First Come First Served Scheduling
- The one drawback in FCFS scheduling do a lengthy seek if the next waiting request to service is at a long distant, even though another request have just arrived present on the same cylinder to which the read-write head is currently positioned.
- It results in random seek pattern, if the requests are uniformly distributed throughout the disk.
- The positional relationships between requests are ignored in this method. No optimization of seek pattern is attempted.
- When there are only few requests or in light load situation this method is appreciable.
FCFS saturate the device and response times become large. It offers small variance. Fig above depicts this method of scheduling.In the figure the scheduling is based on First come first served basis. The number indicates the order of request arrived
Shortest Seek Time First Scheduling
- SSTF seek patterns tend to be highly localized with the result that the innermost and outermost tracks can receive poor service compared with the mid-range tracks.
- It has high throughput compared to FCFS.
- The mean response time is minimized for moderate loads.
- The drawback in this scheduling method is results in higher variances because of the discrimination against the outermost and innermost tracks.
- This higher variance is tolerable when considering higher throughput and less response time.
- It is mainly used in batch processing systems where throughput is important.
Fig above explains this disk scheduling scheme.
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