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/ 4.0 / 4.0 / untmp / DEVDIV_TFS / Dev10 / Releases / RTMRel / ndp / cdf / src / System.Runtime.DurableInstancing / System / Runtime / Collections / HopperCache.cs / 1305376 / HopperCache.cs
//------------------------------------------------------------ // Copyright (c) Microsoft Corporation. All rights reserved. //----------------------------------------------------------- namespace System.Runtime.Collections { using System; using System.Collections; using System.Threading; using System.Diagnostics; // This cache works like a MruCache, but operates loosely and without locks in the mainline path. // // It consists of three 'hoppers', which are Hashtables (chosen for their nice threading characteristics - reading // doesn't require a lock). Items enter the cache in the second hopper. On lookups, cache hits result in the // cache entry being promoted to the first hopper. When the first hopper is full, the third hopper is dropped, // and the first and second hoppers are shifted down, leaving an empty first hopper. If the second hopper is // full when a new cache entry is added, the third hopper is dropped, the second hopper is shifted down, and a // new second hopper is slotted in to become the new item entrypoint. // // Items can only be added and looked up. There's no way to remove an item besides through attrition. // // This cache has a built-in concept of weakly-referenced items (which can be enabled or disabled in the // constructor). It needs this concept since the caller of the cache can't remove dead cache items itself. // A weak HopperCache will simply ignore dead entries. // // This structure allows cache lookups to be almost lock-free. The only time the first hopper is written to // is when a cache entry is promoted. Promoting a cache entry is not critical - it's ok to skip a promotion. // Only one promotion is allowed at a time. If a second is attempted, it is skipped. This allows promotions // to be synchronized with just an Interlocked call. // // New cache entries go into the second hopper, which requires a lock, as does shifting the hoppers down. // // The hopperSize parameter determines the size of the first hopper. When it reaches this size, the hoppers // are shifted. The second hopper is allowed to grow to twice this size. This is because it needs room to get // new cache entries into the system, and the second hopper typically starts out 'full'. Entries are never added // directly to the third hopper. // // 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 // with a different value. The new value will not necessarily overwrite the old value. // // If a cache entry is about to be promoted from the third hopper, and in the mean time the third hopper has been // shifted away, an intervening GetValue for the same key might return null, even though the item is still in // the cache and a later GetValue might find it. So it's very important never to add the same key to the cache // with two different values, even if GetValue returns null for the key in-between the first add and the second. // (If this particular behavior is a problem, it may be possible to tighten up, but it's not necessary for the // current use of HopperCache - UriPrefixTable.) class HopperCache { readonly int hopperSize; readonly bool weak; Hashtable outstandingHopper; Hashtable strongHopper; Hashtable limitedHopper; int promoting; LastHolder mruEntry; public HopperCache(int hopperSize, bool weak) { Fx.Assert(hopperSize > 0, "HopperCache hopperSize must be positive."); this.hopperSize = hopperSize; this.weak = weak; this.outstandingHopper = new Hashtable(hopperSize * 2); this.strongHopper = new Hashtable(hopperSize * 2); this.limitedHopper = new Hashtable(hopperSize * 2); } // Calls to Add must be synchronized. public void Add(object key, object value) { Fx.Assert(key != null, "HopperCache key cannot be null."); Fx.Assert(value != null, "HopperCache value cannot be null."); // Special-case DBNull since it can never be collected. if (this.weak && !object.ReferenceEquals(value, DBNull.Value)) { value = new WeakReference(value); } Fx.Assert(this.strongHopper.Count <= this.hopperSize * 2, "HopperCache strongHopper is bigger than it's allowed to get."); if (this.strongHopper.Count >= this.hopperSize * 2) { Hashtable recycled = this.limitedHopper; recycled.Clear(); recycled.Add(key, value); // The try/finally is here to make sure these happen without interruption. try { } finally { this.limitedHopper = this.strongHopper; this.strongHopper = recycled; } } else { // We do nothing to prevent things from getting added multiple times. Also may be writing over // a dead weak entry. this.strongHopper[key] = value; } } // Calls to GetValue do not need to be synchronized, but the object used to synchronize the Add calls // must be passed in. It's sometimes used. public object GetValue(object syncObject, object key) { Fx.Assert(key != null, "Can't look up a null key."); WeakReference weakRef; object value; // The MruCache does this so we have to too. LastHolder last = this.mruEntry; if (last != null && key.Equals(last.Key)) { if (this.weak && (weakRef = last.Value as WeakReference) != null) { value = weakRef.Target; if (value != null) { return value; } this.mruEntry = null; } else { return last.Value; } } // Try the first hopper. object origValue = this.outstandingHopper[key]; value = this.weak && (weakRef = origValue as WeakReference) != null ? weakRef.Target : origValue; if (value != null) { this.mruEntry = new LastHolder(key, origValue); return value; } // Try the subsequent hoppers. origValue = this.strongHopper[key]; value = this.weak && (weakRef = origValue as WeakReference) != null ? weakRef.Target : origValue; if (value == null) { origValue = this.limitedHopper[key]; value = this.weak && (weakRef = origValue as WeakReference) != null ? weakRef.Target : origValue; if (value == null) { // Still no value? It's not here. return null; } } this.mruEntry = new LastHolder(key, origValue); // If we can get the promoting semaphore, move up to the outstanding hopper. int wasPromoting = 1; try { try { } finally { // This is effectively a lock, which is why it uses lock semantics. If the Interlocked call // were 'lost', the cache wouldn't deadlock, but it would be permanently broken. wasPromoting = Interlocked.CompareExchange(ref this.promoting, 1, 0); } // Only one thread can be inside this 'if' at a time. if (wasPromoting == 0) { Fx.Assert(this.outstandingHopper.Count <= this.hopperSize, "HopperCache outstandingHopper is bigger than it's allowed to get."); if (this.outstandingHopper.Count >= this.hopperSize) { lock (syncObject) { Hashtable recycled = this.limitedHopper; recycled.Clear(); recycled.Add(key, origValue); // The try/finally is here to make sure these happen without interruption. try { } finally { this.limitedHopper = this.strongHopper; this.strongHopper = this.outstandingHopper; this.outstandingHopper = recycled; } } } else { // It's easy for this to happen twice with the same key. // // It's important that no one else can be shifting the current oustandingHopper // during this operation. We are only allowed to modify the *current* outstandingHopper // while holding the pseudo-lock, which would be violated if it could be shifted out from // under us (and potentially added to by Add in a ----). this.outstandingHopper[key] = origValue; } } } finally { if (wasPromoting == 0) { this.promoting = 0; } } return value; } class LastHolder { readonly object key; readonly object value; internal LastHolder(object key, object value) { this.key = key; this.value = value; } internal object Key { get { return this.key; } } internal object Value { get { return this.value; } } } } } // File provided for Reference Use Only by Microsoft Corporation (c) 2007.
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