Re: Queues when accessing disks

From: M. Edward Borasky (znmeb_at_cesmail.net)
Date: 12/31/04

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    To: Felipe Erias <charles.swann@gmail.com>
Date:	Fri, 31 Dec 2004 12:26:44 -0800

    On Fri, 2004-12-31 at 04:32 +0100, Felipe Erias wrote:
    > Hi,
    >
    > I'm trying to apply queuing theory to the study of the GNU/Linux kernel.
    > Right now, I'm focusing in the queue of processes that appears when they
    > try to access an I/O device (specifically, an IDE HD). When they want to
    > read data, it behaves as a usual queue: several clients (processes) that
    > require attention from a server (disk / driver / ...). The case when they want
    > to write data is a bit more tricky, because of the cache buffers used by the OS,
    > and maybe could be modelized by a network of queues. Both cases are
    > interesting for my work, but I'll take the reading one first, just
    > because it seems
    > a bit more simple 'a priori'.
    >
    > To modelize the queue, I need to get some information:
    > - what processes claim attention from the disk
    > - when they do it
    > - when they begin to be served
    > - when they finish being served
    >
    > To get all this information, maybe I could hack my kernel a bit to write
    > a line to a log on every access to the HD, or account the IRQs from
    > the IDE channels... I also have the feeling that this queuing problem could
    > dissappear o became more hidden if DMA were enabled.
    >
    > To be true, I'm a bit lost and that's why I ask for your help.
    >
    > Yours sincerely,
    >
    > Felipe Erias

    I have a similar goal -- building an operational queueing model of a
    running Linux system. At this point, though, I'm not really trying to do
    per-process work, just model the system overall, including processors,
    I/O and virtual memory. As I noted in a recent posting, I'm currently
    blocked by the incorrect statistics gathered by a 2.4 kernel in
    "/proc/partitions". You can compute the throughputs, but not the average
    wait and service times, or the utilizations, which depend on the service
    times.

    I'm rather strongly considering establishing a mailing list just devoted
    to this topic -- queuing models of Linux kernels based on statistics the
    kernel collects. If such a list already exists, please let me know -- I
    don't want to re-invent any wheels, but there *is* a wheel I do want to
    invent! Please e-mail me off-list if you know of such a list or would
    join in if I create it.

    Thanks,

    Ed Borasky
    http://www.borasky-research.net/

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