Question

Most future processor chips will contain multiple cores whose references to shared memory are accelerated by caches. These will usually be kept transparent to software via a coherence protocol. Many protocols are based on snooping or directories. Nevertheless, multicore chips will differ regarding interconnect, small or large number of cores, flat vs. hierarchical, ect

a) Discuss what factors that favor snooping coherence and why.

b) Discuss what factors that favor directory coherence and why.

Answer 1

SNOOPING COHERENCE:

It is a process where the individual can cache monitor address lines for accesses to memory locations.It send all requests for data to all processors, which snoop to see if they have a copy and respond accordingly.It works well with bus and is generally used for small scale machines.

Snooping is much more faster if enough bandwidth is available because all the transactions are a request/ response observed by all the processors.It has a low average miss latency for cache to cache misses.This particular coherence is also simple to implement.

DIRECTORY COHERENCE:

In a directory-based coherence system, the data being shared is placed in a common directory that maintains the coherence between caches. It keeps track of what is being shared in one centralized place.It sends point to point request to processors and its scaling is better than the snooping mechanism.They have longer latencies and are generally used for large scale systems.They allow flexibility to use any scalable point to point network.