Finding hit ratio

Question

Cache access time Tc = 100 ns
Memory access time Tm = 500 ns
If the effective access time is 10% greater than the cache access time, what is the hit ratio H?
(A) 89%
(B) 91%
(C) 98%
(D) 95%

Answer 1

effective access time =cache hit ratio*cache access time+ cache miss ratio *(cache access time +main memory access time) 

effective access time =10% greater the cache access time ==>110

                                    let cache hit ratio is h

110 = h*100ns +(1-h) (100+500)

110= 100h+600-600h

500h= 490

h= 490/500= .98 = 98%

Comments

Can I know from where that formula:
110 = h*100ns +(1-h) (100+500)
i.e.
Effective Access Time = H*Tc+(1-H)(Tc+Tm)
came?

 

As far as my learning, when TLB hit occurs, we directly access main meory for actual required page.
When TLB miss occurs, we access main memory first for page table and then second time for actual required page.
So I feel the formula is:

Effective Access Time = H(Tc+Tm)+(1-H)(Tc+2Tm)

 

Galvin page link: https://books.google.co.in/books?id=s0MUQn7cxA4C&pg=PA294

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this question is not related  to TLB

TLB & CACHE are different things...TLB requires hardware implementation...

never depend on formula...gate question are not based on formula.....u should read concepts..not formula..

and concept says if there is miss in cache then only we have to access main memory...

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ohh so this is general caching problem!!! Thanks a lot.

Answer 2

its all about two level memory access. Here firsty cpu will check in cache memory if miss occurs then it will go to main memory

but this is not mentioned here ,so we use simultaneous access of cache and main memory so,

110 = h*100 + (1-h)*500

h=97.5% 

hence answer is close to (c)

we should not use 600 for cache miss until it is mentioned. I am using hierarchical access(look aside cache).

Comments

dear 2-level chache is used when multiple cache level are there...here only 1 cache is there

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@cse23 your approach is wrong.

Answer 3

Given -

T_cache= 100 ns

T_main  =500 ns

EAT(Effective Access Time) = 110

H=?

110 = 100H +(1- H)*600

=> H = 98%

Option - C

Comments

Can I know from where that formula:
110 = h*100ns +(1-h) (100+500)
i.e.
Effective Access Time = H*Tc+(1-H)(Tc+Tm)
came?

 

 

 

As far as my learning, when TLB hit occurs, we directly access main meory for actual required page.
When TLB miss occurs, we access main memory first for page table and then second time for actual required page.
So I feel the formula is:

 

Effective Access Time = H(Tc+Tm)+(1-H)(Tc+2Tm)

Galvin page link: https://books.google.co.in/books?id=s0MUQn7cxA4C&pg=PA294

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thank you. nicely explained.

Answer 4

effective access time =cache hit ratio*cache access time+ cache miss ratio *(cache access time +main memory access time)