| | | 1 | | // Licensed to the .NET Foundation under one or more agreements. |
| | | 2 | | // The .NET Foundation licenses this file to you under the MIT license. |
| | | 3 | | |
| | | 4 | | using System; |
| | | 5 | | using System.Collections; |
| | | 6 | | using System.Threading; |
| | | 7 | | |
| | | 8 | | namespace CoreWCF.Runtime.Collections |
| | | 9 | | { |
| | | 10 | | // This cache works like a MruCache, but operates loosely and without locks in the mainline path. |
| | | 11 | | // |
| | | 12 | | // It consists of three 'hoppers', which are Hashtables (chosen for their nice threading characteristics - reading |
| | | 13 | | // doesn't require a lock). Items enter the cache in the second hopper. On lookups, cache hits result in the |
| | | 14 | | // cache entry being promoted to the first hopper. When the first hopper is full, the third hopper is dropped, |
| | | 15 | | // and the first and second hoppers are shifted down, leaving an empty first hopper. If the second hopper is |
| | | 16 | | // full when a new cache entry is added, the third hopper is dropped, the second hopper is shifted down, and a |
| | | 17 | | // new second hopper is slotted in to become the new item entrypoint. |
| | | 18 | | // |
| | | 19 | | // Items can only be added and looked up. There's no way to remove an item besides through attrition. |
| | | 20 | | // |
| | | 21 | | // This cache has a built-in concept of weakly-referenced items (which can be enabled or disabled in the |
| | | 22 | | // constructor). It needs this concept since the caller of the cache can't remove dead cache items itself. |
| | | 23 | | // A weak HopperCache will simply ignore dead entries. |
| | | 24 | | // |
| | | 25 | | // This structure allows cache lookups to be almost lock-free. The only time the first hopper is written to |
| | | 26 | | // is when a cache entry is promoted. Promoting a cache entry is not critical - it's ok to skip a promotion. |
| | | 27 | | // Only one promotion is allowed at a time. If a second is attempted, it is skipped. This allows promotions |
| | | 28 | | // to be synchronized with just an Interlocked call. |
| | | 29 | | // |
| | | 30 | | // New cache entries go into the second hopper, which requires a lock, as does shifting the hoppers down. |
| | | 31 | | // |
| | | 32 | | // The hopperSize parameter determines the size of the first hopper. When it reaches this size, the hoppers |
| | | 33 | | // are shifted. The second hopper is allowed to grow to twice this size. This is because it needs room to get |
| | | 34 | | // new cache entries into the system, and the second hopper typically starts out 'full'. Entries are never added |
| | | 35 | | // directly to the third hopper. |
| | | 36 | | // |
| | | 37 | | // It's a error on the part of the caller to add the same key to the cache again if it's already in the cache |
| | | 38 | | // with a different value. The new value will not necessarily overwrite the old value. |
| | | 39 | | // |
| | | 40 | | // If a cache entry is about to be promoted from the third hopper, and in the mean time the third hopper has been |
| | | 41 | | // shifted away, an intervening GetValue for the same key might return null, even though the item is still in |
| | | 42 | | // the cache and a later GetValue might find it. So it's very important never to add the same key to the cache |
| | | 43 | | // with two different values, even if GetValue returns null for the key in-between the first add and the second. |
| | | 44 | | // (If this particular behavior is a problem, it may be possible to tighten up, but it's not necessary for the |
| | | 45 | | // current use of HopperCache - UriPrefixTable.) |
