This is an improvement over ARC by combining ARC with Clock (second-chance) cache replacement algorithm. Like ARC, there are two LRU queues and each queue is separated from cache ($$T$$) and directory ($$T+B$$). But instead of a simple queue, the LRU are now implemented in a circular queue with a “hand” pointing to one of the items. And each item has a R-bit to flag that the item has been accessed recently.

The algorithm is as follows:

• If there is a cache hit ($$T_1+T_2$$), assert the R-bit of the item
• If there is a cache miss (not in $$T_1+T_2$$), make room to load the item into the cache
• If $$|T_1+T_2|=c$$, then the cache is full. We loop until a page is moved from $$T_1$$ to $$B_1$$ or from $$T_2$$ to $$B_2$$, as described in the following
• In every loop, if $$|T_1|>p$$, and the item pointed by the “hand” in $$T_1$$ has R-bit asserted, it is moved to the tail (i.e. the one behind the “hand”) position in $$T_2$$ with the R-bit cleared
• Alternatively, if $$|T_1|>p$$, but the item pointed by the “hand” in $$T_1$$ has R-bit cleared, it is moved to the MRU position of B1 and the loop terminates
• Alternatively, if $$|T_1|\le p$$, and the item pointed by the “hand” in $$T_2$$ has R-bit asserted, its R-bit will be cleared and the “hand” moves forward by 1
• Alternatively, if $$|T_1|\le p$$, but the item pointed by the “hand” in $$T_2$$ has R-bit cleared, it is moved to the MRU position of B2 and the loop terminates
• If $$|L_1|=c$$, discard the an object from $$B_1$$ or if $$|L_1+L_2|=2c$$, discard an object from $$B_2$$ according to the second-chance priority
• Now the cache shall have room for the new entry
• If the item is a directory miss, load the object into $$T_1$$ (from $$B_1+B_2$$) with R-bit cleared.
• If the item is a directory hit but a cache miss, adjust the parameter $$p$$ according to ARC and move the object to the “tail” position of $$T_2$$ with its R-bit cleared

Comparing LRU, ARC and CAR:

• LRU captures recency but not frequency. ARC improves over this
• A scan, i.e. a sequence of one-time-use object reference can pollute the LRU. ARC solves the problem by a dual LRU.
• LRU requires items be moved on every access, which is expensive. The Clock algorithm gives a lightweight solution.

And therefore, CAR combines ARC and Clock and it solves all the above problems.

## Bibliographic data

@inproceedings{
title = "CAR: Clock with Adaptive Replacement",
author = "Sorav Bansal and Dharmendra S. Modha",
booktitle = "Proc. 3rd USENIX Conference on File and Storage Technologies (FAST)",
pages = "187--200",
year = "2004",
}