Thu, 21 Aug 2014 16:44:41 +0200
8055098: WB API should be extended to provide information about size and age of object.
Summary: Extend the WhiteBox API to provide information about the size and age of objects. Further add a mechanism to trigger a young GC.
Reviewed-by: tschatzl, sjohanss
Contributed-by: Leonid Mesnik <leonid.mesnik@oracle.com>
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.
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19 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
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23 */
25 #ifndef SHARE_VM_GC_IMPLEMENTATION_G1_G1STRINGDEDUPTABLE_HPP
26 #define SHARE_VM_GC_IMPLEMENTATION_G1_G1STRINGDEDUPTABLE_HPP
28 #include "gc_implementation/g1/g1StringDedupStat.hpp"
29 #include "runtime/mutexLocker.hpp"
31 class G1StringDedupEntryCache;
33 //
34 // Table entry in the deduplication hashtable. Points weakly to the
35 // character array. Can be chained in a linked list in case of hash
36 // collisions or when placed in a freelist in the entry cache.
37 //
38 class G1StringDedupEntry : public CHeapObj<mtGC> {
39 private:
40 G1StringDedupEntry* _next;
41 unsigned int _hash;
42 typeArrayOop _obj;
44 public:
45 G1StringDedupEntry() :
46 _next(NULL),
47 _hash(0),
48 _obj(NULL) {
49 }
51 G1StringDedupEntry* next() {
52 return _next;
53 }
55 G1StringDedupEntry** next_addr() {
56 return &_next;
57 }
59 void set_next(G1StringDedupEntry* next) {
60 _next = next;
61 }
63 unsigned int hash() {
64 return _hash;
65 }
67 void set_hash(unsigned int hash) {
68 _hash = hash;
69 }
71 typeArrayOop obj() {
72 return _obj;
73 }
75 typeArrayOop* obj_addr() {
76 return &_obj;
77 }
79 void set_obj(typeArrayOop obj) {
80 _obj = obj;
81 }
82 };
84 //
85 // The deduplication hashtable keeps track of all unique character arrays used
86 // by String objects. Each table entry weakly points to an character array, allowing
87 // otherwise unreachable character arrays to be declared dead and pruned from the
88 // table.
89 //
90 // The table is dynamically resized to accommodate the current number of table entries.
91 // The table has hash buckets with chains for hash collision. If the average chain
92 // length goes above or below given thresholds the table grows or shrinks accordingly.
93 //
94 // The table is also dynamically rehashed (using a new hash seed) if it becomes severely
95 // unbalanced, i.e., a hash chain is significantly longer than average.
96 //
97 // All access to the table is protected by the StringDedupTable_lock, except under
98 // safepoints in which case GC workers are allowed to access a table partitions they
99 // have claimed without first acquiring the lock. Note however, that this applies only
100 // the table partition (i.e. a range of elements in _buckets), not other parts of the
101 // table such as the _entries field, statistics counters, etc.
102 //
103 class G1StringDedupTable : public CHeapObj<mtGC> {
104 private:
105 // The currently active hashtable instance. Only modified when
106 // the table is resizes or rehashed.
107 static G1StringDedupTable* _table;
109 // Cache for reuse and fast alloc/free of table entries.
110 static G1StringDedupEntryCache* _entry_cache;
112 G1StringDedupEntry** _buckets;
113 size_t _size;
114 uintx _entries;
115 uintx _shrink_threshold;
116 uintx _grow_threshold;
117 bool _rehash_needed;
119 // The hash seed also dictates which hash function to use. A
120 // zero hash seed means we will use the Java compatible hash
121 // function (which doesn't use a seed), and a non-zero hash
122 // seed means we use the murmur3 hash function.
123 jint _hash_seed;
125 // Constants governing table resize/rehash/cache.
