ITPEC FE Morning April 2020 Question 16
When the number of concurrently running programs is increased in a virtual storage system with a small and insufficient main memory capacity, which of the following is the state in which the overhead of the system increases and the processor utilization of the applications decreases?
Explanation
Thrashing — page faults become so frequent that the system spends its time moving pages instead of running programs.
With too little main memory for the programs sharing it, each program keeps too few of its pages in memory. Almost every access needs a page that was just evicted, so a page fault occurs, and loading that page evicts another page some program will soon need. Paging I/O dominates, overhead rises and the CPU time left for applications falls. Adding programs makes it worse; the remedy is fewer concurrent programs or more memory.
Answer (d)
Why not others:
- •(a) Bottleneck — any resource that limits overall throughput; a general term, not the specific paging collapse described
- •(b) Fragmentation — free memory split into small scattered pieces; it wastes space but does not by itself drive up paging overhead
- •(c) Paging — the normal mechanism of moving pages between main memory and disk; thrashing is paging taken to excess
Key rule: Thrashing starts when the programs' working sets no longer fit in main memory together, and from that point adding programs lowers throughput.
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