Question

The fact that there are high cache miss rates for programs with binary executables of 100s of MB and the binary (machine code) instructions compete with the data stream (data is the value of the operands upon which the instructions are to act) implies what for the concepts of locality that justify many cache designs?

Answer 1

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First we should know about What is cache memory, It is a special very high-speed memory. It is used to speedup and synchronising with high-speed CPU. Cache memory is costlier than main memory or disk memory but economical than CPU registers. It is an extremely fast memory type that acts as a buffer between RAM and the CPU. It holds frequently requested data and instructions so that they are immediately available to the CPU when needed. Let me attached the block diagram of cache memory that is

now, when we talked about the cache miss rates which is caused If the processor does not find the memory location in the cache then cache miss occurs. For a cache miss, the cache allocates a new entry and copies in data from main memory, then the request is fulfilled from the contents of the cache.

The performance of cache memory is frequently measured in terms of a quantity called Hit ratio.

Hit ratio = hit / (hit + miss) =  no. of hits/total accesses

We can improve Cache performance using higher cache block size, higher associativity, reduce miss rate, reduce miss penalty, and reduce Reduce the time to hit in the cache.

Cache Mapping:
There are three different types of mapping used for the purpose of cache memory are as follow: Direct mapping, Associative mapping, and Set-Associative mapping. These are explained as following below.

1. Direct Mapping –
   The simplest technique, known as direct mapping, maps each block of main memory into only one possible cache line. or
   In direct mapping, assigned each memory block to a specific line in the cache. If a line is previously taken up by a memory block when a new block needs to be loaded, the old block is trashed. An address space is split into two parts index field and tag field. The cache is used to store the tag field whereas the rest is stored in the main memory. Direct mapping`s performance is directly proportional to the Hit ratio.

   For purposes of cache access, each main memory address can be viewed as consisting of three fields.The least significant w bits identify a unique word or byte within a block of main memory. In most contemporary machines,the address is at the byte level.The remaining s bits specify one of the 2^s blocks of main memory.The cache logic interprets these s bits as a tag of s-r bits (most significant portion) and a line field of r bits.

2. Associative Mapping –
   In this type of mapping the associative memory is used to store content and addresses both of the memory word. Any block can go into any line of the cache. This means that the word id bits are used to identify which word in the block is needed, but the tag becomes all of the remaining bits. This enables the placement of the any word at any place in the cache memory. It is considered to be the fastest and the most flexible mapping form.

3. Set-associative Mapping –
   This form of mapping is a enhanced form of the direct mapping where the drawbacks of direct mapping is removed. Set associative addresses the problem of possible thrashing in the direct mapping method. It does this by saying that instead of having exactly one line that a block can map to in the cache, we will group a few lines together creating a set. Then a block in memory can map to any one of the lines of a specific set..Set-associative mapping allows that each word that is present in the cache can have two or more words in the main memory for the same index address. Set associative cache mapping combines the best of direct and associative cache mapping techniques.

when we talked about the locality of cache memory, here are some overview of it.

Locality of reference –
Since size of cache memory is less as compared to main memory. So to check which part of main memory should be given priority and loaded in cache is decided based on locality of reference.

Types of Locality of reference

1. Spatial Locality of reference
   This says that there is chance that element will be present in the close proximity to the reference point and next time if again searched then more close proximity to the point of reference.
2. Temporal Locality of reference
   In this Least recently used algorithm will be used. Whenever there is page fault occurs within word will not only load word in main memory but complete page fault will be loaded because spatial locality of reference rule says that if you are referring any word next word will be referred in its register that’s why we load complete page table so complete block will be loaded.

So, now you will be able to understand the locality concept and cache concept with ease.

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