| | | 46 | | internal class HopperCache |
| | | 47 | | { |
| | | 48 | | private readonly int _hopperSize; |
| | | 49 | | private readonly bool _weak; |
| | | 50 | | private Hashtable _outstandingHopper; |
| | | 51 | | private Hashtable _strongHopper; |
| | | 52 | | private Hashtable _limitedHopper; |
| | | 53 | | private int _promoting; |
| | | 54 | | private LastHolder _mruEntry; |
| | | 55 | | |
| | | 56 | | |
| | 0 | 57 | | public HopperCache(int hopperSize, bool weak) |
| | | 58 | | { |
| | | 59 | | Fx.Assert(hopperSize > 0, "HopperCache hopperSize must be positive."); |
| | | 60 | | |
| | 0 | 61 | | _hopperSize = hopperSize; |
| | 0 | 62 | | _weak = weak; |
| | | 63 | | |
| | 0 | 64 | | _outstandingHopper = new Hashtable(hopperSize * 2); |
| | 0 | 65 | | _strongHopper = new Hashtable(hopperSize * 2); |
| | 0 | 66 | | _limitedHopper = new Hashtable(hopperSize * 2); |
| | 0 | 67 | | } |
| | | 68 | | |
| | | 69 | | // Calls to Add must be synchronized. |
| | | 70 | | public void Add(object key, object value) |
| | | 71 | | { |
| | | 72 | | Fx.Assert(key != null, "HopperCache key cannot be null."); |
| | | 73 | | Fx.Assert(value != null, "HopperCache value cannot be null."); |
| | | 74 | | |
| | | 75 | | // Special-case DBNull since it can never be collected. |
| | 0 | 76 | | if (_weak && !ReferenceEquals(value, DBNull.Value)) |
| | | 77 | | { |
| | 0 | 78 | | value = new WeakReference(value); |
| | | 79 | | } |
| | | 80 | | |
| | | 81 | | Fx.Assert(_strongHopper.Count <= _hopperSize * 2, |
| | | 82 | | "HopperCache strongHopper is bigger than it's allowed to get."); |
| | | 83 | | |
| | 0 | 84 | | if (_strongHopper.Count >= _hopperSize * 2) |
| | | 85 | | { |
| | 0 | 86 | | Hashtable recycled = _limitedHopper; |
| | 0 | 87 | | recycled.Clear(); |
| | 0 | 88 | | recycled.Add(key, value); |
| | | 89 | | |
| | | 90 | | // The try/finally is here to make sure these happen without interruption. |
| | 0 | 91 | | try { } |
| | | 92 | | finally |
| | | 93 | | { |
| | 0 | 94 | | _limitedHopper = _strongHopper; |
| | 0 | 95 | | _strongHopper = recycled; |
| | 0 | 96 | | } |
| | | 97 | | } |
| | | 98 | | else |
| | | 99 | | { |
| | | 100 | | // We do nothing to prevent things from getting added multiple times. Also may be writing over |
| | | 101 | | // a dead weak entry. |
| | 0 | 102 | | _strongHopper[key] = value; |
| | | 103 | | } |
| | 0 | 104 | | } |
| | | 105 | | |
| | | 106 | | // Calls to GetValue do not need to be synchronized, but the object used to synchronize the Add calls |
| | | 107 | | // must be passed in. It's sometimes used. |
| | | 108 | | public object GetValue(object syncObject, object key) |
| | | 109 | | { |
| | | 110 | | Fx.Assert(key != null, "Can't look up a null key."); |
| | | 111 | | |
| | | 112 | | WeakReference weakRef; |
| | | 113 | | object value; |
| | | 114 | | |
| | | 115 | | // The MruCache does this so we have to too. |
| | 0 | 116 | | LastHolder last = _mruEntry; |
| | 0 | 117 | | if (last != null && key.Equals(last.Key)) |
| | | 118 | | { |
| | 0 | 119 | | if (_weak && (weakRef = last.Value as WeakReference) != null) |
| | | 120 | | { |
| | 0 | 121 | | value = weakRef.Target; |
| | 0 | 122 | | if (value != null) |
| | | 123 | | { |
| | 0 | 124 | | return value; |
| | | 125 | | } |
| | 0 | 126 | | _mruEntry = null; |
| | | 127 | | } |
| | | 128 | | else |
| | | 129 | | { |
| | 0 | 130 | | return last.Value; |
| | | 131 | | } |
| | | 132 | | } |
| | | 133 | | |
| | | 134 | | // Try the first hopper. |
| | 0 | 135 | | object origValue = _outstandingHopper[key]; |
| | 0 | 136 | | value = _weak && (weakRef = origValue as WeakReference) != null ? weakRef.Target : origValue; |
| | 0 | 137 | | if (value != null) |
