MadeEasy Subject Test: CO & Architecture - Cache Memory

Question

A cache memory is 30 times faster than main memory (MM) and 50% of the time cache is referred for the execution of instruction. The performance is gained by introducing this cache is ________.

What I did

EMAT = 0.5(M/30)+0.5(M/30+M) = 32M/60

speed up=  (M)/32M/60)

       =60/32=1.875

answer given is 1.90-1.97 (using amdhal’s law)

Comments

@adarsh_1997 could you please help! where m i going wrong?

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@eyeamgj @shaik masthan @Gupta731

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I added as an answer..please check

Answer 1

Initially I was also getting 1.88.
Then I realized memory is accessed twice [Taking a general case] , so I approached it differently .
Let time to access main memory = x units
So time to access cache = x/30 [Since 30 times faster]
In non cache environment , total time of memory access for 1 instruction = 2x
In cache environment :-
For instruction = 1/2[x/30] +1/2x
For 2nd memory operation = 1/2[x/30] +1/2x
Therefor total time in cached environment = x/30 + x units

Performance gain = 2x / (x/30 + x units) = 60/31 = 1.94

Comments

sorry brother But I don't think this is the approach here @prashant jha 1

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I might be wrong  :) Learning just :D

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But why do you think it is wrong , I might get where I am going wrong :)

Answer 2

Suppose main memory access takes $t_m$ and cache access takes $\frac{t_m}{30}$ as it is 30 times faster.

when the cache was not there, access took $t_m$ time.

now when the cache is introduced it takes $0.5\frac{t_m}{30}+0.5t_m =\frac{t_m}{60}+\frac{t_m}{2}$

Now speed up will be $\frac{earlier\ time}{new\ time}$

$=\frac{t_m}{\frac{t_m}{60}+\frac{t_m}{2}}$

$=\frac{t_m}{\frac{t_m+30t_m}{60}}$

$=\frac{t_m\times60}{31t_m}$

$\frac{60}{31}=1.93$

Comments

Yes, I have read about it in the book. I know the theory part but I still don't know which one to use when. I just use intuition.

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@Gupta731 https://gateoverflow.in/14480/formula-write-back-write-through-access-time-parallel-serial

Hope this would help.

 

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@Gurdeep Saini Could you please help?

Answer 3

let x be a factor, when multiplying with the x we will get the actual memory reference time;

effective time before cache was introduced = 1 x (30 x) = 30x

effective time after cache is introduced = 0.5( 1 x) + 0.5 ( 30 x) = 15.5x

since 50% time it is referencing cache.

performance gain = 30x /15.5x = 1.93