Multilevel Cache Organisation | Computer Architecture & Organisation (CAO) - Computer Science Engineering (CSE) PDF Download

Introduction

• Cache is a random access memory used by the CPU to reduce the average time taken to access memory. 
• Multilevel Caches is one of the techniques to improve Cache Performance by reducing the “MISS PENALTY”. Miss Penalty refers to the extra time required to bring the data into cache from the Main memory whenever there is a “miss” in the cache.

For clear understanding let us consider an example where the CPU requires 10 Memory References for accessing the desired information and consider this scenario in the following 3 cases of System design :

Case 1 : System Design without Cache Memory

In this case, the CPU needs to access the main memory 10 times to access the desired information. 

Case 2 : System Design with Cache Memory

Here the CPU at first checks whether the desired data is present in the Cache Memory or not i.e. whether there is a “hit” in cache or “miss” in the cache. Suppose there is 3 miss in Cache Memory then the Main Memory will be accessed only 3 times. We can see that here the miss penalty is reduced because the Main Memory is accessed a lesser number of times than that in the previous case. 

Case 3 : System Design with Multilevel Cache Memory

Here the Cache performance is optimized further by introducing multilevel Caches. As shown in the above figure, we are considering 2 level Cache Design. Suppose there is 3 miss in the L1 Cache Memory and out of these 3 misses there is 2 miss in the L2 Cache Memory then the Main Memory will be accessed only 2 times. It is clear that here the Miss Penalty is reduced considerably than that in the previous case thereby improving the Performance of Cache Memory. 

Note: We can observe from the above 3 cases that we are trying to decrease the number of Main Memory References and thus decreasing the Miss Penalty in order to improve the overall System Performance. Also, it is important to note that in the Multilevel Cache Design, L1 Cache is attached to the CPU and it is small in size but fast. Although, L2 Cache is attached to the Primary Cache i.e. L1 Cache and it is larger in size and slower but still faster than the Main Memory.

Effective Access Time = Hit rate * Cache access time

                                                  + Miss rate * Lower level access time

Average access Time For Multilevel Cache:(T_avg) 
T_avg = H₁ * C₁ + (1 – H₁) * (H₂ * C₂ +(1 – H₂) *M )
where
H₁ is the Hit rate in the L₁ caches.
H₂ is the Hit rate in the L₂ cache.
C₁ is the Time to access information in the L₁ caches.
C₂ is the Miss penalty to transfer information from the L₂ cache to an L₁ cache.
M is the Miss penalty to transfer information from the main memory to the L₂ cache. 

Example: Find the Average memory access time for a processor with a 2 ns clock cycle time, a miss rate of 0.04 misses per instruction, a missed penalty of 25 clock cycles, and a cache access time (including hit detection) of 1 clock cycle. Also, assume that the read and write miss penalties are the same and ignore other write stalls. 

Average Memory access time(AMAT)= Hit Time + Miss Rate * Miss Penalty.
Hit Time = 1 clock cycle (Hit time = Hit rate * access time) but here Hit time is directly given so,
Miss rate = 0.04
Miss Penalty= 25 clock cycle (this is the time taken by the above level of memory after the hit)
so, AMAT= 1 + 0.04 * 25
AMAT= 2 clock cycle
according to question 1 clock cycle = 2 ns
AMAT = 4ns