"Stephen C. Tweedie" wrote:
> On Sun, 15 Nov 1998 14:13:31 -0500, Chip Salzenberg
> <chip@perlsupport.com> said:
>
> > According to Alan Cox:
> >> Chip:
> >> > But that's a *one-way* elevator. Ideal elevators are two-way,
> >> > aren't they?
> >>
> >> If you go for the shortest movement alone you starve the edges of the disks.
>
> > I said "two-way", not "shortest movement".
>
> Two-way elevators _do_ have this problem. A block in the middle
> cylinder of the disk gets serviced twice per full two-way pass, once in
> each direction. Maximum wait time is half a pass; average is a
> quarter. For a block at the edge of the disk, it will get serviced only
> when the two-way pass approaches close to the edge of the disk, and will
> not get another chance for a full pass, average wait half a pass.
>
> It's not a total stavation issue, but it _does_ give you a significant
> unfairness.
Well a better model is when you assume that there's much more blocks in between
edges of the disk than on the edges. The unfairness you described exists, but it's
not that significant - definitely more blocks might be served quicker by the
two-way elevator, with the peak best for the center of the drive (and obviously
worst serving times for the edges :)
See ya!
-- Tomek,
"In theory there's no difference between theory and practice, but in practice...."
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