| /* |
| * Copyright (c) 2014, 2017, Oracle and/or its affiliates. All rights reserved. |
| * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER. |
| * |
| * This code is free software; you can redistribute it and/or modify it |
| * under the terms of the GNU General Public License version 2 only, as |
| * published by the Free Software Foundation. |
| * |
| * This code is distributed in the hope that it will be useful, but WITHOUT |
| * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or |
| * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License |
| * version 2 for more details (a copy is included in the LICENSE file that |
| * accompanied this code). |
| * |
| * You should have received a copy of the GNU General Public License version |
| * 2 along with this work; if not, write to the Free Software Foundation, |
| * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA. |
| * |
| * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA |
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| */ |
| |
| #ifndef SHARE_VM_GC_G1_G1STRINGDEDUP_HPP |
| #define SHARE_VM_GC_G1_G1STRINGDEDUP_HPP |
| |
| // |
| // String Deduplication |
| // |
| // String deduplication aims to reduce the heap live-set by deduplicating identical |
| // instances of String so that they share the same backing character array. |
| // |
| // The deduplication process is divided in two main parts, 1) finding the objects to |
| // deduplicate, and 2) deduplicating those objects. The first part is done as part of |
| // a normal GC cycle when objects are marked or evacuated. At this time a check is |
| // applied on each object to check if it is a candidate for deduplication. If so, the |
| // object is placed on the deduplication queue for later processing. The second part, |
| // processing the objects on the deduplication queue, is a concurrent phase which |
| // starts right after the stop-the-wold marking/evacuation phase. This phase is |
| // executed by the deduplication thread, which pulls deduplication candidates of the |
| // deduplication queue and tries to deduplicate them. |
| // |
| // A deduplication hashtable is used to keep track of all unique character arrays |
| // used by String objects. When deduplicating, a lookup is made in this table to see |
| // if there is already an identical character array somewhere on the heap. If so, the |
| // String object is adjusted to point to that character array, releasing the reference |
| // to the original array allowing it to eventually be garbage collected. If the lookup |
| // fails the character array is instead inserted into the hashtable so that this array |
| // can be shared at some point in the future. |
| // |
| // Candidate selection |
| // |
| // An object is considered a deduplication candidate if all of the following |
| // statements are true: |
| // |
| // - The object is an instance of java.lang.String |
| // |
| // - The object is being evacuated from a young heap region |
| // |
| // - The object is being evacuated to a young/survivor heap region and the |
| // object's age is equal to the deduplication age threshold |
| // |
| // or |
| // |
| // The object is being evacuated to an old heap region and the object's age is |
| // less than the deduplication age threshold |
| // |
| // Once an string object has been promoted to an old region, or its age is higher |
| // than the deduplication age threshold, is will never become a candidate again. |
| // This approach avoids making the same object a candidate more than once. |
| // |
| // Interned strings are a bit special. They are explicitly deduplicated just before |
| // being inserted into the StringTable (to avoid counteracting C2 optimizations done |
| // on string literals), then they also become deduplication candidates if they reach |
| // the deduplication age threshold or are evacuated to an old heap region. The second |
| // attempt to deduplicate such strings will be in vain, but we have no fast way of |
| // filtering them out. This has not shown to be a problem, as the number of interned |
| // strings is usually dwarfed by the number of normal (non-interned) strings. |
| // |
