What is the disadvantages of using round-robin scheduling?

Answer 1

Disadvantage:

§ Very important jobs wait in line.

§ Largest job take enough time for completion.

§ Setting the quantum too short causes too many context switches and lower the CPU efficiency.

§ Setting the quantum too long may cause poor response time and approximates FCFS.

Answer 2

The solution depends on what you perceive as a disadvantage of round-robin scheduling, which in turn depends on the exact application. For example, many might quote the fixed scheduling scheme as a disadvantage, because it doesn't generally allow processing a higher priority item faster than a lower priority item. However, other applications will benefit from equal treatment without regard for priority, as this scheme grants equal opportunity to all.

For example, if accelerated processing of selected high priority items is required, one could implement a multi-tier round robin scheduler, with one tier per priority level. For example, there could be one high priority ring of items A (A1, A2, A3, ...., Aa), a medium priority ring of items B (B1, B2, B3, ..., Bb) and a ring of low priority tasks C (C1, C2, C3, ..., Cc).

Each ring could maintain a marker of the next item to handle within that ring, MA, MB and MC. In each round-robin cycle, one could allow MA to advance three positions (process A1, A2, and A3), MB to advance two positions (process B1, B2) and MC one position (process C1). In the next cycle, you'd process A4, A5, A6, B3, B4, C2, and so forth, thus granting different weight (priority) to different items while maintaining the basic round-robin characteristics. Because each level is allowed a fixed number of steps each cycle, busy high priority items can't starve the low priority ones - this is often regarded as one of the key advantages of round robin scheduling.