mono/sgen/sgen-gray.c

   1 /**
   2  * \file
   3  * Gray queue management.
   4  *
   5  * Copyright 2001-2003 Ximian, Inc
   6  * Copyright 2003-2010 Novell, Inc.
   7  * Copyright (C) 2012 Xamarin Inc
   8  *
   9  * Licensed under the MIT license. See LICENSE file in the project root for full license information.
  10  */
  11 #include "config.h"
  12 #ifdef HAVE_SGEN_GC
  13
  14 #include "mono/sgen/sgen-gc.h"
  15 #include "mono/sgen/sgen-protocol.h"
  16
  17 #ifdef HEAVY_STATISTICS
  18 guint64 stat_gray_queue_section_alloc;
  19 guint64 stat_gray_queue_section_free;
  20 guint64 stat_gray_queue_enqueue_fast_path;
  21 guint64 stat_gray_queue_dequeue_fast_path;
  22 guint64 stat_gray_queue_enqueue_slow_path;
  23 guint64 stat_gray_queue_dequeue_slow_path;
  24 #endif
  25
  26 #define GRAY_QUEUE_LENGTH_LIMIT 64
  27
  28 #ifdef SGEN_CHECK_GRAY_OBJECT_SECTIONS
  29 #define STATE_TRANSITION(s,o,n) do {                                    \
  30                 int __old = (o);                                        \
  31                 if (mono_atomic_cas_i32 ((volatile int*)&(s)->state, (n), __old) != __old) \
  32                         g_assert_not_reached ();                        \
  33         } while (0)
  34 #define STATE_SET(s,v)          (s)->state = (v)
  35 #define STATE_ASSERT(s,v)       g_assert ((s)->state == (v))
  36 #else
  37 #define STATE_TRANSITION(s,o,n)
  38 #define STATE_SET(s,v)
  39 #define STATE_ASSERT(s,v)
  40 #endif
  41
  42 /*
  43  * Whenever we dispose a gray queue, we save its free list.  Then, in the next collection,
  44  * we reuse that free list for the new gray queue.
  45  */
  46 static GrayQueueSection *last_gray_queue_free_list;
  47
  48 void
  49 sgen_gray_object_alloc_queue_section (SgenGrayQueue *queue, gboolean is_parallel)
  50 {
  51         GrayQueueSection *section;
  52
  53         if (queue->free_list) {
  54                 /* Use the previously allocated queue sections if possible */
  55                 section = queue->free_list;
  56                 queue->free_list = section->next;
  57                 STATE_TRANSITION (section, GRAY_QUEUE_SECTION_STATE_FREE_LIST, GRAY_QUEUE_SECTION_STATE_FLOATING);
  58         } else {
  59                 HEAVY_STAT (stat_gray_queue_section_alloc ++);
  60
  61                 /* Allocate a new section */
  62                 section = (GrayQueueSection *)sgen_alloc_internal (INTERNAL_MEM_GRAY_QUEUE);
  63                 STATE_SET (section, GRAY_QUEUE_SECTION_STATE_FLOATING);
  64         }
  65
  66         /* Section is empty */
  67         section->size = 0;
  68
  69         STATE_TRANSITION (section, GRAY_QUEUE_SECTION_STATE_FLOATING, GRAY_QUEUE_SECTION_STATE_ENQUEUED);
  70
  71         /* Link it with the others */
  72         section->next = queue->first;
  73         section->prev = NULL;
  74         if (queue->first)
  75                 queue->first->prev = section;
  76         else
  77                 queue->last = section;
  78         queue->first = section;
  79         queue->cursor = section->entries - 1;
  80
  81         if (is_parallel) {
  82                 mono_memory_write_barrier ();
  83                 /*
  84                  * FIXME
  85                  * we could probably optimize the code to only rely on the write barrier
  86                  * for synchronization with the stealer thread. Additionally we could also
  87                  * do a write barrier once every other gray queue change, and request
  88                  * to have a minimum of sections before stealing, to keep consistency.
  89                  */
  90                 mono_atomic_inc_i32 (&queue->num_sections);
  91         } else {
  92                 queue->num_sections++;
  93         }
  94 }
  95
  96 void
  97 sgen_gray_object_free_queue_section (GrayQueueSection *section)
  98 {
  99         HEAVY_STAT (stat_gray_queue_section_free ++);
 100
 101         STATE_TRANSITION (section, GRAY_QUEUE_SECTION_STATE_FLOATING, GRAY_QUEUE_SECTION_STATE_FREED);
 102         sgen_free_internal (section, INTERNAL_MEM_GRAY_QUEUE);
 103 }
 104
 105 /*
 106  * The following two functions are called in the inner loops of the
 107  * collector, so they need to be as fast as possible.  We have macros
 108  * for them in sgen-gc.h.
