1 /* 2 * Copyright (c) 2014, 2015, 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 #include "precompiled.hpp" 26 #include "classfile/altHashing.hpp" 27 #include "classfile/javaClasses.inline.hpp" 28 #include "gc/g1/g1CollectedHeap.inline.hpp" 29 #include "gc/g1/g1SATBCardTableModRefBS.hpp" 30 #include "gc/g1/g1StringDedup.hpp" 31 #include "gc/g1/g1StringDedupTable.hpp" 32 #include "gc/shared/gcLocker.hpp" 33 #include "memory/padded.inline.hpp" 34 #include "oops/oop.inline.hpp" 35 #include "oops/typeArrayOop.hpp" 36 #include "runtime/mutexLocker.hpp" 37 38 // 39 // Freelist in the deduplication table entry cache. Links table 40 // entries together using their _next fields. 41 // 42 class G1StringDedupEntryFreeList : public CHeapObj<mtGC> { 43 private: 44 G1StringDedupEntry* _list; 45 size_t _length; 46 47 public: 48 G1StringDedupEntryFreeList() : 49 _list(NULL), 50 _length(0) { 51 } 52 53 void add(G1StringDedupEntry* entry) { 54 entry->set_next(_list); 55 _list = entry; 56 _length++; 57 } 58 59 G1StringDedupEntry* remove() { 60 G1StringDedupEntry* entry = _list; 61 if (entry != NULL) { 62 _list = entry->next(); 63 _length--; 64 } 65 return entry; 66 } 67 68 size_t length() { 69 return _length; 70 } 71 }; 72 73 // 74 // Cache of deduplication table entries. This cache provides fast allocation and 75 // reuse of table entries to lower the pressure on the underlying allocator. 76 // But more importantly, it provides fast/deferred freeing of table entries. This 77 // is important because freeing of table entries is done during stop-the-world 78 // phases and it is not uncommon for large number of entries to be freed at once. 79 // Tables entries that are freed during these phases are placed onto a freelist in 80 // the cache. The deduplication thread, which executes in a concurrent phase, will 81 // later reuse or free the underlying memory for these entries. 82 // 83 // The cache allows for single-threaded allocations and multi-threaded frees. 84 // Allocations are synchronized by StringDedupTable_lock as part of a table 85 // modification. 86 // 87 class G1StringDedupEntryCache : public CHeapObj<mtGC> { 88 private: 89 // One freelist per GC worker to allow lock less freeing of 90 // entries while doing a parallel scan of the table. Using 91 // PaddedEnd to avoid false sharing. 92 PaddedEnd<G1StringDedupEntryFreeList>* _lists; 93 size_t _nlists; 94 95 public: 96 G1StringDedupEntryCache(); 97 ~G1StringDedupEntryCache(); 98 99 // Get a table entry from the cache freelist, or allocate a new 100 // entry if the cache is empty. 101 G1StringDedupEntry* alloc(); 102 103 // Insert a table entry into the cache freelist. 104 void free(G1StringDedupEntry* entry, uint worker_id); 105 106 // Returns current number of entries in the cache. 107 size_t size(); 108 109 // If the cache has grown above the given max size, trim it down 110 // and deallocate the memory occupied by trimmed of entries. 111 void trim(size_t max_size); 112 }; 113 114 G1StringDedupEntryCache::G1StringDedupEntryCache() { 115 _nlists = MAX2(ParallelGCThreads, (size_t)1); 116 _lists = PaddedArray<G1StringDedupEntryFreeList, mtGC>::create_unfreeable((uint)_nlists); 117 } 118 119 G1StringDedupEntryCache::~G1StringDedupEntryCache() { 120 ShouldNotReachHere(); 121 } 122 123 G1StringDedupEntry* G1StringDedupEntryCache::alloc() { 124 for (size_t i = 0; i < _nlists; i++) { 125 G1StringDedupEntry* entry = _lists[i].remove(); 126 if (entry != NULL) { 127 return entry; 128 } 129 } 130 return new G1StringDedupEntry(); 131 } 132 133 void G1StringDedupEntryCache::free(G1StringDedupEntry* entry, uint worker_id) { 134 assert(entry->obj() != NULL, "Double free"); 135 assert(worker_id < _nlists, "Invalid worker id"); 136 entry->set_obj(NULL); 137 entry->set_hash(0); 138 _lists[worker_id].add(entry); 139 } 140 141 size_t G1StringDedupEntryCache::size() { 