On Sun, 15 Nov 1998 11:17:19 -0600 (EST), kwrohrer@ce.mediaone.net said:
> And lo, Chip Salzenberg saith unto me:
>>
>> But that's a *one-way* elevator. Ideal elevators are two-way, aren't they?
> Depends on what you call "ideal". Bottom-up elevators are fairer to the
> first and last parts of the disk than back-and-forth elevators are. But I
> haven't seen any papers on how fair is "fair enough" in the general case.
> I'd think not allowing processes to add more requests to the current pass
> would be more important, and more likely necessary, than choosing one-way
> over back-and-forth, for "typical" tasks. But I freely admit that my sense
> of "typical" is intuitive, not concrete or even anecdotal...
Allowing processes to add new tasks to the queue is essential for good
performance. As long as the elevator is uni-directional, it doesn't
make things any less fair: over the long term, every request gets served
within the space of exactly one pass and no individual requests get
priority.
--Stephen
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