Give two reasons why caches are useful. What problems do they solve?
What problems do they cause? If a cache can be made as large as the
device for which it is caching (for instance, a cache as large as a disk),
why not make it that large and eliminate the device?.
Caches are useful when two or more components need to ex-change data, and the components perform transfers at differing speeds. Caches solve the transfer problem by providing a buffer of intermediate speed between the components. If the fast device finds the data it needs in the cache, it need not wait for the slower device. The data in the cache must be kept consistent with the data in the components. If a component has a data value change, and the datum is also in the cache, the cache must also be updated. This is especially a problem on multiprocessor systems where more than one process may be accessing a datum.
A component may be eliminated by an equal-sized cache, but only if:
(a) the cache and the component have equivalent state-saving capacity (that is, if the component retains its data when electricity is removed, the cache must retain data as well),
(b) the cache is affordable, because faster storage tends to be more expensive.
How the problem of maintaining coherence of
cached data manifests itself in the following processing environments:
a. Single-processor systems
b. Multiprocessor systems
c. Distributed systems
In single-processor systems, the memory needs to be updated when a processor issues updates to cached values. These updates can be performed immediately or in a lazy manner. In a multiprocessor system, different processors might be caching the same memory location in its local caches. When updates are made, the other cached locations need to be invalidated or updated. In distributed systems, consistency of cached memory values is not an issue. However, consistency problems might arise when a client caches file data.