LU cache

The least-used cache (sometimes called "Least Frequently Used") is a variant of LRU that evicts items that are used least frequently once its threshold has been exceeded.

In simple terms, the LU cache will remove the element in the cache that has been accessed the least, regardless of time.

Before reading this page please make sure that you've read and understand the Basic Usage patterns.

General usage

To create a core.cache LUCache instance you should always use its associated constructor function lu-cache-factory with an optional :threshold parameter:

    (ns your.lib 
      (:require [clojure.core.cache :as cache]))
	
    (def C (cache/lu-cache-factory {} :threshold 2))
    
    (-> C (assoc :a 1) (assoc :b 2))
	;=> {:a 1, :b 2}

note: the default :threshold value is 32

At this point the cache has not yet crossed the item threshold of 2, but if you execute yet another call the story will change:

    (-> C (assoc :a 1) (assoc :b 2) (assoc :c 3))
	;=> {:b 2, :c 3}

At this point the operation of the LU cache looks exactly the same at the FIFO cache. However, the difference becomes apparent when a given cache item is "touched":

    (-> C (assoc :a 1) 
          (assoc :b 2)
          (.hit :b)
          (.hit :b)
          (.hit :a)       ;; ensure :a is used most recently
          (assoc :c 3))
    
    ;=> {:c 3, :b 2}

As you see, hitting the key :b twice marks it as more important that :a even though the latter was "touched" more recently. That is, when the threshold is passed, the cache will expel the Least Used element in favor of the element accessed most often.

All caveats apply regarding the proper usage patterns.

LU cache use cases

The LU cache eviction policy is very simple to understand and works well in general.

Advantages to using an LU cache

There are a few reasons why you might want to use an LU cache:

• Performs pretty well if access patterns rarely change
• Its logic is easy to understand
• It's reasonably fast
• It works well with data subject to temporal locality concerns

Disadvantages to using an LU cache

• It performs poorly when access patterns change
• It requires some historical data to operate
• It requires a larger cache to increase efficiency

As always, you should measure your system's characteristics to determine the best eviction strategy for your purposes.