madeeasy test series

Question

Consider 3 processes P₀, P₁ and P₂ to be scheduled as per the SRTF algorithm. The process ‘P₀’ is known to be scheduled first and when ‘P₀ has been running ‘6’ units of time, then the process ‘P₂’ has arrived. The process ‘P₂’ has run for ‘4’ unit of time,then the process ‘P₁’ has arrived and completed running in ‘5’ units of time. Then the minimum burst time of ‘P₀’ is _______ (in units).

Please explain how to solve this. Ans is given 17 

Answer 1

Let P0's BT be x units and P2's BT be y units.

Now, when P0 has finished 6 units, means left is x-6. But at this moment P2 has been given next chance. Since it is SRTF, so P2 getting chance/priority more than P0  means P2's BT must have been less than the remaining BT of P0. Otherwise P0 would have continued.

So, y< x-6 ....(i)

Now, P2 executed for 4 units after which P1 came and it was given the CPU time. P1 takes 5 units.

This means P1's BT< the remaining BT of P2. Otherwise P2 would have continued.

The remaining BT of P2 is y-4.

So, 5< y-4.

y> 9  hence min. Value of y is 10.

From equation (i),

y<x-6 => 10<x-6 => x> 16 => min(x)= 17.

Comments

@MiNiPanda   nice and precise explanation thanks

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Thank you :P

Answer 2

here in the above question arrival time for p1,p2,p3 are 0,10,6 and

here they using SRTF scheduling algorithm

1st p0 is executing for 6 unit and then p2 is scheduling means p2 have less remaining time than p0

2nd p2 is executing for 4 unit and then p1 is arrived and run complete for 5 unit, so we can say that

p2 may have 5(p1 burst time)+4(p2 1st execution time)+1(at least 1 more )=10 unit,

like that for p0 10(p2)+6(p0)+1(unit more)=17 unit.

Comments

p2 may have 5(p1 burst time)+4(p2 1st execution time)+1(at least 1 more )=10 unit,   can you explain this line in detail ?  @meethunjadhav

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p2 is arriving after p0 and before p1 hence p0>p2>p1

So,p2 have 4(p2 burst time)+6(p1 is running for 5 unit means it arrive early than p2 means p2 must have more than p1 burst time that's why 6  )=10 unit