104 int _global_pushes; 1105 int _global_pops; 1106 int _global_max_size; 1107 1108 int _global_transfers_to; 1109 int _global_transfers_from; 1110 1111 int _regions_claimed; 1112 int _objs_found_on_bitmap; 1113 1114 int _satb_buffers_processed; 1115 #endif // _MARKING_STATS_ 1116 1117 // it updates the local fields after this task has claimed 1118 // a new region to scan 1119 void setup_for_region(HeapRegion* hr); 1120 // it brings up-to-date the limit of the region 1121 void update_region_limit(); 1122 1123 // called when either the words scanned or the refs visited limit 1124 // has been reached 1125 void reached_limit(); 1126 // recalculates the words scanned and refs visited limits 1127 void recalculate_limits(); 1128 // decreases the words scanned and refs visited limits when we reach 1129 // an expensive operation 1130 void decrease_limits(); 1131 // it checks whether the words scanned or refs visited reached their 1132 // respective limit and calls reached_limit() if they have 1133 void check_limits() { 1134 if (_words_scanned >= _words_scanned_limit || 1135 _refs_reached >= _refs_reached_limit) { 1136 reached_limit(); 1137 } 1138 } 1139 // this is supposed to be called regularly during a marking step as 1140 // it checks a bunch of conditions that might cause the marking step 1141 // to abort 1142 void regular_clock_call(); 1143 bool concurrent() { return _concurrent; } 1144 1145 public: 1146 // It resets the task; it should be called right at the beginning of 1147 // a marking phase. 1148 void reset(CMBitMap* _nextMarkBitMap); 1149 // it clears all the fields that correspond to a claimed region. 1150 void clear_region_fields(); 1151 1152 void set_concurrent(bool concurrent) { _concurrent = concurrent; } 1153 1154 // The main method of this class which performs a marking step 1155 // trying not to exceed the given duration. However, it might exit 1156 // prematurely, according to some conditions (i.e. SATB buffers are 1157 // available for processing). 1158 void do_marking_step(double target_ms, 1159 bool do_termination, 1160 bool is_serial); 1161 1162 // These two calls start and stop the timer 1163 void record_start_time() { 1164 _elapsed_time_ms = os::elapsedTime() * 1000.0; 1165 } 1166 void record_end_time() { 1167 _elapsed_time_ms = os::elapsedTime() * 1000.0 - _elapsed_time_ms; 1168 } 1169 1170 // returns the worker ID associated with this task. 1171 uint worker_id() { return _worker_id; } 1172 1173 // From TerminatorTerminator. It determines whether this task should 1174 // exit the termination protocol after it's entered it. 1175 virtual bool should_exit_termination(); 1176 1177 // Resets the local region fields after a task has finished scanning a 1178 // region; or when they have become stale as a result of the region 1179 // being evacuated. 1180 void giveup_current_region(); 1181 1182 HeapWord* finger() { return _finger; } 1183 1184 bool has_aborted() { return _has_aborted; } 1185 void set_has_aborted() { _has_aborted = true; } 1186 void clear_has_aborted() { _has_aborted = false; } 1187 bool has_timed_out() { return _has_timed_out; } 1188 bool claimed() { return _claimed; } 1189 1190 void set_cm_oop_closure(G1CMOopClosure* cm_oop_closure); 1191 1192 // It grays the object by marking it and, if necessary, pushing it 1193 // on the local queue 1194 inline void deal_with_reference(oop obj); 1195 1196 // It scans an object and visits its children. 1197 void scan_object(oop obj); 1198 1199 // It pushes an object on the local queue. 1200 inline void push(oop obj); 1201 1202 // These two move entries to/from the global stack. 1203 void move_entries_to_global_stack(); 1204 void get_entries_from_global_stack(); 1205 1206 // It pops and scans objects from the local queue. If partially is 1207 // true, then it stops when the queue size is of a given limit. If 1208 // partially is false, then it stops when the queue is empty. 1209 void drain_local_queue(bool partially); 1210 // It moves entries from the global stack to the local queue and 1211 // drains the local queue. If partially is true, then it stops when 1212 // both the global stack and the local queue reach a given size. If 1213 // partially if false, it tries to empty them totally. 1214 void drain_global_stack(bool partially); 1215 // It keeps picking SATB buffers and processing them until no SATB 1216 // buffers are available. 1217 void drain_satb_buffers(); 1218 1219 // moves the local finger to a new location 1220 inline void move_finger_to(HeapWord* new_finger) { 1221 assert(new_finger >= _finger && new_finger < _region_limit, "invariant"); 1222 _finger = new_finger; 1223 } 1224 1225 CMTask(uint worker_id, ConcurrentMark *cm, 1226 size_t* marked_bytes, BitMap* card_bm, 1227 CMTaskQueue* task_queue, CMTaskQueueSet* task_queues); 1228 1229 // it prints statistics associated with this task 1230 void print_stats(); 1231 1232 #if _MARKING_STATS_ 1233 void increase_objs_found_on_bitmap() { ++_objs_found_on_bitmap; } 1234 #endif // _MARKING_STATS_ 1235 }; 1236 1237 // Class that's used to to print out per-region liveness 1238 // information. It's currently used at the end of marking and also 1239 // after we sort the old regions at the end of the cleanup operation. 1240 class G1PrintRegionLivenessInfoClosure: public HeapRegionClosure { 1241 private: 1242 outputStream* _out; 1243 1244 // Accumulators for these values. 1245 size_t _total_used_bytes; 1246 size_t _total_capacity_bytes; 1247 size_t _total_prev_live_bytes; 1248 size_t _total_next_live_bytes; 1249 1250 // These are set up when we come across a "stars humongous" region 1251 // (as this is where most of this information is stored, not in the 1252 // subsequent "continues humongous" regions). After that, for every 1253 // region in a given humongous region series we deduce the right 1254 // values for it by simply subtracting the appropriate amount from 1255 // these fields. All these values should reach 0 after we've visited 1256 // the last region in the series. 1257 size_t _hum_used_bytes; 1258 size_t _hum_capacity_bytes; 1259 size_t _hum_prev_live_bytes; 1260 size_t _hum_next_live_bytes; 1261 1262 // Accumulator for the remembered set size 1263 size_t _total_remset_bytes; 1264 1265 // Accumulator for strong code roots memory size 1266 size_t _total_strong_code_roots_bytes; 1267 1268 static double perc(size_t val, size_t total) { 1269 if (total == 0) { 1270 return 0.0; 1271 } else { 1272 return 100.0 * ((double) val / (double) total); 1273 } 1274 } 1275 1276 static double bytes_to_mb(size_t val) { 1277 return (double) val / (double) M; 1278 } 1279 1280 // See the .cpp file. 1281 size_t get_hum_bytes(size_t* hum_bytes); 1282 void get_hum_bytes(size_t* used_bytes, size_t* capacity_bytes, 1283 size_t* prev_live_bytes, size_t* next_live_bytes); 1284 1285 public: 1286 // The header and footer are printed in the constructor and 1287 // destructor respectively. 1288 G1PrintRegionLivenessInfoClosure(outputStream* out, const char* phase_name); 1289 virtual bool doHeapRegion(HeapRegion* r); 1290 ~G1PrintRegionLivenessInfoClosure(); 1291 }; 1292 1293 #endif // SHARE_VM_GC_IMPLEMENTATION_G1_CONCURRENTMARK_HPP