| // For additional information on string deduplication, please see JEP 192, |
| // http://openjdk.java.net/jeps/192 |
| // |
| |
| #include "memory/allocation.hpp" |
| #include "oops/oop.hpp" |
| |
| class OopClosure; |
| class BoolObjectClosure; |
| class ThreadClosure; |
| class outputStream; |
| class G1StringDedupTable; |
| class G1StringDedupUnlinkOrOopsDoClosure; |
| class G1GCPhaseTimes; |
| |
| // |
| // Main interface for interacting with string deduplication. |
| // |
| class G1StringDedup : public AllStatic { |
| private: |
| // Single state for checking if both G1 and string deduplication is enabled. |
| static bool _enabled; |
| |
| // Candidate selection policies, returns true if the given object is |
| // candidate for string deduplication. |
| static bool is_candidate_from_mark(oop obj); |
| static bool is_candidate_from_evacuation(bool from_young, bool to_young, oop obj); |
| |
| public: |
| // Returns true if both G1 and string deduplication is enabled. |
| static bool is_enabled() { |
| return _enabled; |
| } |
| |
| // Initialize string deduplication. |
| static void initialize(); |
| |
| // Stop the deduplication thread. |
| static void stop(); |
| |
| // Immediately deduplicates the given String object, bypassing the |
| // the deduplication queue. |
| static void deduplicate(oop java_string); |
| |
| // Enqueues a deduplication candidate for later processing by the deduplication |
| // thread. Before enqueuing, these functions apply the appropriate candidate |
| // selection policy to filters out non-candidates. |
| static void enqueue_from_mark(oop java_string); |
| static void enqueue_from_evacuation(bool from_young, bool to_young, |
| unsigned int queue, oop java_string); |
| |
| static void oops_do(OopClosure* keep_alive); |
| static void parallel_unlink(G1StringDedupUnlinkOrOopsDoClosure* unlink, uint worker_id); |
| static void unlink_or_oops_do(BoolObjectClosure* is_alive, OopClosure* keep_alive, |
| bool allow_resize_and_rehash, G1GCPhaseTimes* phase_times = NULL); |
| |
| static void threads_do(ThreadClosure* tc); |
| static void print_worker_threads_on(outputStream* st); |
| static void verify(); |
| }; |
| |
| // |
| // This closure encapsulates the state and the closures needed when scanning |
| // the deduplication queue and table during the unlink_or_oops_do() operation. |
| // A single instance of this closure is created and then shared by all worker |
| // threads participating in the scan. The _next_queue and _next_bucket fields |
| // provide a simple mechanism for GC workers to claim exclusive access to a |
| // queue or a table partition. |
| // |
| class G1StringDedupUnlinkOrOopsDoClosure : public StackObj { |
| private: |
| BoolObjectClosure* _is_alive; |
| OopClosure* _keep_alive; |
| G1StringDedupTable* _resized_table; |
| G1StringDedupTable* _rehashed_table; |
| size_t _next_queue; |
| size_t _next_bucket; |
| |
| public: |
| G1StringDedupUnlinkOrOopsDoClosure(BoolObjectClosure* is_alive, |
| OopClosure* keep_alive, |
| bool allow_resize_and_rehash); |
| ~G1StringDedupUnlinkOrOopsDoClosure(); |
| |
| bool is_resizing() { |
| return _resized_table != NULL; |
| } |
| |
| G1StringDedupTable* resized_table() { |
| return _resized_table; |
| } |
| |
| bool is_rehashing() { |
| return _rehashed_table != NULL; |
| } |
| |
| // Atomically claims the next available queue for exclusive access by |
| // the current thread. Returns the queue number of the claimed queue. |
| size_t claim_queue(); |
| |
| // Atomically claims the next available table partition for exclusive |
| // access by the current thread. Returns the table bucket number where |
| // the claimed partition starts. |
| size_t claim_table_partition(size_t partition_size); |
| |
| // Applies and returns the result from the is_alive closure, or |
| // returns true if no such closure was provided. |
| bool is_alive(oop o) { |
| if (_is_alive != NULL) { |
| return _is_alive->do_object_b(o); |
| } |
| return true; |
| } |
| |
| // Applies the keep_alive closure, or does nothing if no such |
| // closure was provided. |
| void keep_alive(oop* p) { |
| if (_keep_alive != NULL) { |
| _keep_alive->do_oop(p); |
| } |
| } |
| }; |
| |
| #endif // SHARE_VM_GC_G1_G1STRINGDEDUP_HPP |