142 size_t size = 0; 143 for (size_t i = 0; i < _nlists; i++) { 144 size += _lists[i].length(); 145 } 146 return size; 147 } 148 149 void G1StringDedupEntryCache::trim(size_t max_size) { 150 size_t cache_size = 0; 151 for (size_t i = 0; i < _nlists; i++) { 152 G1StringDedupEntryFreeList* list = &_lists[i]; 153 cache_size += list->length(); 154 while (cache_size > max_size) { 155 G1StringDedupEntry* entry = list->remove(); 156 assert(entry != NULL, "Should not be null"); 157 cache_size--; 158 delete entry; 159 } 160 } 161 } 162 163 G1StringDedupTable* G1StringDedupTable::_table = NULL; 164 G1StringDedupEntryCache* G1StringDedupTable::_entry_cache = NULL; 165 166 const size_t G1StringDedupTable::_min_size = (1 << 10); // 1024 167 const size_t G1StringDedupTable::_max_size = (1 << 24); // 16777216 168 const double G1StringDedupTable::_grow_load_factor = 2.0; // Grow table at 200% load 169 const double G1StringDedupTable::_shrink_load_factor = _grow_load_factor / 3.0; // Shrink table at 67% load 170 const double G1StringDedupTable::_max_cache_factor = 0.1; // Cache a maximum of 10% of the table size 171 const uintx G1StringDedupTable::_rehash_multiple = 60; // Hash bucket has 60 times more collisions than expected 172 const uintx G1StringDedupTable::_rehash_threshold = (uintx)(_rehash_multiple * _grow_load_factor); 173 174 uintx G1StringDedupTable::_entries_added = 0; 175 uintx G1StringDedupTable::_entries_removed = 0; 176 uintx G1StringDedupTable::_resize_count = 0; 177 uintx G1StringDedupTable::_rehash_count = 0; 178 179 G1StringDedupTable::G1StringDedupTable(size_t size, jint hash_seed) : 180 _size(size), 181 _entries(0), 182 _grow_threshold((uintx)(size * _grow_load_factor)), 183 _shrink_threshold((uintx)(size * _shrink_load_factor)), 184 _rehash_needed(false), 185 _hash_seed(hash_seed) { 186 assert(is_power_of_2(size), "Table size must be a power of 2"); 187 _buckets = NEW_C_HEAP_ARRAY(G1StringDedupEntry*, _size, mtGC); 188 memset(_buckets, 0, _size * sizeof(G1StringDedupEntry*)); 189 } 190 191 G1StringDedupTable::~G1StringDedupTable() { 192 FREE_C_HEAP_ARRAY(G1StringDedupEntry*, _buckets); 193 } 194 195 void G1StringDedupTable::create() { 196 assert(_table == NULL, "One string deduplication table allowed"); 197 _entry_cache = new G1StringDedupEntryCache(); 198 _table = new G1StringDedupTable(_min_size); 199 } 200 201 void G1StringDedupTable::add(typeArrayOop value, unsigned int hash, G1StringDedupEntry** list) { 202 G1StringDedupEntry* entry = _entry_cache->alloc(); 203 entry->set_obj(value); 204 entry->set_hash(hash); 205 entry->set_next(*list); 206 *list = entry; 207 _entries++; 208 } 209 210 void G1StringDedupTable::remove(G1StringDedupEntry** pentry, uint worker_id) { 211 G1StringDedupEntry* entry = *pentry; 212 *pentry = entry->next(); 213 _entry_cache->free(entry, worker_id); 214 } 215 216 void G1StringDedupTable::transfer(G1StringDedupEntry** pentry, G1StringDedupTable* dest) { 217 G1StringDedupEntry* entry = *pentry; 218 *pentry = entry->next(); 219 unsigned int hash = entry->hash(); 220 size_t index = dest->hash_to_index(hash); 221 G1StringDedupEntry** list = dest->bucket(index); 222 entry->set_next(*list); 223 *list = entry; 224 } 225 226 bool G1StringDedupTable::equals(typeArrayOop value1, typeArrayOop value2) { 227 return (value1 == value2 || 228 (value1->length() == value2->length() && 229 (!memcmp(value1->base(T_CHAR), 230 value2->base(T_CHAR), 231 value1->length() * sizeof(jchar))))); 232 } 233 234 typeArrayOop G1StringDedupTable::lookup(typeArrayOop value, unsigned int hash, 235 G1StringDedupEntry** list, uintx &count) { 236 for (G1StringDedupEntry* entry = *list; entry != NULL; entry = entry->next()) { 237 if (entry->hash() == hash) { 238 typeArrayOop existing_value = entry->obj(); 239 if (equals(value, existing_value)) { 240 // Match found 241 return existing_value; 