[lkml]   [2007]   [Apr]   [15]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
Messages in this thread
    SubjectRe: [Announce] [patch] Modular Scheduler Core and Completely Fair Scheduler [CFS]

    * Matt Mackall <> wrote:

    > Look at what happened with I/O scheduling. Opening things up to some
    > new ideas by making it possible to select your I/O scheduler took us
    > from 10 years of stagnation to healthy, competitive development, which
    > gave us a substantially better I/O scheduler.

    actually, 2-3 years ago we already had IO schedulers, and my opinion
    against plugsched back then (also shared by Nick and Linus) was very
    much considering them. There are at least 4 reasons why I/O schedulers
    are different from CPU schedulers:

    1) CPUs are a non-persistent resource shared by _all_ tasks and
    workloads in the system. Disks are _persistent_ resources very much
    attached to specific workloads. (If tasks had to be 'persistent' to
    the CPU they were started on we'd have much different scheduling
    technology, and there would be much less complexity.) More analogous
    to CPU schedulers would perhaps be VM/MM schedulers, and those tend
    to be hard to modularize in a technologically sane way too. (and
    unlike disks there's no good generic way to attach VM/MM schedulers
    to particular workloads.) So it's apples to oranges.

    in practice it comes down to having one good scheduler that runs all
    workloads on a system reasonably well. And given that a very large
    portion of system runs mixed workloads, the demand for one good
    scheduler is pretty high. While i can run with mixed IO schedulers
    just fine.

    2) plugsched did not allow on the fly selection of schedulers, nor did
    it allow a per CPU selection of schedulers. IO schedulers you can
    change per disk, on the fly, making them much more useful in
    practice. Also, IO schedulers (while definitely not being slow!) are
    alot less performance sensitive than CPU schedulers.

    3) I/O schedulers are pretty damn clean code, and plugsched, at least
    the last version i saw of it, didnt come even close.

    4) the good thing that happened to I/O, after years of stagnation isnt
    I/O schedulers. The good thing that happened to I/O is called Jens
    Axboe. If you care about the I/O subystem then print that name out
    and hang it on the wall. That and only that is what mattered.

    all in one, while there are definitely uses (embedded would like to have
    a smaller/different scheduler, etc.), the technical case for
    modularization for the sake of selectability is alot lower for CPU
    schedulers than it is for I/O schedulers.

    nor was the non-modularity of some piece of code ever an impediment to
    competition. May i remind you of the pretty competitive SLAB allocator
    landscape, resulting in things like the SLOB allocator, written by
    yourself? ;-)

    To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
    the body of a message to
    More majordomo info at
    Please read the FAQ at

     \ /
      Last update: 2007-04-15 22:51    [W:0.023 / U:5.300 seconds]
    ©2003-2017 Jasper Spaans. hosted at Digital OceanAdvertise on this site