 109  */
 110
 111 void
 112 sgen_gray_object_enqueue (SgenGrayQueue *queue, GCObject *obj, SgenDescriptor desc, gboolean is_parallel)
 113 {
 114         GrayQueueEntry entry = SGEN_GRAY_QUEUE_ENTRY (obj, desc);
 115
 116         HEAVY_STAT (stat_gray_queue_enqueue_slow_path ++);
 117
 118         SGEN_ASSERT (9, obj, "enqueueing a null object");
 119         //sgen_check_objref (obj);
 120
 121 #ifdef SGEN_CHECK_GRAY_OBJECT_ENQUEUE
 122         if (queue->enqueue_check_func)
 123                 queue->enqueue_check_func (obj);
 124 #endif
 125
 126         if (G_UNLIKELY (!queue->first || queue->cursor == GRAY_LAST_CURSOR_POSITION (queue->first))) {
 127                 if (queue->first) {
 128                         /*
 129                          * We don't actively update the section size with each push/pop. For the first
 130                          * section we determine the size from the cursor position. For the reset of the
 131                          * sections we need to have the size set.
 132                          */
 133                         queue->first->size = SGEN_GRAY_QUEUE_SECTION_SIZE;
 134                 }
 135
 136                 sgen_gray_object_alloc_queue_section (queue, is_parallel);
 137         }
 138         STATE_ASSERT (queue->first, GRAY_QUEUE_SECTION_STATE_ENQUEUED);
 139         SGEN_ASSERT (9, queue->cursor <= GRAY_LAST_CURSOR_POSITION (queue->first), "gray queue %p overflow, first %p, cursor %p", queue, queue->first, queue->cursor);
 140         *++queue->cursor = entry;
 141
 142 #ifdef SGEN_HEAVY_BINARY_PROTOCOL
 143         sgen_binary_protocol_gray_enqueue (queue, queue->cursor, obj);
 144 #endif
 145 }
 146
 147 /*
 148  * We attempt to spread the objects in the gray queue across a number
 149  * of sections. If the queue has more sections, then it's already spread,
 150  * if it doesn't have enough sections, then we allocate as many as we
 151  * can.
 152  */
 153 void
 154 sgen_gray_object_spread (SgenGrayQueue *queue, int num_sections)
 155 {
 156         GrayQueueSection *section_start, *section_end;
 157         int total_entries = 0, num_entries_per_section;
 158         int num_sections_final;
 159
 160         if (queue->num_sections >= num_sections)
 161                 return;
 162
 163         if (!queue->first)
 164                 return;
 165
 166         /* Compute number of elements in the gray queue */
 167         queue->first->size = queue->cursor - queue->first->entries + 1;
 168         total_entries = queue->first->size;
 169         for (section_start = queue->first->next; section_start != NULL; section_start = section_start->next) {
 170                 SGEN_ASSERT (0, section_start->size == SGEN_GRAY_QUEUE_SECTION_SIZE, "We expect all section aside from the first one to be full");
 171                 total_entries += section_start->size;
 172         }
 173
 174         /* Compute how many sections we should have and elements per section */
 175         num_sections_final = (total_entries > num_sections) ? num_sections : total_entries;
 176         num_entries_per_section = total_entries / num_sections_final;
 177
 178         /* Allocate all needed sections */
 179         while (queue->num_sections < num_sections_final)
 180                 sgen_gray_object_alloc_queue_section (queue, TRUE);
 181
 182         /* Spread out the elements in the sections. By design, sections at the end are fuller. */
 183         section_start = queue->first;
 184         section_end = queue->last;
 185         while (section_start != section_end) {
 186                 /* We move entries from end to start, until they meet */
 187                 while (section_start->size < num_entries_per_section) {
 188                         GrayQueueEntry entry;
 189                         if (section_end->size <= num_entries_per_section) {
 190                                 section_end = section_end->prev;
 191                                 if (section_end == section_start)
 192                                         break;
 193                         }
 194                         if (section_end->size <= num_entries_per_section)
 195                                 break;
 196
 197                         section_end->size--;
 198                         entry = section_end->entries [section_end->size];
 199                         section_start->entries [section_start->size] = entry;
 200                         section_start->size++;
 201                 }
 202                 section_start = section_start->next;