242 } 243 } 244 count++; 245 } 246 247 // Not found 248 return NULL; 249 } 250 251 typeArrayOop G1StringDedupTable::lookup_or_add_inner(typeArrayOop value, unsigned int hash) { 252 size_t index = hash_to_index(hash); 253 G1StringDedupEntry** list = bucket(index); 254 uintx count = 0; 255 256 // Lookup in list 257 typeArrayOop existing_value = lookup(value, hash, list, count); 258 259 // Check if rehash is needed 260 if (count > _rehash_threshold) { 261 _rehash_needed = true; 262 } 263 264 if (existing_value == NULL) { 265 // Not found, add new entry 266 add(value, hash, list); 267 268 // Update statistics 269 _entries_added++; 270 } 271 272 return existing_value; 273 } 274 275 unsigned int G1StringDedupTable::hash_code(typeArrayOop value) { 276 unsigned int hash; 277 int length = value->length(); 278 const jchar* data = (jchar*)value->base(T_CHAR); 279 280 if (use_java_hash()) { 281 hash = java_lang_String::hash_code(data, length); 282 } else { 283 hash = AltHashing::murmur3_32(_table->_hash_seed, data, length); 284 } 285 286 return hash; 287 } 288 289 void G1StringDedupTable::deduplicate(oop java_string, G1StringDedupStat& stat) { 290 assert(java_lang_String::is_instance(java_string), "Must be a string"); 291 No_Safepoint_Verifier nsv; 292 293 stat.inc_inspected(); 294 295 typeArrayOop value = java_lang_String::value(java_string); 296 if (value == NULL) { 297 // String has no value 298 stat.inc_skipped(); 299 return; 300 } 301 302 unsigned int hash = 0; 303 304 if (use_java_hash()) { 305 // Get hash code from cache 306 hash = java_lang_String::hash(java_string); 307 } 308 309 if (hash == 0) { 310 // Compute hash 311 hash = hash_code(value); 312 stat.inc_hashed(); 313 } 314 315 if (use_java_hash() && hash != 0) { 316 // Store hash code in cache 317 java_lang_String::set_hash(java_string, hash); 318 } 319 320 typeArrayOop existing_value = lookup_or_add(value, hash); 321 if (existing_value == value) { 322 // Same value, already known 323 stat.inc_known(); 324 return; 325 } 326 327 // Get size of value array 328 uintx size_in_bytes = value->size() * HeapWordSize; 329 stat.inc_new(size_in_bytes); 330 331 if (existing_value != NULL) { 332 // Enqueue the reference to make sure it is kept alive. Concurrent mark might 333 // otherwise declare it dead if there are no other strong references to this object. 334 G1SATBCardTableModRefBS::enqueue(existing_value); 335 336 // Existing value found, deduplicate string 337 java_lang_String::set_value(java_string, existing_value); 338 339 if (G1CollectedHeap::heap()->is_in_young(value)) { 340 stat.inc_deduped_young(size_in_bytes); 341 } else { 342 stat.inc_deduped_old(size_in_bytes); 343 } 344 } 345 } 346 347 G1StringDedupTable* G1StringDedupTable::prepare_resize() { 348 size_t size = _table->_size; 349 350 // Check if the hashtable needs to be resized 351 if (_table->_entries > _table->_grow_threshold) { 352 // Grow table, double the size 353 size *= 2; 354 if (size > _max_size) { 355 // Too big, don't resize 356 return NULL; 357 } 358 } else if (_table->_entries < _table->_shrink_threshold) { 359 // Shrink table, half the size 360 size /= 2; 361 if (size < _min_size) { 362 // Too small, don't resize 363 return NULL; 364 } 365 } else if (StringDeduplicationResizeALot) { 366 // Force grow 367 size *= 2; 368 if (size > _max_size) { 369 // Too big, force shrink instead 370 size /= 4; 371 } 372 } else { 373 // Resize not needed 374 return NULL; 375 } 376 377 // Update statistics 378 _resize_count++; 379 380 // Allocate the new table. The new table will be populated by workers 381 // calling unlink_or_oops_do() and finally installed by finish_resize(). 