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    SubjectRe: (reiserfs) Re: More on 2.2.18pre2aa2
    James Sutherland wrote:
    > In terms of latency, I'd suggest we aim to keep the device in use all the
    > time we have outstanding requests: every time the device is ready to
    > accept a request, we feed it the "next" one in the queue; until it is free
    > again, requests pile up in the queue, being sorted, merged etc.
    >
    > This should perform very well under light loads - requests just get passed
    > to the device as fast as they can be handled - and under very heavy loads
    > - we have a large queue in play, with plenty of scope for reordering,
    > merges etc.

    The "large queue" goes against the whole point of this exercise - that
    is that if there are many items in the "queue" being sorted then
    unlucky requests can end up waiting a long time to get serviced.
    You want to have some reasonable break-off point where you just
    decide to make an elevator swipe in order to avoid starving those
    requests.

    Keep in mind that I'm using the word "queue" in a confusing manner
    here - we're talking about how much we'll try to elevator-sort in
    a go, not the actual queue of all requests. I've been calling this
    a queue because from a performance standpoint that's what it is,
    so I think it helps to think of the problem that way.

    Oh yeah one other thing about huge elevator queues to think about
    (although its not too pertinant these days). A friend of mine had
    a job 15 years or so ago to working on a commercial UNIX flavor.
    They were having some severe I/O performance problems under high
    loads. The problem was that they were trying to sort and merge
    all the pending requests. When I/O was heavy this ended up
    chewing all the CPU time and the kernel wouldn't actually be
    able to keep up sending requests to the disk :-) However, keep
    in mind that this was int the bad-old-days when "hard drive"
    meant a rack-mount Eagle and the elevator was expected to consider
    things like head positioning and rotational speed when doing
    a sort...

    -Mitch
    -
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