CSC/ECE 506 Spring 2013/8c da

Snoop Filters

Introduction

One of the issues with large systems with multiple processors having shared memory and each processor having its own private cache is the cache coherence problem. The non-coherent view of values of a single data item in these different caches is referred to as the cache coherence problem. A protocol which ensures a coherent view of cached values as seen by multiple processors is referred to as cache coherence protocol.

Snooping is the process where the individual caches monitor address lines for accesses to memory locations that they have cached. When a write operation is observed to a location that a cache has a copy of, the cache controller invalidates its own copy of the snooped memory location. Many cache coherence protocols require a hardware component to monitor the bus transactions.

Hardware support for implementing cache coherence protocols in a bus-based multiprocessor system at each node can be provided using coherence controller. The coherence controller has a component called the snooper. The role of the snooper is to snoop each bus transaction involved in the cache coherence transaction. For each snooped bus transaction, the coherence controller checks the cache tag array to see if it has the block that is involved in the transaction, checks the current state of the block (if the block is found), and reacts accordingly by responding with data or by changing the state of the block.

In most well-optimized programs, much data is not shared among threads. So most of the time, most snooped bus transactions do not find the block in the local cache. Even in that case, snooper snooped the bus transaction and and checked the cache tag to determine whether the cache has the block, thus incurred unnecessary work. There is the possibility that contention can occur between processor and the snooper to access the cache tag. One possible solution to reduce contention between the processor and the snooper is to introduce a snoop filter, which determines whether a snooper needs to check the cache tag or not. By reducing the number of snooped transactions the need to check the cache tags, contention and power consumption can be reduced.

Why Snoop Filtering?

With the advent of modern computers built with multiple processing cores and shared memory programming model it has become necessary to use cache coherence protocols to maintain coherence between different caches attached to individual processing units and many coherence protocols are snoop based.