 203         }
 204
 205         queue->cursor = queue->first->entries + queue->first->size - 1;
 206         queue->num_sections = num_sections_final;
 207 }
 208
 209 GrayQueueEntry
 210 sgen_gray_object_dequeue (SgenGrayQueue *queue, gboolean is_parallel)
 211 {
 212         GrayQueueEntry entry;
 213
 214         HEAVY_STAT (stat_gray_queue_dequeue_slow_path ++);
 215
 216         if (sgen_gray_object_queue_is_empty (queue)) {
 217                 entry.obj = NULL;
 218                 return entry;
 219         }
 220
 221         STATE_ASSERT (queue->first, GRAY_QUEUE_SECTION_STATE_ENQUEUED);
 222         SGEN_ASSERT (9, queue->cursor >= GRAY_FIRST_CURSOR_POSITION (queue->first), "gray queue %p underflow", queue);
 223
 224         entry = *queue->cursor--;
 225
 226 #ifdef SGEN_HEAVY_BINARY_PROTOCOL
 227         sgen_binary_protocol_gray_dequeue (queue, queue->cursor + 1, entry.obj);
 228 #endif
 229
 230         if (G_UNLIKELY (queue->cursor < GRAY_FIRST_CURSOR_POSITION (queue->first))) {
 231                 GrayQueueSection *section;
 232                 gint32 old_num_sections = 0;
 233
 234                 if (is_parallel)
 235                         old_num_sections = mono_atomic_dec_i32 (&queue->num_sections);
 236                 else
 237                         queue->num_sections--;
 238
 239                 if (is_parallel && old_num_sections <= 0) {
 240                         mono_os_mutex_lock (&queue->steal_mutex);
 241                 }
 242
 243                 section = queue->first;
 244                 queue->first = section->next;
 245                 if (queue->first) {
 246                         queue->first->prev = NULL;
 247                 } else {
 248                         queue->last = NULL;
 249                         SGEN_ASSERT (0, !old_num_sections, "Why do we have an inconsistent number of sections ?");
 250                 }
 251                 section->next = queue->free_list;
 252
 253                 STATE_TRANSITION (section, GRAY_QUEUE_SECTION_STATE_ENQUEUED, GRAY_QUEUE_SECTION_STATE_FREE_LIST);
 254
 255                 queue->free_list = section;
 256                 queue->cursor = queue->first ? queue->first->entries + queue->first->size - 1 : NULL;
 257
 258                 if (is_parallel && old_num_sections <= 0) {
 259                         mono_os_mutex_unlock (&queue->steal_mutex);
 260                 }
 261         }
 262
 263         return entry;
 264 }
 265
 266 GrayQueueSection*
 267 sgen_gray_object_dequeue_section (SgenGrayQueue *queue)
 268 {
 269         GrayQueueSection *section;
 270
 271         if (!queue->first)
 272                 return NULL;
 273
 274         /* We never steal from this queue */
 275         queue->num_sections--;
 276
 277         section = queue->first;
 278         queue->first = section->next;
 279         if (queue->first)
 280                 queue->first->prev = NULL;
 281         else
 282                 queue->last = NULL;
 283
 284         section->next = NULL;
 285         section->size = queue->cursor - section->entries + 1;
 286
 287         queue->cursor = queue->first ? queue->first->entries + queue->first->size - 1 : NULL;
 288
 289         STATE_TRANSITION (section, GRAY_QUEUE_SECTION_STATE_ENQUEUED, GRAY_QUEUE_SECTION_STATE_FLOATING);
 290
 291         return section;
 292 }
 293
 294 GrayQueueSection*
 295 sgen_gray_object_steal_section (SgenGrayQueue *queue)
 296 {
 297         gint32 sections_remaining;
 298         GrayQueueSection *section = NULL;
 299
 300         /*
 301          * With each push/pop into the queue we increment the number of sections.
 302          * There is only one thread accessing the top (the owner) and potentially
 303          * multiple workers trying to steal sections from the bottom, so we need
 304          * to lock. A num sections decrement from the owner means that the first
 305          * section is reserved, while a decrement by the stealer means that the
 306          * last section is reserved. If after we decrement the num sections, we
 307          * have at least one more section present, it means we can't race with
 308          * the other thread. If this is not the case the steal end abandons the
 309          * pop, setting back the num_sections, while the owner end will take a
 310          * lock to make sure we are not racing with the stealer (since the stealer
 311          * might have popped an entry and be in the process of updating the entry
 312          * that the owner is trying to pop.