382 return new G1StringDedupTable(size, _table->_hash_seed); 383 } 384 385 void G1StringDedupTable::finish_resize(G1StringDedupTable* resized_table) { 386 assert(resized_table != NULL, "Invalid table"); 387 388 resized_table->_entries = _table->_entries; 389 390 // Free old table 391 delete _table; 392 393 // Install new table 394 _table = resized_table; 395 } 396 397 void G1StringDedupTable::unlink_or_oops_do(G1StringDedupUnlinkOrOopsDoClosure* cl, uint worker_id) { 398 // The table is divided into partitions to allow lock-less parallel processing by 399 // multiple worker threads. A worker thread first claims a partition, which ensures 400 // exclusive access to that part of the table, then continues to process it. To allow 401 // shrinking of the table in parallel we also need to make sure that the same worker 402 // thread processes all partitions where entries will hash to the same destination 403 // partition. Since the table size is always a power of two and we always shrink by 404 // dividing the table in half, we know that for a given partition there is only one 405 // other partition whoes entries will hash to the same destination partition. That 406 // other partition is always the sibling partition in the second half of the table. 407 // For example, if the table is divided into 8 partitions, the sibling of partition 0 408 // is partition 4, the sibling of partition 1 is partition 5, etc. 409 size_t table_half = _table->_size / 2; 410 411 // Let each partition be one page worth of buckets 412 size_t partition_size = MIN2(table_half, os::vm_page_size() / sizeof(G1StringDedupEntry*)); 413 assert(table_half % partition_size == 0, "Invalid partition size"); 414 415 // Number of entries removed during the scan 416 uintx removed = 0; 417 418 for (;;) { 419 // Grab next partition to scan 420 size_t partition_begin = cl->claim_table_partition(partition_size); 421 size_t partition_end = partition_begin + partition_size; 422 if (partition_begin >= table_half) { 423 // End of table 424 break; 425 } 426 427 // Scan the partition followed by the sibling partition in the second half of the table 428 removed += unlink_or_oops_do(cl, partition_begin, partition_end, worker_id); 429 removed += unlink_or_oops_do(cl, table_half + partition_begin, table_half + partition_end, worker_id); 430 } 431 432 // Delayed update avoid contention on the table lock 433 if (removed > 0) { 434 MutexLockerEx ml(StringDedupTable_lock, Mutex::_no_safepoint_check_flag); 435 _table->_entries -= removed; 436 _entries_removed += removed; 437 } 438 } 439 440 uintx G1StringDedupTable::unlink_or_oops_do(G1StringDedupUnlinkOrOopsDoClosure* cl, 441 size_t partition_begin, 442 size_t partition_end, 443 uint worker_id) { 444 uintx removed = 0; 445 for (size_t bucket = partition_begin; bucket < partition_end; bucket++) { 446 G1StringDedupEntry** entry = _table->bucket(bucket); 447 while (*entry != NULL) { 448 oop* p = (oop*)(*entry)->obj_addr(); 449 if (cl->is_alive(*p)) { 450 cl->keep_alive(p); 451 if (cl->is_resizing()) { 452 // We are resizing the table, transfer entry to the new table 453 _table->transfer(entry, cl->resized_table()); 454 } else { 455 if (cl->is_rehashing()) { 456 // We are rehashing the table, rehash the entry but keep it 457 // in the table. We can't transfer entries into the new table 458 // at this point since we don't have exclusive access to all 459 // destination partitions. finish_rehash() will do a single 460 // threaded transfer of all entries. 461 typeArrayOop value = (typeArrayOop)*p; 462 unsigned int hash = hash_code(value); 463 (*entry)->set_hash(hash); 464 } 465 466 // Move to next entry 467 entry = (*entry)->next_addr(); 468 } 469 } else { 470 // Not alive, remove entry from table 471 _table->remove(entry, worker_id); 472 removed++; 473 } 474 } 475 } 476 477 return removed; 478 } 479 480 G1StringDedupTable* G1StringDedupTable::prepare_rehash() { 481 if (!