function gc() {
	var node;
	var N;
	var M;
	var v;

	node = cache.first();
	N = cache.length;
	while ( node ) {
		v = node.value;
		if ( !v.persist ) {
			cache.remove( node );
		}
		node = node.next;
	}
	M = cache.length;
	return {
		'size': M,
		'removed': N - M
	};
}
Performs garbage collection on the index cache.

@private
@returns {Object} garbage collection results