[PATCH]sched: Isochronous class v2 for unprivileged soft rt scheduling

From: Con Kolivas (kernel_at_kolivas.org)
Date: 01/19/05

  • Next message: George Anzinger: "Re: [PATCH] dynamic tick patch" 
    Date:	Thu, 20 Jan 2005 09:39:45 +1100
To: linux <linux-kernel@vger.kernel.org>

    This is version 2 of the SCHED_ISO patch with the yield bug fixed and
    code cleanups.

    This patch for 2.6.11-rc1 provides a method of providing real time
    scheduling to unprivileged users which increasingly is desired for
    multimedia workloads.

    It does this by adding a new scheduling class called SCHED_ISO or
    Isochronous scheduling which means "same time" scheduling. This class
    does not require superuser privileges and is starvation free. The
    scheduling class no. 4 was chosen since there are quite a few userspace
    applications already supporting 3 and 4 for SCHED_BATCH and SCHED_ISO
    respectively on non-mainline kernels. As a way of immediately providing
    support for current userspace apps, any unprivileged user starting an
    application requesting SCHED_RR or SCHED_FIFO will be demoted to
    SCHED_ISO. This may or may not be worth removing later.

    The SCHED_ISO class runs as SCHED_RR effectively at a priority just
    above all SCHED_NORMAL tasks and below all true real time tasks. Once a
    cpu usage limit is exceeded by tasks of this class (per cpu), SCHED_ISO
    tasks will then run as SCHED_NORMAL until the cpu usage drops to 90% of
    the cpu limit.

    By default the cpu limit is set to 70% which literature suggests should
    provide good real time behaviour for most applications without gross
    unfairness. This cpu limit is calculated as a decaying average over 5
    seconds. These limits are configurable with the tunables
    /proc/sys/kernel/iso_cpu
    /proc/sys/kernel/iso_period

    iso_cpu can be set to 100 which would allow all unprivileged users
    access to unrestricted SCHED_RR behaviour. OSX provides a similar class
    to SCHED_ISO and uses 90% as its cpu limit.

    The sysrq-n combination which converts all user real-time tasks to
    SCHED_NORMAL also will affect SCHED_ISO tasks.

    Currently the round robin interval is set to 10ms which is a cache
    friendly timeslice. It may be worth making this configurable or smaller,
    and it would also be possible to implement SCHED_ISO of a FIFO nature as
    well.

    For testing, the userspace tool schedtool available here:
    http://freequaos.host.sk/schedtool/
    can be used as a wrapper to start SCHED_ISO tasks
    schedtool -I -e xmms
    for example

    Patch also available here:
    http://ck.kolivas.org/patches/SCHED_ISO/

    Signed-off-by: Con Kolivas <kernel@kolivas.org>

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  • Next message: George Anzinger: "Re: [PATCH] dynamic tick patch"

    Relevant Pages

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