8029075: String deduplication in G1

Implementation of JEP 192, http://openjdk.java.net/jeps/192

Reviewed-by: brutisso, tschatzl, coleenp

hotspot/src/share/vm/gc_implementation/g1/g1StringDedupQueue.cpp

@@ -0,0 +1,162 @@
+/*
+ * Copyright (c) 2014, 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.
+ *
+ */
+
+#include "precompiled.hpp"
+#include "classfile/javaClasses.hpp"
+#include "gc_implementation/g1/g1StringDedupQueue.hpp"
+#include "memory/gcLocker.hpp"
+#include "runtime/mutexLocker.hpp"
+#include "utilities/stack.inline.hpp"
+
+G1StringDedupQueue* G1StringDedupQueue::_queue = NULL;
+const size_t        G1StringDedupQueue::_max_size = 1000000; // Max number of elements per queue
+const size_t        G1StringDedupQueue::_max_cache_size = 0; // Max cache size per queue
+
+G1StringDedupQueue::G1StringDedupQueue() :
+  _cursor(0),
+  _empty(true),
+  _dropped(0) {
+  _nqueues = MAX2(ParallelGCThreads, (size_t)1);
+  _queues = NEW_C_HEAP_ARRAY(G1StringDedupWorkerQueue, _nqueues, mtGC);
+  for (size_t i = 0; i < _nqueues; i++) {
+    new (_queues + i) G1StringDedupWorkerQueue(G1StringDedupWorkerQueue::default_segment_size(), _max_cache_size, _max_size);
+  }
+}
+
+G1StringDedupQueue::~G1StringDedupQueue() {
+  ShouldNotReachHere();
+}
+
+void G1StringDedupQueue::create() {
+  assert(_queue == NULL, "One string deduplication queue allowed");
+  _queue = new G1StringDedupQueue();
+}
+
+void G1StringDedupQueue::wait() {
+  MonitorLockerEx ml(StringDedupQueue_lock, Mutex::_no_safepoint_check_flag);
+  while (_queue->_empty) {
+    ml.wait(Mutex::_no_safepoint_check_flag);
+  }
+}
+
+void G1StringDedupQueue::push(uint worker_id, oop java_string) {
+  assert(SafepointSynchronize::is_at_safepoint(), "Must be at safepoint");
+  assert(worker_id < _queue->_nqueues, "Invalid queue");
+
+  // Push and notify waiter
+  G1StringDedupWorkerQueue& worker_queue = _queue->_queues[worker_id];
+  if (!worker_queue.is_full()) {
+    worker_queue.push(java_string);
+    if (_queue->_empty) {
+      MonitorLockerEx ml(StringDedupQueue_lock, Mutex::_no_safepoint_check_flag);
+      if (_queue->_empty) {
+        // Mark non-empty and notify waiter
+        _queue->_empty = false;
+        ml.notify();
+      }
+    }
+  } else {
+    // Queue is full, drop the string and update the statistics
+    Atomic::inc_ptr(&_queue->_dropped);
+  }
+}
+
+oop G1StringDedupQueue::pop() {
+  assert(!SafepointSynchronize::is_at_safepoint(), "Must not be at safepoint");
+  No_Safepoint_Verifier nsv;
+
+  // Try all queues before giving up
+  for (size_t tries = 0; tries < _queue->_nqueues; tries++) {
+    // The cursor indicates where we left of last time
+    G1StringDedupWorkerQueue* queue = &_queue->_queues[_queue->_cursor];
+    while (!queue->is_empty()) {
+      oop obj = queue->pop();
+      // The oop we pop can be NULL if it was marked
+      // dead. Just ignore those and pop the next oop.
+      if (obj != NULL) {
+        return obj;
+      }
+    }
+
+    // Try next queue
+    _queue->_cursor = (_queue->_cursor + 1) % _queue->_nqueues;
+  }
+
+  // Mark empty
+  _queue->_empty = true;
+
+  return NULL;
+}
+
+void G1StringDedupQueue::unlink_or_oops_do(G1StringDedupUnlinkOrOopsDoClosure* cl) {
+  // A worker thread first claims a queue, which ensures exclusive
+  // access to that queue, then continues to process it.
+  for (;;) {
+    // Grab next queue to scan
+    size_t queue = cl->claim_queue();
+    if (queue >= _queue->_nqueues) {
+      // End of queues
+      break;
+    }
+
+    // Scan the queue
+    unlink_or_oops_do(cl, queue);
+  }
+}
+
+void G1StringDedupQueue::unlink_or_oops_do(G1StringDedupUnlinkOrOopsDoClosure* cl, size_t queue) {
+  assert(queue < _queue->_nqueues, "Invalid queue");
+  StackIterator<oop, mtGC> iter(_queue->_queues[queue]);
+  while (!iter.is_empty()) {
+    oop* p = iter.next_addr();
+    if (*p != NULL) {
+      if (cl->is_alive(*p)) {
+        cl->keep_alive(p);
+      } else {
+        // Clear dead reference
+        *p = NULL;
+      }
+    }
+  }
+}
+
+void G1StringDedupQueue::print_statistics(outputStream* st) {
+  st->print_cr(
+    "   [Queue]\n"
+    "      [Dropped: "UINTX_FORMAT"]", _queue->_dropped);
+}
+
+void G1StringDedupQueue::verify() {
+  for (size_t i = 0; i < _queue->_nqueues; i++) {
+    StackIterator<oop, mtGC> iter(_queue->_queues[i]);
+    while (!iter.is_empty()) {
+      oop obj = iter.next();
+      if (obj != NULL) {
+        guarantee(Universe::heap()->is_in_reserved(obj), "Object must be on the heap");
+        guarantee(!obj->is_forwarded(), "Object must not be forwarded");
+        guarantee(java_lang_String::is_instance(obj), "Object must be a String");
+      }
+    }
+  }
+}