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