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SubjectRe: elevator algorithm bug in ll_rw_blk.c

On Sun, 15 Nov 1998 14:13:31 -0500, Chip Salzenberg
<> 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


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