Fix LRU cache index stalling
The old LRU implementation stored a list's indices to reference items in that list, but deletions from the list made indices point to the wrong slot. Functionally, it meant that deleted slots were not guaranteed to be the oldest slot. Using a linked list fixes that.
This commit is contained in:
Thaddee Tyl
parent
11b6e06f2f
commit
5dd58142cb
119
lib/lru-cache.js
119
lib/lru-cache.js
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@ -1,75 +1,102 @@
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// Cache any data with a timestamp,
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// remove only the oldest data.
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// In-memory KV, remove the oldest data when the capacity is reached.
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var typeEnum = {
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unit: 0,
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heap: 1,
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};
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function Cache(size, type) {
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if (!this instanceof Cache) { return new Cache(size, type); }
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function CacheSlot(key, value) {
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this.key = key;
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this.value = value;
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this.older = null; // Newest slot that is older than this slot.
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this.newer = null; // Oldest slot that is newer than this slot.
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}
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function Cache(capacity, type) {
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if (!(this instanceof Cache)) { return new Cache(capacity, type); }
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type = type || 'unit';
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this.size = size;
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this.capacity = capacity;
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this.type = typeEnum[type];
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if (this.type === typeEnum.unit) { this.size -= 1; }
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// `cache` contains {content, index}.
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// - content: the actual data that is cached.
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// - index: the position in `order` of the data.
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this.cache = Object.create(null);
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this.order = []; // list of cache keys from oldest to newest.
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this.cache = new Map(); // Maps cache keys to CacheSlots.
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this.newest = null; // Newest slot in the cache.
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this.oldest = null;
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}
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Cache.prototype = {
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set: function addToCache(cacheIndex, cached) {
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if (this.cache[cacheIndex] !== undefined) {
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this.order.splice(this.cache[cacheIndex].index, 1);
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// Put the new element at the end of `order`.
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this.cache[cacheIndex].index = this.order.length;
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this.cache[cacheIndex].content = cached;
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this.order.push(cacheIndex);
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} else {
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// If the cache is full, remove the oldest data
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// (ie, the data requested longest ago.)
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var numberToRemove = this.limitReached();
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if (numberToRemove > this.order.length) { numberToRemove = this.order.length; }
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for (var i = 0; i < numberToRemove; i++) {
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// Remove `order`'s oldest element, the first.
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delete this.cache[this.order[0]];
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this.order.shift();
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set: function addToCache(cacheKey, cached) {
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var slot = this.cache.get(cacheKey);
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if (slot === undefined) {
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slot = new CacheSlot(cacheKey, cached);
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this.cache.set(cacheKey, slot);
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}
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this.makeNewest(slot);
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var numItemsToRemove = this.limitReached();
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if (numItemsToRemove > 0) {
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for (var i = 0; i < numItemsToRemove; i++) {
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this.removeOldest();
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}
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this.cache[cacheIndex] = {
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index: this.order.length,
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content: cached,
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};
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this.order.push(cacheIndex);
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}
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},
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get: function getFromCache(cacheIndex) {
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if (this.cache[cacheIndex] !== undefined) {
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this.order.splice(this.cache[cacheIndex].index, 1);
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// Put the new element at the end of `order`.
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this.cache[cacheIndex].index = this.order.length;
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this.order.push(cacheIndex);
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return this.cache[cacheIndex].content;
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} else { return; }
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get: function getFromCache(cacheKey) {
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var slot = this.cache.get(cacheKey);
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if (slot !== undefined) {
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this.makeNewest(slot);
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return slot.value;
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}
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},
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has: function hasInCache(cacheIndex) {
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return this.cache[cacheIndex] !== undefined;
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has: function hasInCache(cacheKey) {
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return this.cache.has(cacheKey);
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},
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makeNewest: function makeNewestSlot(slot) {
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var previousNewest = this.newest;
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if (previousNewest === slot) { return; }
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var older = slot.older;
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var newer = slot.newer;
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if (older !== null) {
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older.newer = newer;
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} else if (newer !== null) {
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this.oldest = newer;
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}
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if (newer !== null) {
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newer.older = older;
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}
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this.newest = slot;
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if (previousNewest !== null) {
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slot.older = previousNewest;
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slot.newer = null;
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previousNewest.newer = slot;
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} else {
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// If previousNewest is null, the cache used to be empty.
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this.oldest = slot;
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}
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},
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removeOldest: function removeOldest() {
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var cacheKey = this.oldest.key;
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if (this.oldest !== null) {
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this.oldest = this.oldest.newer;
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if (this.oldest !== null) {
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this.oldest.older = null;
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}
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}
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this.cache.delete(cacheKey);
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},
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// Returns the number of elements to remove if we're past the limit.
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limitReached: function heuristic() {
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if (this.type === typeEnum.unit) {
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// Remove the excess.
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return Math.max(0, (this.order.length - this.size));
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return Math.max(0, (this.cache.size - this.capacity));
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} else if (this.type === typeEnum.heap) {
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if (getHeapSize() >= this.size) {
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if (getHeapSize() >= this.capacity) {
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console.log('LRU HEURISTIC heap:', getHeapSize());
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// Remove half of them.
