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