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    SubjectRe: [ext3][kernels >= 2.6.20.7 at least] KDE going comatose when FS is under heavy write load (massive starvation)

    * Linus Torvalds <torvalds@linux-foundation.org> wrote:

    > Even with a good software IO scheduler, when you have disks that do
    > tagged queueing, if you fill up the disk queue with a few dozen
    > (depends on the disk what the queue limit is) huge write requests, it
    > doesn't really matter if the _software_ queuing then gives a big
    > advantage to reads coming in. They'll _still_ be waiting for a long
    > time, especially since you don't know what the disk firmware is going
    > to do.

    by far the largest advantage of tagged queueing is when we go from 1
    pending request to 2 pending requests. The rest helps too for certain
    workloads (especially benchmarks), but if the IRQ handling is fast
    enough, having just 2 is more than enough to get 80% of the advantage of
    say of hardware-queue with a depth of 64.

    So perhaps if there's any privileged reads going on then we should limit
    writes to a depth of 2 at most, with some timeout mechanism that would
    gradually allow the deepening of the hardware queue, as long as no
    highprio reads come inbetween? With 2 pending requests and even assuming
    worst-case seeks the user-visible latency would be on the order of 20-30
    msecs, which is at the edge of human perception. The problem comes when
    a hardware queue of 32-64 entries starves that one highprio read which
    then results in a 2+ seconds latency.

    Ingo
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