What is a cache hit and a cache miss? Why would context-switching cause cache miss?

Question

From the 11th Chapter(Performance and Scalability) and the section named Context Switching of the JCIP book:

When a new thread is switched in, the data it needs is unlikely to be in the local processor cache, so a context-switch causes a flurry of cache misses, and thus threads run a little more slowly when they are first scheduled.

1. Can someone explain in an easy to understand way the concept of cache miss and its probable opposite (cache hit)?
2. Why context-switching would cause a lot of cache miss?

Answer 1

Can someone explain in an easy to understand way the concept of cache miss and its probable opposite (cache hit)?

A cache miss, generally, is when something is looked up in the cache and is not found – the cache did not contain the item being looked up. The cache hit is when you look something up in a cache and it was storing the item and is able to satisfy the query.

Why context-switching would cause a lot of cache miss?

In terms of memory, each processor has a memory cache – a high speed copy of small portions of main memory. When a new thread is context switched into a processor, the local cache memory is empty or it doesn't correspond to the data needed for the thread. This means that all (or most) memory lookups made by that new thread result in cache misses because the data that it needs is not stored in the local memory cache. The hardware has to then make a number of requests to main memory to fill up the local memory cache which causes the thread to initially run slower.

Answer 2

Whenever the processor wants to fetch data from main memory, first it will look at the cache buffer to see whether the corresponding address is present in the buffer. If it is there, it will perform the operation by using the cache; no need to fetch from the main memory. This is called a "Cache hit".

If the address is not present in the cache, it is called a "Cache miss". If a cache miss has occurred, that means the processor has go to main memory to fetch the address and it takes some more time.