232 lines
8.2 KiB
C++
232 lines
8.2 KiB
C++
/*
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* Copyright (c) 2014, 2015, Oracle and/or its affiliates. All rights reserved.
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* DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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*
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* This code is free software; you can redistribute it and/or modify it
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* under the terms of the GNU General Public License version 2 only, as
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* published by the Free Software Foundation.
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*
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* This code is distributed in the hope that it will be useful, but WITHOUT
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* ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
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* FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
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* version 2 for more details (a copy is included in the LICENSE file that
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* accompanied this code).
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*
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* You should have received a copy of the GNU General Public License version
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* 2 along with this work; if not, write to the Free Software Foundation,
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* Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
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*
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* Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
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* or visit www.oracle.com if you need additional information or have any
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* questions.
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*
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*/
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#ifndef SHARE_VM_GC_G1_G1STRINGDEDUPTABLE_HPP
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#define SHARE_VM_GC_G1_G1STRINGDEDUPTABLE_HPP
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#include "gc/g1/g1StringDedupStat.hpp"
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#include "runtime/mutexLocker.hpp"
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class G1StringDedupEntryCache;
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class G1StringDedupUnlinkOrOopsDoClosure;
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//
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// Table entry in the deduplication hashtable. Points weakly to the
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// character array. Can be chained in a linked list in case of hash
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// collisions or when placed in a freelist in the entry cache.
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//
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class G1StringDedupEntry : public CHeapObj<mtGC> {
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private:
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G1StringDedupEntry* _next;
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unsigned int _hash;
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typeArrayOop _obj;
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public:
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G1StringDedupEntry() :
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_next(NULL),
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_hash(0),
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_obj(NULL) {
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}
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G1StringDedupEntry* next() {
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return _next;
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}
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G1StringDedupEntry** next_addr() {
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return &_next;
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}
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void set_next(G1StringDedupEntry* next) {
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_next = next;
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}
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unsigned int hash() {
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return _hash;
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}
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void set_hash(unsigned int hash) {
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_hash = hash;
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}
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typeArrayOop obj() {
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return _obj;
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}
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typeArrayOop* obj_addr() {
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return &_obj;
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}
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void set_obj(typeArrayOop obj) {
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_obj = obj;
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}
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};
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//
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// The deduplication hashtable keeps track of all unique character arrays used
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// by String objects. Each table entry weakly points to an character array, allowing
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// otherwise unreachable character arrays to be declared dead and pruned from the
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// table.
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//
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// The table is dynamically resized to accommodate the current number of table entries.
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// The table has hash buckets with chains for hash collision. If the average chain
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// length goes above or below given thresholds the table grows or shrinks accordingly.
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//
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// The table is also dynamically rehashed (using a new hash seed) if it becomes severely
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// unbalanced, i.e., a hash chain is significantly longer than average.
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//
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// All access to the table is protected by the StringDedupTable_lock, except under
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// safepoints in which case GC workers are allowed to access a table partitions they
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// have claimed without first acquiring the lock. Note however, that this applies only
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// the table partition (i.e. a range of elements in _buckets), not other parts of the
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// table such as the _entries field, statistics counters, etc.
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//
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class G1StringDedupTable : public CHeapObj<mtGC> {
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private:
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// The currently active hashtable instance. Only modified when
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// the table is resizes or rehashed.
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static G1StringDedupTable* _table;
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// Cache for reuse and fast alloc/free of table entries.
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static G1StringDedupEntryCache* _entry_cache;
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G1StringDedupEntry** _buckets;
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size_t _size;
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uintx _entries;
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uintx _shrink_threshold;
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uintx _grow_threshold;
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bool _rehash_needed;
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// The hash seed also dictates which hash function to use. A
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// zero hash seed means we will use the Java compatible hash
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// function (which doesn't use a seed), and a non-zero hash
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// seed means we use the murmur3 hash function.