| | | 138 | | { |
| | 0 | 139 | | _mruEntry = new LastHolder(key, origValue); |
| | 0 | 140 | | return value; |
| | | 141 | | } |
| | | 142 | | |
| | | 143 | | // Try the subsequent hoppers. |
| | 0 | 144 | | origValue = _strongHopper[key]; |
| | 0 | 145 | | value = _weak && (weakRef = origValue as WeakReference) != null ? weakRef.Target : origValue; |
| | 0 | 146 | | if (value == null) |
| | | 147 | | { |
| | 0 | 148 | | origValue = _limitedHopper[key]; |
| | 0 | 149 | | value = _weak && (weakRef = origValue as WeakReference) != null ? weakRef.Target : origValue; |
| | 0 | 150 | | if (value == null) |
| | | 151 | | { |
| | | 152 | | // Still no value? It's not here. |
| | 0 | 153 | | return null; |
| | | 154 | | } |
| | | 155 | | } |
| | | 156 | | |
| | 0 | 157 | | _mruEntry = new LastHolder(key, origValue); |
| | | 158 | | |
| | | 159 | | // If we can get the promoting semaphore, move up to the outstanding hopper. |
| | 0 | 160 | | int wasPromoting = 1; |
| | | 161 | | try |
| | | 162 | | { |
| | 0 | 163 | | try { } |
| | | 164 | | finally |
| | | 165 | | { |
| | | 166 | | // This is effectively a lock, which is why it uses lock semantics. If the Interlocked call |
| | | 167 | | // were 'lost', the cache wouldn't deadlock, but it would be permanently broken. |
| | 0 | 168 | | wasPromoting = Interlocked.CompareExchange(ref _promoting, 1, 0); |
| | 0 | 169 | | } |
| | | 170 | | |
| | | 171 | | // Only one thread can be inside this 'if' at a time. |
| | 0 | 172 | | if (wasPromoting == 0) |
| | | 173 | | { |
| | | 174 | | Fx.Assert(_outstandingHopper.Count <= _hopperSize, |
| | | 175 | | "HopperCache outstandingHopper is bigger than it's allowed to get."); |
| | | 176 | | |
| | 0 | 177 | | if (_outstandingHopper.Count >= _hopperSize) |
| | | 178 | | { |
| | 0 | 179 | | lock (syncObject) |
| | | 180 | | { |
| | 0 | 181 | | Hashtable recycled = _limitedHopper; |
| | 0 | 182 | | recycled.Clear(); |
| | 0 | 183 | | recycled.Add(key, origValue); |
| | | 184 | | |
| | | 185 | | // The try/finally is here to make sure these happen without interruption. |
| | 0 | 186 | | try { } |
| | | 187 | | finally |
| | | 188 | | { |
| | 0 | 189 | | _limitedHopper = _strongHopper; |
| | 0 | 190 | | _strongHopper = _outstandingHopper; |
| | 0 | 191 | | _outstandingHopper = recycled; |
| | 0 | 192 | | } |
| | | 193 | | } |
| | | 194 | | } |
| | | 195 | | else |
| | | 196 | | { |
| | | 197 | | // It's easy for this to happen twice with the same key. |
| | | 198 | | // |
| | | 199 | | // It's important that no one else can be shifting the current oustandingHopper |
| | | 200 | | // during this operation. We are only allowed to modify the *current* outstandingHopper |
| | | 201 | | // while holding the pseudo-lock, which would be violated if it could be shifted out from |
| | | 202 | | // under us (and potentially added to by Add in a race). |
| | 0 | 203 | | _outstandingHopper[key] = origValue; |
| | | 204 | | } |
| | | 205 | | } |
| | 0 | 206 | | } |
| | | 207 | | finally |
| | | 208 | | { |
| | 0 | 209 | | if (wasPromoting == 0) |
| | | 210 | | { |
| | 0 | 211 | | _promoting = 0; |
| | | 212 | | } |
| | 0 | 213 | | } |
| | | 214 | | |
| | 0 | 215 | | return value; |
| | | 216 | | } |
| | | 217 | | |
| | | 218 | | private class LastHolder |
| | | 219 | | { |
| | 0 | 220 | | internal LastHolder(object key, object value) |
| | | 221 | | { |
| | 0 | 222 | | Key = key; |
| | 0 | 223 | | Value = value; |
| | 0 | 224 | | } |
| | | 225 | | |
| | 0 | 226 | | internal object Key { get; private set; } |
| | | 227 | | |
| | 0 | 228 | | internal object Value { get; private set; } |
| | | 229 | | } |
| | | 230 | | } |
| | | 231 | | } |