Saturday, August 27, 2011

linked hash map with LRU size control

I was asked to implement a generic LRU cache with size control. When size reaches a threshold, the LRU item must be kicked out. Optionally thread safe.

I came up with a concurrent hash map to provide fast get() and put(). Then I realized the kickout() operation would require a full scan of O(n) unless I provide an auxiliary data structure to physically
store the nodes in lastAccess order. I came up with a sorted map (concurrent skip list map) of {time stamp --> key}. My idea was to __adjust__ the physical positions within the aux at every get() and put(). My "adjust" implementation basically removes an item from the aux and re-insert.

Alternatively, the key object could be decorated in a stampedKey objects with a mutable timestamp field and an immutable key field.

That was my back-of-envelope design. Let's look at the standard solution -- a LinkedHashMap with "A special constructor to create a linked hash map whose order of iteration is the order in which its
entries were last accessed, from least-recently accessed to most-recently (access-order). This kind of map is well-suited to building LRU caches. Invoking the put or get method results in an access to the corresponding entry (assuming it exists after the invocation completes). The putAll method generates one entry access
for each mapping in the specified map, in the order that key-value mappings are provided by the specified map's entry set iterator. No other methods generate entry accesses. In particular, operations on collection-views do not affect the order of iteration of the backing map."

Apparently that too uses a regular hash table + an aux which is a linked list. (Linked list allows 2 nodes with identical time stamp. For my design to accommodate this, the stampedKey decorator might be stored
in the aux.)

Without looking at its source code, I believe the adjustment uses remove-reinsert on the aux.

Q: removing (not kickout()) from linked list is O(n). How do you avoid that? Remember any remove() on the cache must hit both the hash table and the aux.
%%A: the hash table entry holds a pointer to the node on the linked list

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