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return (this.order.length >> 1);
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return (this.cache.size >> 1);
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} else { return 0; }
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} else {
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console.error("Unknown heuristic '" + this.type + "' for LRU cache.");
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137
test/lru-cache.js
Normal file
137
test/lru-cache.js
Normal file
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@ -0,0 +1,137 @@
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var LRU = require('../lib/lru-cache.js');
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module.exports = [
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["should support being called without new", function(done, assert) {
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var cache = LRU(1);
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assert(cache instanceof LRU);
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done();
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}],
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["should support a zero capacity", function(done, assert) {
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var cache = new LRU(0);
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cache.set('key', 'value');
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assert.equal(cache.cache.size, 0);
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done();
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}],
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["should support a one capacity", function(done, assert) {
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var cache = new LRU(1);
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cache.set('key1', 'value1');
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assert.equal(cache.cache.size, 1);
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assert.equal(cache.newest, cache.cache.get('key1'));
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assert.equal(cache.oldest, cache.cache.get('key1'));
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cache.set('key2', 'value2');
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assert.equal(cache.cache.size, 1);
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assert.equal(cache.newest, cache.cache.get('key2'));
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assert.equal(cache.oldest, cache.cache.get('key2'));
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assert.equal(cache.get('key1'), undefined);
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assert.equal(cache.get('key2'), 'value2');
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done();
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}],
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["should remove the oldest element when reaching capacity",
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function(done, assert) {
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var cache = new LRU(2);
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cache.set('key1', 'value1');
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cache.set('key2', 'value2');
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cache.set('key3', 'value3');
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var slot1 = cache.cache.get('key1');
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var slot2 = cache.cache.get('key2');
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var slot3 = cache.cache.get('key3');
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assert.equal(cache.cache.size, 2);
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assert.equal(cache.oldest, slot2);
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assert.equal(cache.newest, slot3);
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assert.equal(slot2.older, null);
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assert.equal(slot2.newer, slot3);
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assert.equal(slot3.older, slot2);
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assert.equal(slot3.newer, null);
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assert.equal(cache.cache.get('key1'), undefined);
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assert.equal(cache.get('key1'), undefined);
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assert.equal(cache.get('key2'), 'value2');
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assert.equal(cache.get('key3'), 'value3');
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done();
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}],
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["should make sure that resetting a key in cache makes it newest",
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function(done, assert) {
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var cache = new LRU(2);
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cache.set('key', 'value');
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cache.set('key2', 'value2');
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assert.equal(cache.oldest, cache.cache.get('key'));
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assert.equal(cache.newest, cache.cache.get('key2'));
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cache.set('key', 'value');
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assert.equal(cache.oldest, cache.cache.get('key2'));
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assert.equal(cache.newest, cache.cache.get('key'));
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done();
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}],
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["should make sure that getting a key in cache makes it newest",
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function(done, assert) {
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// When the key is oldest.
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var cache = new LRU(2);
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cache.set('key1', 'value1');
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cache.set('key2', 'value2');
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var slot1 = cache.cache.get('key1');
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var slot2 = cache.cache.get('key2');
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assert.equal(cache.oldest, slot1);
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assert.equal(cache.newest, slot2);
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assert.equal(slot1.older, null);
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assert.equal(slot1.newer, slot2);
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assert.equal(slot2.older, slot1);
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assert.equal(slot2.newer, null);
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assert.equal(cache.get('key1'), 'value1');
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var slot1 = cache.cache.get('key1');
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var slot2 = cache.cache.get('key2');
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assert.equal(cache.oldest, slot2);
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assert.equal(cache.newest, slot1);
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assert.equal(slot2.older, null);
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assert.equal(slot2.newer, slot1);
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assert.equal(slot1.older, slot2);
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assert.equal(slot1.newer, null);
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// When the key is newest.
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cache = new LRU(2);
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cache.set('key1', 'value1');
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cache.set('key2', 'value2');
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assert.equal(cache.get('key2'), 'value2');
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var slot1 = cache.cache.get('key1');
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var slot2 = cache.cache.get('key2');
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assert.equal(cache.oldest, slot1);
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assert.equal(cache.newest, slot2);
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assert.equal(slot1.older, null);
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assert.equal(slot1.newer, slot2);
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assert.equal(slot2.older, slot1);
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assert.equal(slot2.newer, null);
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// When the key is in the middle.
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cache = new LRU(3);
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cache.set('key1', 'value1');
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cache.set('key2', 'value2');
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cache.set('key3', 'value3');
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var slot1 = cache.cache.get('key1');
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var slot2 = cache.cache.get('key2');
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var slot3 = cache.cache.get('key3');
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assert.equal(cache.oldest, slot1);
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assert.equal(cache.newest, slot3);
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assert.equal(slot1.older, null);
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assert.equal(slot1.newer, slot2);
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assert.equal(slot2.older, slot1);
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assert.equal(slot2.newer, slot3);
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assert.equal(slot3.older, slot2);
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assert.equal(slot3.newer, null);
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assert.equal(cache.get('key2'), 'value2');
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var slot1 = cache.cache.get('key1');
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var slot2 = cache.cache.get('key2');
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var slot3 = cache.cache.get('key3');
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assert.equal(cache.oldest, slot1);
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assert.equal(cache.newest, slot2);
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assert.equal(slot1.older, null);
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assert.equal(slot1.newer, slot3);
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assert.equal(slot3.older, slot1);
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assert.equal(slot3.newer, slot2);
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assert.equal(slot2.older, slot3);
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assert.equal(slot2.newer, null);
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done();
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}],
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];
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@ -142,4 +142,6 @@ test('The server', [
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server.kill();
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server.on('exit', function() { done(); });
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}],
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]);});
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]);})
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.then(function() { test('The LRU cache', require('./lru-cache.js')); });
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