_table->_rehash_needed && !StringDeduplicationRehashALot) { 482 // Rehash not needed 483 return NULL; 484 } 485 486 // Update statistics 487 _rehash_count++; 488 489 // Compute new hash seed 490 _table->_hash_seed = AltHashing::compute_seed(); 491 492 // Allocate the new table, same size and hash seed 493 return new G1StringDedupTable(_table->_size, _table->_hash_seed); 494 } 495 496 void G1StringDedupTable::finish_rehash(G1StringDedupTable* rehashed_table) { 497 assert(rehashed_table != NULL, "Invalid table"); 498 499 // Move all newly rehashed entries into the correct buckets in the new table 500 for (size_t bucket = 0; bucket < _table->_size; bucket++) { 501 G1StringDedupEntry** entry = _table->bucket(bucket); 502 while (*entry != NULL) { 503 _table->transfer(entry, rehashed_table); 504 } 505 } 506 507 rehashed_table->_entries = _table->_entries; 508 509 // Free old table 510 delete _table; 511 512 // Install new table 513 _table = rehashed_table; 514 } 515 516 void G1StringDedupTable::verify() { 517 for (size_t bucket = 0; bucket < _table->_size; bucket++) { 518 // Verify entries 519 G1StringDedupEntry** entry = _table->bucket(bucket); 520 while (*entry != NULL) { 521 typeArrayOop value = (*entry)->obj(); 522 guarantee(value != NULL, "Object must not be NULL"); 523 guarantee(G1CollectedHeap::heap()->is_in_reserved(value), "Object must be on the heap"); 524 guarantee(!value->is_forwarded(), "Object must not be forwarded"); 525 guarantee(value->is_typeArray(), "Object must be a typeArrayOop"); 526 unsigned int hash = hash_code(value); 527 guarantee((*entry)->hash() == hash, "Table entry has inorrect hash"); 528 guarantee(_table->hash_to_index(hash) == bucket, "Table entry has incorrect index"); 529 entry = (*entry)->next_addr(); 530 } 531 532 // Verify that we do not have entries with identical oops or identical arrays. 533 // We only need to compare entries in the same bucket. If the same oop or an 534 // identical array has been inserted more than once into different/incorrect 535 // buckets the verification step above will catch that. 536 G1StringDedupEntry** entry1 = _table->bucket(bucket); 537 while (*entry1 != NULL) { 538 typeArrayOop value1 = (*entry1)->obj(); 539 G1StringDedupEntry** entry2 = (*entry1)->next_addr(); 540 while (*entry2 != NULL) { 541 typeArrayOop value2 = (*entry2)->obj(); 542 guarantee(!equals(value1, value2), "Table entries must not have identical arrays"); 543 entry2 = (*entry2)->next_addr(); 544 } 545 entry1 = (*entry1)->next_addr(); 546 } 547 } 548 } 549 550 void G1StringDedupTable::trim_entry_cache() { 551 MutexLockerEx ml(StringDedupTable_lock, Mutex::_no_safepoint_check_flag); 552 size_t max_cache_size = (size_t)(_table->_size * _max_cache_factor); 553 _entry_cache->trim(max_cache_size); 554 } 555 556 void G1StringDedupTable::print_statistics(outputStream* st) { 557 st->print_cr( 558 " [Table]\n" 559 " [Memory Usage: "G1_STRDEDUP_BYTES_FORMAT_NS"]\n" 560 " [Size: "SIZE_FORMAT", Min: "SIZE_FORMAT", Max: "SIZE_FORMAT"]\n" 561 " [Entries: "UINTX_FORMAT", Load: "G1_STRDEDUP_PERCENT_FORMAT_NS", Cached: " UINTX_FORMAT ", Added: "UINTX_FORMAT", Removed: "UINTX_FORMAT"]\n" 562 " [Resize Count: "UINTX_FORMAT", Shrink Threshold: "UINTX_FORMAT"("G1_STRDEDUP_PERCENT_FORMAT_NS"), Grow Threshold: "UINTX_FORMAT"("G1_STRDEDUP_PERCENT_FORMAT_NS")]\n" 563 " [Rehash Count: "UINTX_FORMAT", Rehash Threshold: "UINTX_FORMAT", Hash Seed: 0x%x]\n" 564 " [Age Threshold: "UINTX_FORMAT"]", 565 G1_STRDEDUP_BYTES_PARAM(_table->_size * sizeof(G1StringDedupEntry*) + (_table->_entries + _entry_cache->size()) * sizeof(G1StringDedupEntry)), 566 _table->_size, _min_size, _max_size, 567 _table->_entries, (double)_table->_entries / (double)_table->_size * 100.0, _entry_cache->size(), _entries_added, _entries_removed, 568 _resize_count, _table->_shrink_threshold, _shrink_load_factor * 100.0, _table->_grow_threshold, _grow_load_factor * 100.0, 569 _rehash_count, _rehash_threshold, _table->_hash_seed, 570 StringDeduplicationAgeThreshold); 571 }