Directory Based Protocol for Cache Coherency Issues & Challenges

Abstract

Today's systems are designed with Multi Core Architecture. The idea behind this is to achieve high system throughput. Once the Processor clock speed reached its saturation, designers opted for having multiple cores. Each Core or Processor equipped with their own private cache memory. But under Chip Multiprocessor, where all the processor have access to shared memory, having respective cache memory will result with Cache Coherency Problem. Cache Coherency Problem is nothing but maintaining data consistency in spite of allowing multiple processor to have a access to common memory. This problem is addressed by software or hardware. The later method is usually preferred than the former, so as to get rid of programming issues. In hardware method 2 options are usually adopted like Snoopy Protocol or Directory Protocol. In Directory Protocol, for each block of data there is a directory entry that contains a number of pointers. The purpose of this number is to mention the locations of block copies. Each directory entry also contains a dirty bit to specify whether a unique cache has a permission or not to write the associated block of data. There are three primary categories of directory-based protocol: Centralized Directories, Flat Directories, and Hierarchical Directories. The important advantage of directory based protocols is that they scale much better than snoopy protocols. In addition to this it has the advantage of ability to exploit arbitrary point-to-point interconnects. But mean time it also has the overhead in terms of the storage and manipulation of directory state. This paper explains different Directory Based implementation and its advantages over Snoopy Protocol.