mono/sgen/sgen-gray.c

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