  ### OS : 2004 GATE Questions

Questions : 1

QID : 311GATE-2004, 2M

Consider a system with a two-level paging scheme in which a regular memory access takes 150 nanoseconds, and servicing a page fault takes 8 milliseconds. An average instruction takes 100 nanoseconds of CPU time, and two memory accesses. The TLB hit ratio is 90%, and the page fault rate is one in every 10,000 instructions. What is the effective average instruction execution time?

Options:
A) 645 nanoseconds
B) 1050 nanoseconds
C) 1215 nanoseconds
D) 1230 nanoseconds
Ans : Option D
Solution :
Main Memory Access Time = m = 150 ns

Time for One Instruction = i = 100 ns + 2 mem accesses
= 100ns + 2*150 ns
= 400 ns

TLB Hit Ratio = 90%
so, out of 10,000 instructions, TLB is missed 1000 times
TLB access time is not given, consider its ZERO

Page Fault Rate = 1/10000
So, out of 10,000 instructions, only for 1 instruction page fault occurs.
Page Fault Service time = 8ms = 8000,000 ns

Lets calculate Total Execution time for all 10,000 instructions including TLB and Page Fault overhead.

Total Time = Actual Instruction Time
+ Overhead when TLB is missed

= [10,000 * 400 ]
+ [1000 * 300] (because its 2 level paging, its TLB + 2m)
+ [1 * 8000000]

= 1230,0000 ns

So, Average = 1230,0000 / 10,000
= 1230 ns

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