Question

Given six memory partitions of 800 KB,500 KB,350KB,200KB,450KB and 100 KB (in order), how would the first-fit, best-fit and worst-fit algorithms place processes of size 315KB,600KB, 345KB, 110KB and 400KB(in order) i) Which algorithm makes the most efficient use of memory? ii) Are there any internal and external fragmentation? If yes, how it can be solved?

Answer 1

Solution:

a) First-fit

The first-fit algorithm selects the first free partition that is large enough to accommodate the request. First-fit would allocate in the following manner: 315 KB => 800 KB partition, leaves a 485 KB partition 600 KB => would not be able to allocate, no partition large enough! 345 KB => 500 KB partition, leaves a 155 KB partition 110 KB => 350 KB partition, leaves a 240 KB partition 400 KB => 450 KB partition, leaves a 50 KB partition

b) Best-fit

The best-fit algorithm selects the partition whose size is closest in size (and large enough) to the requested size. Best-fit would allocate in the following manner: 315 KB => 350 KB, leaving a 35 KB partition 600 KB => 800 KB, leaving a 200 KB partition 345 KB => 450 KB, leaving a 105 KB partition 110 KB => 200 KB, leaving a 90 KB partition 400 KB => 500 KB, leaving a 100 KB partition

c) Worst-fit

The worst-fit algorithm effectively selects the largest partition for each request. Worst-fit would allocate in the following manner: 315 KB => 800 KB, leaving a 485 KB partition 600 KB => would not be allowed to allocate as no partition is large enough! 345 KB => 500 KB, leaving a 155 KB partition 110 KB => 450 KB, leaving a 340 KB partition 400 KB => 485 KB, leaving a 85 KB partition (new partition 485KB = 800KB - 315KB)

(i) The best-fit algorithm performed the best of the three algorithms, as it was the only algorithm to meet all the memory requests.

(ii) Yes, internal and external fragmentation both are present in the above problem.

Internal fragmentation can be reduced effectively by dividing the memory into variable sized blocks and assigning the best-fit block to the process requesting for the memory. External fragmentation can be eliminated by compaction, paging, and segmentation so that memory can be allocated in a non-contiguous manner to a process.

In a nutshell, both internal and external fragmentation are natural process related to empty memory space or memory being wasted. However, the problem with both cases cannot be resolved entirely but can be mitigated to some extent with solutions given above.

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