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jint _hash_seed;
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// Constants governing table resize/rehash/cache.
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static const size_t _min_size;
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static const size_t _max_size;
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static const double _grow_load_factor;
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static const double _shrink_load_factor;
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static const uintx _rehash_multiple;
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static const uintx _rehash_threshold;
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static const double _max_cache_factor;
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// Table statistics, only used for logging.
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static uintx _entries_added;
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static uintx _entries_removed;
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static uintx _resize_count;
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static uintx _rehash_count;
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G1StringDedupTable(size_t size, jint hash_seed = 0);
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~G1StringDedupTable();
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// Returns the hash bucket at the given index.
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G1StringDedupEntry** bucket(size_t index) {
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return _buckets + index;
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}
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// Returns the hash bucket index for the given hash code.
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size_t hash_to_index(unsigned int hash) {
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return (size_t)hash & (_size - 1);
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}
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// Adds a new table entry to the given hash bucket.
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void add(typeArrayOop value, unsigned int hash, G1StringDedupEntry** list);
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// Removes the given table entry from the table.
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void remove(G1StringDedupEntry** pentry, uint worker_id);
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// Transfers a table entry from the current table to the destination table.
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void transfer(G1StringDedupEntry** pentry, G1StringDedupTable* dest);
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// Returns an existing character array in the given hash bucket, or NULL
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// if no matching character array exists.
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typeArrayOop lookup(typeArrayOop value, unsigned int hash,
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G1StringDedupEntry** list, uintx &count);
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// Returns an existing character array in the table, or inserts a new
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// table entry if no matching character array exists.
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typeArrayOop lookup_or_add_inner(typeArrayOop value, unsigned int hash);
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// Thread safe lookup or add of table entry
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static typeArrayOop lookup_or_add(typeArrayOop value, unsigned int hash) {
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// Protect the table from concurrent access. Also note that this lock
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// acts as a fence for _table, which could have been replaced by a new
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// instance if the table was resized or rehashed.
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MutexLockerEx ml(StringDedupTable_lock, Mutex::_no_safepoint_check_flag);
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return _table->lookup_or_add_inner(value, hash);
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}
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// Returns true if the hashtable is currently using a Java compatible
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// hash function.
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static bool use_java_hash() {
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return _table->_hash_seed == 0;
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}
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static bool equals(typeArrayOop value1, typeArrayOop value2);
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// Computes the hash code for the given character array, using the
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// currently active hash function and hash seed.
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static unsigned int hash_code(typeArrayOop value);
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static uintx unlink_or_oops_do(G1StringDedupUnlinkOrOopsDoClosure* cl,
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size_t partition_begin,
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size_t partition_end,
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uint worker_id);
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public:
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static void create();
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// Deduplicates the given String object, or adds its backing
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// character array to the deduplication hashtable.
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static void deduplicate(oop java_string, G1StringDedupStat& stat);
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// If a table resize is needed, returns a newly allocated empty
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// hashtable of the proper size.
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static G1StringDedupTable* prepare_resize();
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// Installs a newly resized table as the currently active table
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// and deletes the previously active table.
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static void finish_resize(G1StringDedupTable* resized_table);
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// If a table rehash is needed, returns a newly allocated empty
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// hashtable and updates the hash seed.
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static G1StringDedupTable* prepare_rehash();
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// Transfers rehashed entries from the currently active table into
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// the new table. Installs the new table as the currently active table
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// and deletes the previously active table.
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static void finish_rehash(G1StringDedupTable* rehashed_table);
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// If the table entry cache has grown too large, trim it down according to policy
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static void trim_entry_cache();
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static void unlink_or_oops_do(G1StringDedupUnlinkOrOopsDoClosure* cl, uint worker_id);
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static void print_statistics(outputStream* st);
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static void verify();
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};
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#endif // SHARE_VM_GC_G1_G1STRINGDEDUPTABLE_HPP
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