--- /dev/null Thu Jan 01 00:00:00 1970 +0000
+++ b/hotspot/src/share/vm/gc/g1/g1StringDedup.hpp Wed May 13 15:16:06 2015 +0200
@@ -0,0 +1,204 @@
+/*
+ * Copyright (c) 2014, 2015, 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
+ * or visit www.oracle.com if you need additional information or have any
+ * questions.
+ *
+ */
+
+#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"
+#include "runtime/atomic.hpp"
+
+class OopClosure;
+class BoolObjectClosure;
+class ThreadClosure;
+class outputStream;
+class G1StringDedupTable;
+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 unlink(BoolObjectClosure* is_alive);
+ 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