Messages in this thread | | | Date | Fri, 10 Mar 2000 03:07:37 +0100 | From | Jamie Lokier <> | Subject | Re: elevator messages in 2.3.50 |
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Andrea Arcangeli wrote: > >I don't know how you reach this conclusion. Can you explain? > > > >I can see that if you never receive requests during holdoff then of > >course you lose. [..] > > You answered your question yourself. > > Consider the worst case of the heuristic. Its worts case is definitely the > common simple case. Harming it is a no way IMHO.
That's what I don't see. That the worst case is the _common_ simple case. Sure it's the simpler case, but I see random paging as a common case which is the best case for this heuristic.
You _might_ be right, but do you know for sure? I.e. has it ever been measured?
Two more things:
- Using numbers plucked from the air, the heuristic's worst case will reduce throughput by at most 2%, and the best case can improve throughput by several times. So the "typical average behaviour" may improve even if the heuristic doesn't reorder requests most of the time.
- You can make it self-tuning w.r.t. whether it is worth doing according to recent access patterns. To do this, if a request arrives within 100us if a long seek request being issued, and the request which arrived would have been a short seek, then the heuristic would have been a gain. It may be that monitoring the occurrence of that indicates the future likelihood of the heuristic being a gain. (Again, need's measurement).
In this way even the worst case you are thinking of is avoided.
-- Jamie
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