 313          */
 314
 315         if (queue->num_sections <= 1)
 316                 return NULL;
 317
 318         /* Give up if there is contention on the last section */
 319         if (mono_os_mutex_trylock (&queue->steal_mutex) != 0)
 320                 return NULL;
 321
 322         sections_remaining = mono_atomic_dec_i32 (&queue->num_sections);
 323         if (sections_remaining <= 0) {
 324                 /* The section that we tried to steal might be the head of the queue. */
 325                 mono_atomic_inc_i32 (&queue->num_sections);
 326         } else {
 327                 /* We have reserved for us the tail section of the queue */
 328                 section = queue->last;
 329                 SGEN_ASSERT (0, section, "Why we don't have any sections to steal?");
 330                 SGEN_ASSERT (0, !section->next, "Why aren't we stealing the tail?");
 331                 queue->last = section->prev;
 332                 section->prev = NULL;
 333                 SGEN_ASSERT (0, queue->last, "Why are we stealing the last section?");
 334                 queue->last->next = NULL;
 335
 336                 STATE_TRANSITION (section, GRAY_QUEUE_SECTION_STATE_ENQUEUED, GRAY_QUEUE_SECTION_STATE_FLOATING);
 337         }
 338
 339         mono_os_mutex_unlock (&queue->steal_mutex);
 340         return section;
 341 }
 342
 343 void
 344 sgen_gray_object_enqueue_section (SgenGrayQueue *queue, GrayQueueSection *section, gboolean is_parallel)
 345 {
 346         STATE_TRANSITION (section, GRAY_QUEUE_SECTION_STATE_FLOATING, GRAY_QUEUE_SECTION_STATE_ENQUEUED);
 347
 348         if (queue->first)
 349                 queue->first->size = queue->cursor - queue->first->entries + 1;
 350
 351         section->next = queue->first;
 352         section->prev = NULL;
 353         if (queue->first)
 354                 queue->first->prev = section;
 355         else
 356                 queue->last = section;
 357         queue->first = section;
 358         queue->cursor = queue->first->entries + queue->first->size - 1;
 359 #ifdef SGEN_CHECK_GRAY_OBJECT_ENQUEUE
 360         if (queue->enqueue_check_func) {
 361                 int i;
 362                 for (i = 0; i < section->size; ++i)
 363                         queue->enqueue_check_func (section->entries [i].obj);
 364         }
 365 #endif
 366         if (is_parallel) {
 367                 mono_memory_write_barrier ();
 368                 mono_atomic_inc_i32 (&queue->num_sections);
 369         } else {
 370                 queue->num_sections++;
 371         }
 372 }
 373
 374 void
 375 sgen_gray_object_queue_trim_free_list (SgenGrayQueue *queue)
 376 {
 377         GrayQueueSection *section, *next;
 378         int i = 0;
 379         for (section = queue->free_list; section && i < GRAY_QUEUE_LENGTH_LIMIT - 1; section = section->next) {
 380                 STATE_ASSERT (section, GRAY_QUEUE_SECTION_STATE_FREE_LIST);
 381                 i ++;
 382         }
 383         if (!section)
 384                 return;
 385         while (section->next) {
 386                 next = section->next;
 387                 section->next = next->next;
 388                 STATE_TRANSITION (next, GRAY_QUEUE_SECTION_STATE_FREE_LIST, GRAY_QUEUE_SECTION_STATE_FLOATING);
 389                 sgen_gray_object_free_queue_section (next);
 390         }
 391 }
 392
 393 void
 394 sgen_gray_object_queue_init (SgenGrayQueue *queue, GrayQueueEnqueueCheckFunc enqueue_check_func, gboolean reuse_free_list)
 395 {
 396         memset (queue, 0, sizeof (SgenGrayQueue));
 397
 398 #ifdef SGEN_CHECK_GRAY_OBJECT_ENQUEUE
 399         queue->enqueue_check_func = enqueue_check_func;
 400 #endif
 401
 402         mono_os_mutex_init (&queue->steal_mutex);
 403
 404         if (reuse_free_list) {
 405                 queue->free_list = last_gray_queue_free_list;
 406                 last_gray_queue_free_list = NULL;
 407         }
 408 }
 409
 410 void
 411 sgen_gray_object_queue_dispose (SgenGrayQueue *queue)
 412 {
 413         SGEN_ASSERT (0, sgen_gray_object_queue_is_empty (queue), "Why are we disposing a gray queue that's not empty?");
 414
 415         /* Free the extra sections allocated during the last collection */
