1 /*
   2  * Copyright (c) 2016, 2019, 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.  Oracle designates this
   8  * particular file as subject to the "Classpath" exception as provided
   9  * by Oracle in the LICENSE file that accompanied this code.
  10  *
  11  * This code is distributed in the hope that it will be useful, but WITHOUT
  12  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
  13  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
  14  * version 2 for more details (a copy is included in the LICENSE file that
  15  * accompanied this code).
  16  *
  17  * You should have received a copy of the GNU General Public License version
  18  * 2 along with this work; if not, write to the Free Software Foundation,
  19  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
  20  *
  21  * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
  22  * or visit www.oracle.com if you need additional information or have any
  23  * questions.
  24  */
  25 package jdk.jfr.internal;
  26 
  27 import java.util.concurrent.ConcurrentHashMap;
  28 import java.util.concurrent.atomic.AtomicLong;
  29 
  30 import sun.misc.Unsafe;
  31 
  32 public final class StringPool {
  33 
  34     private static final Unsafe unsafe = Unsafe.getUnsafe();
  35 
  36     static final int MIN_LIMIT = 16;
  37     static final int MAX_LIMIT = 128; /* 0 MAX means disabled */
  38     private static final long epochAddress;
  39     private static final SimpleStringIdPool sp = new SimpleStringIdPool();
  40     static {
  41         epochAddress = JVM.getJVM().getEpochAddress();
  42         sp.reset();
  43     }
  44     public static long addString(String s) {
  45         return sp.addString(s);
  46     }
  47     private static boolean getCurrentEpoch() {
  48         return unsafe.getByte(epochAddress) == 1;
  49     }
  50     private static class SimpleStringIdPool {
  51         /* string id index */
  52         private final AtomicLong sidIdx = new AtomicLong();
  53         /* epoch of cached strings */
  54         private boolean poolEpoch;
  55         /* the cache */
  56         private final ConcurrentHashMap<String, Long> cache;
  57         /* max size */
  58         private final int MAX_SIZE = 32*1024;
  59         /* max size bytes*/
  60         private final long MAX_SIZE_UTF16 = 16*1024*1024;
  61         /* max size bytes*/
  62         private long currentSizeUTF16;
  63 
  64         /* looking at a biased data set 4 is a good value */
  65         private final String[] preCache = new String[]{"", "" , "" ,""};
  66         /* index of oldest */
  67         private int preCacheOld = 0;
  68         /* loop mask */
  69         private static final int preCacheMask = 0x03;
  70 
  71         SimpleStringIdPool() {
  72             cache = new ConcurrentHashMap<>(MAX_SIZE, 0.75f);
  73         }
  74         void reset() {
  75             reset(getCurrentEpoch());
  76         }
  77         private void reset(boolean epoch) {
  78             this.cache.clear();
  79             this.poolEpoch = epoch;
  80             this.currentSizeUTF16 = 0;
  81         }
  82         private long addString(String s) {
  83             boolean currentEpoch = getCurrentEpoch();
  84             if (poolEpoch == currentEpoch) {
  85                 /* pool is for current chunk */
  86                 Long lsid = this.cache.get(s);
  87                 if (lsid != null) {
  88                     return lsid.longValue();
  89                 }
  90             } else {
  91                 /* pool is for an old chunk */
  92                 reset(currentEpoch);
  93             }
  94             if (!preCache(s)) {
  95                 /* we should not pool this string */
  96                 return -1;
  97             }
  98             if (cache.size() > MAX_SIZE || currentSizeUTF16 > MAX_SIZE_UTF16) {
  99                 /* pool was full */
 100                 reset(currentEpoch);
 101             }
 102             return storeString(s);
 103         }
 104 
 105         private long storeString(String s) {
 106             long sid = this.sidIdx.getAndIncrement();
 107             /* we can race but it is ok */
 108             this.cache.put(s, sid);
 109             boolean currentEpoch;
 110             synchronized(SimpleStringIdPool.class) {
 111                 currentEpoch = JVM.addStringConstant(poolEpoch, sid, s);
 112                 currentSizeUTF16 += s.length();
 113             }
 114             /* did we write in chunk that this pool represent */
 115             return currentEpoch == poolEpoch ? sid : -1;
 116         }
 117         private boolean preCache(String s) {
 118             if (preCache[0].equals(s)) {
 119                 return true;
 120             }
 121             if (preCache[1].equals(s)) {
 122                 return true;
 123             }
 124             if (preCache[2].equals(s)) {
 125                 return true;
 126             }
 127             if (preCache[3].equals(s)) {
 128                 return true;
 129             }
 130             preCacheOld = (preCacheOld - 1) & preCacheMask;
 131             preCache[preCacheOld] = s;
 132             return false;
 133         }
 134     }
 135 }