Messages in this thread | | | Date | Thu, 18 Mar 1999 16:05:26 -0500 | From | Arvind Sankar <> | Subject | Re: disk head scheduling |
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On Thu, Mar 18, 1999 at 08:50:36PM +0100, Ingo Molnar wrote: > > On Thu, 18 Mar 1999, Arvind Sankar wrote: > > > I have a currently working algo which is greedy instead. i.e. for a new > > request, it computes the place where its insertion will produce the least > > incremental cost (where cost is the number of additional sectors to seek). > > It seems to work for me. No idea how to compare the two algos, though. Can > > somebody help me out here... > > be careful. These are all well-known problems, and your algorithm (also > called SSF (smallest-seek-first) scheduling) is not fair enough. It has to > be coupled with a mechanizm that provides fairness. (ie. guarantees that a > request will not 'hang around' for too long time, unfairly blocking a > process just because the position of the sector is unfortunate)
yeah. besides, it isn't really better than elevator, since doing reads while going backward on a disk is worse than just going to the start and doing all of them in the forward direction.
Reverting now :)
> > > Another point is that IN_ORDER seems to be called only for two requests > > on the same device, so no idea why it compares the device numbers. > > no, all requests (for all devices) are in a single 'queue'. (Per-major > device queues is candidate 2.3 feature, it's really simple)
oh. I realized that it was at least per-major, but it's global, huh?
-- arvind
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