 416         sgen_gray_object_queue_trim_free_list (queue);
 417
 418         SGEN_ASSERT (0, !last_gray_queue_free_list, "Are we disposing two gray queues after another?");
 419         last_gray_queue_free_list = queue->free_list;
 420
 421         mono_os_mutex_destroy (&queue->steal_mutex);
 422
 423         /* just to make sure */
 424         memset (queue, 0, sizeof (SgenGrayQueue));
 425 }
 426
 427 void
 428 sgen_gray_object_queue_deinit (SgenGrayQueue *queue)
 429 {
 430         g_assert (!queue->first);
 431         while (queue->free_list) {
 432                 GrayQueueSection *next = queue->free_list->next;
 433                 STATE_TRANSITION (queue->free_list, GRAY_QUEUE_SECTION_STATE_FREE_LIST, GRAY_QUEUE_SECTION_STATE_FLOATING);
 434                 sgen_gray_object_free_queue_section (queue->free_list);
 435                 queue->free_list = next;
 436         }
 437 }
 438
 439 static void
 440 lock_section_queue (SgenSectionGrayQueue *queue)
 441 {
 442         if (!queue->locked)
 443                 return;
 444
 445         mono_os_mutex_lock (&queue->lock);
 446 }
 447
 448 static void
 449 unlock_section_queue (SgenSectionGrayQueue *queue)
 450 {
 451         if (!queue->locked)
 452                 return;
 453
 454         mono_os_mutex_unlock (&queue->lock);
 455 }
 456
 457 void
 458 sgen_section_gray_queue_init (SgenSectionGrayQueue *queue, gboolean locked, GrayQueueEnqueueCheckFunc enqueue_check_func)
 459 {
 460         g_assert (sgen_section_gray_queue_is_empty (queue));
 461
 462         queue->locked = locked;
 463         if (locked) {
 464                 mono_os_mutex_init_recursive (&queue->lock);
 465         }
 466
 467 #ifdef SGEN_CHECK_GRAY_OBJECT_ENQUEUE
 468         queue->enqueue_check_func = enqueue_check_func;
 469 #endif
 470 }
 471
 472 gboolean
 473 sgen_section_gray_queue_is_empty (SgenSectionGrayQueue *queue)
 474 {
 475         return !queue->first;
 476 }
 477
 478 GrayQueueSection*
 479 sgen_section_gray_queue_dequeue (SgenSectionGrayQueue *queue)
 480 {
 481         GrayQueueSection *section;
 482
 483         lock_section_queue (queue);
 484
 485         if (queue->first) {
 486                 section = queue->first;
 487                 queue->first = section->next;
 488
 489                 STATE_TRANSITION (section, GRAY_QUEUE_SECTION_STATE_ENQUEUED, GRAY_QUEUE_SECTION_STATE_FLOATING);
 490
 491                 section->next = NULL;
 492         } else {
 493                 section = NULL;
 494         }
 495
 496         unlock_section_queue (queue);
 497
 498         return section;
 499 }
 500
 501 void
 502 sgen_section_gray_queue_enqueue (SgenSectionGrayQueue *queue, GrayQueueSection *section)
 503 {
 504         STATE_TRANSITION (section, GRAY_QUEUE_SECTION_STATE_FLOATING, GRAY_QUEUE_SECTION_STATE_ENQUEUED);
 505
 506         lock_section_queue (queue);
 507
 508         section->next = queue->first;
 509         queue->first = section;
 510 #ifdef SGEN_CHECK_GRAY_OBJECT_ENQUEUE
 511         if (queue->enqueue_check_func) {
 512                 int i;
 513                 for (i = 0; i < section->size; ++i)
 514                         queue->enqueue_check_func (section->entries [i].obj);
 515         }
 516 #endif
 517
 518         unlock_section_queue (queue);
 519 }
 520
 521 void
 522 sgen_init_gray_queues (void)
 523 {
 524 #ifdef HEAVY_STATISTICS
 525         mono_counters_register ("Gray Queue alloc section", MONO_COUNTER_GC | MONO_COUNTER_ULONG, &stat_gray_queue_section_alloc);
 526         mono_counters_register ("Gray Queue free section", MONO_COUNTER_GC | MONO_COUNTER_ULONG, &stat_gray_queue_section_free);
 527         mono_counters_register ("Gray Queue enqueue fast path", MONO_COUNTER_GC | MONO_COUNTER_ULONG, &stat_gray_queue_enqueue_fast_path);
 528         mono_counters_register ("Gray Queue dequeue fast path", MONO_COUNTER_GC | MONO_COUNTER_ULONG, &stat_gray_queue_dequeue_fast_path);
 529         mono_counters_register ("Gray Queue enqueue slow path", MONO_COUNTER_GC | MONO_COUNTER_ULONG, &stat_gray_queue_enqueue_slow_path);
 530         mono_counters_register ("Gray Queue dequeue slow path", MONO_COUNTER_GC | MONO_COUNTER_ULONG, &stat_gray_queue_dequeue_slow_path);
 531 #endif
 532 }
 533 #endif