126 static const size_t _min_size;
127 static const size_t _max_size;
128 static const double _grow_load_factor;
129 static const double _shrink_load_factor;
130 static const uintx _rehash_multiple;
131 static const uintx _rehash_threshold;
132 static const double _max_cache_factor;
134 // Table statistics, only used for logging.
135 static uintx _entries_added;
136 static uintx _entries_removed;
137 static uintx _resize_count;
138 static uintx _rehash_count;
140 G1StringDedupTable(size_t size, jint hash_seed = 0);
141 ~G1StringDedupTable();
143 // Returns the hash bucket at the given index.
144 G1StringDedupEntry** bucket(size_t index) {
145 return _buckets + index;
146 }
148 // Returns the hash bucket index for the given hash code.
149 size_t hash_to_index(unsigned int hash) {
150 return (size_t)hash & (_size - 1);
151 }
153 // Adds a new table entry to the given hash bucket.
154 void add(typeArrayOop value, unsigned int hash, G1StringDedupEntry** list);
156 // Removes the given table entry from the table.
157 void remove(G1StringDedupEntry** pentry, uint worker_id);
159 // Transfers a table entry from the current table to the destination table.
160 void transfer(G1StringDedupEntry** pentry, G1StringDedupTable* dest);
162 // Returns an existing character array in the given hash bucket, or NULL
163 // if no matching character array exists.
164 typeArrayOop lookup(typeArrayOop value, unsigned int hash,
165 G1StringDedupEntry** list, uintx &count);
167 // Returns an existing character array in the table, or inserts a new
168 // table entry if no matching character array exists.
169 typeArrayOop lookup_or_add_inner(typeArrayOop value, unsigned int hash);
171 // Thread safe lookup or add of table entry
172 static typeArrayOop lookup_or_add(typeArrayOop value, unsigned int hash) {
173 // Protect the table from concurrent access. Also note that this lock
174 // acts as a fence for _table, which could have been replaced by a new
175 // instance if the table was resized or rehashed.
176 MutexLockerEx ml(StringDedupTable_lock, Mutex::_no_safepoint_check_flag);
177 return _table->lookup_or_add_inner(value, hash);
178 }
180 // Returns true if the hashtable is currently using a Java compatible
181 // hash function.
182 static bool use_java_hash() {
183 return _table->_hash_seed == 0;
184 }
186 static bool equals(typeArrayOop value1, typeArrayOop value2);
188 // Computes the hash code for the given character array, using the
189 // currently active hash function and hash seed.
190 static unsigned int hash_code(typeArrayOop value);
192 static uintx unlink_or_oops_do(G1StringDedupUnlinkOrOopsDoClosure* cl,
193 size_t partition_begin,
194 size_t partition_end,
195 uint worker_id);
197 public:
198 static void create();
200 // Deduplicates the given String object, or adds its backing
201 // character array to the deduplication hashtable.
202 static void deduplicate(oop java_string, G1StringDedupStat& stat);
204 // If a table resize is needed, returns a newly allocated empty
205 // hashtable of the proper size.
206 static G1StringDedupTable* prepare_resize();
208 // Installs a newly resized table as the currently active table
209 // and deletes the previously active table.
210 static void finish_resize(G1StringDedupTable* resized_table);
212 // If a table rehash is needed, returns a newly allocated empty
213 // hashtable and updates the hash seed.
214 static G1StringDedupTable* prepare_rehash();
216 // Transfers rehashed entries from the currently active table into
217 // the new table. Installs the new table as the currently active table
218 // and deletes the previously active table.
219 static void finish_rehash(G1StringDedupTable* rehashed_table);
221 // If the table entry cache has grown too large, trim it down according to policy
222 static void trim_entry_cache();
224 static void unlink_or_oops_do(G1StringDedupUnlinkOrOopsDoClosure* cl, uint worker_id);
226 static void print_statistics(outputStream* st);
227 static void verify();
228 };
230 #endif // SHARE_VM_GC_IMPLEMENTATION_G1_G1STRINGDEDUPTABLE_HPP