Test by Bikram | Mock GATE | Test 2 | Question: 6

Question

The designers of a cache system wants to reduce the number of cache misses that occur in a certain group of programs.

Which of the following statements is/are correct regarding what designers can do?

1. If compulsory misses are most common, then the designers should consider increasing the cache line size to take better advantage of locality.
2. If capacity misses are most common, then the designers should consider increasing the total cache size so it can contain more lines.
3. If conflict misses are most common, then the designers should consider increasing the cache’s associativity, in order to provide more flexibility when a collision occurs.

• A.     I, II, and III
• B.     I and II only
• C.     II and III only
• D.     III only

Comments

No..you need to keep the cache size as constant and increase associativity, then chances of confict misses will decrease because any line or block of main memory can be mapped anywhere in the cache. A fully associative cache will have least no of conflict misses with a cache of same size.

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@surojit wow..I never thought like that.thanks.

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watch this video from 15:36 to answer this type of questions...

Answer 1

I) True  - By increasing the block size (and fixed cache size) number of blocks will reduce , so less compulsory misses.

II) True - Capacity Misses occurs when Working Set Size > Cache Size , So increase the cache size and Capacity misses decreases.

III) True  - By increasing associativity conflict misses ( as in one block being replaced by another block) will decrease as there will be more empty blocks to choose from that set due to increased associativity.

Answer 2

Increasing the cache line size brings in more from memory when a miss occurs. If accessing a certain byte suggests that near by bytes are likely to be accessed soon (locality), then increasing the cache line essentially prefetches those other bytes. This, in turn, prevent  later cache miss on those other bytes.

If misses occur because the cache is too small, then the designers should increase the size .

Conflict misses occur when multiple memory locations are repeatedly accessed but map to the same cache location.

Consequently, when they are accessed, they keep kicking one another out of the cache. Increasing the associativity implies that each chunk of the cache is effectively doubled so that more than one memory item can rest in the same cache chunk.

Answer 3

The more the line size, the less are the compulsory misses, because more words could fit into a line.

The more the cache size, the less are the capacity misses. This is obvious.

Associativity tells you the amount of location a word/block could associate to. In other words, the higher the associativity, the more the possible locations for a block to be mapped to, hence conflict misses are reduced.

 

All are True. Option A