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    SubjectRe: [RFD PATCH 0/4] cpu: Bulk CPU Hotplug support.
    On Tue, Jun 16, 2009 at 01:37:15PM +0530, Vaidyanathan Srinivasan wrote:
    > * Andrew Morton <akpm@linux-foundation.org> [2009-06-15 23:23:18]:
    >
    > > On Tue, 16 Jun 2009 11:08:39 +0530 Gautham R Shenoy <ego@in.ibm.com> wrote:
    > >
    > > > Currently on a ppc64 box with 16 CPUs, the time taken for
    > > > a individual cpu-hotplug operation is as follows.
    > > >
    > > > # time echo 0 > /sys/devices/system/cpu/cpu2/online
    > > > real 0m0.025s
    > > > user 0m0.000s
    > > > sys 0m0.002s
    > > >
    > > > # time echo 1 > /sys/devices/system/cpu/cpu2/online
    > > > real 0m0.021s
    > > > user 0m0.000s
    > > > sys 0m0.000s
    > >
    > > Surprised. Do people really online and offline CPUs frequently enough
    > > for this to be a problem?
    >
    > Certainly not for hardware faults or hardware replacement, but
    > cpu-hotplug interface is useful for changing system configuration to
    > meet different objectives like
    >
    > * Reduce system capacity to reduce average power and reduce heat
    >
    > * Increasing number of cores and threads in a CPU package is leading
    > to multiple cpu offline/online operations for any perceivable effect
    >
    > * Dynamically change CPU configurations in virtualized environments

    Perhaps also reducing boot-up time? If I am correctly interpreting the
    above numbers, an eight-CPU system would be consuming 175 milliseconds
    bringing up the seven non-boot CPUs. Reducing this by 150 milliseconds
    might be of interest to some people. ;-)

    Thanx, Paul

    > Ref:
    >
    > [1] Saving power by cpu evacuation sched_max_capacity_pct=n
    > http://lkml.org/lkml/2009/5/13/173
    >
    > [2] Make offline cpus to go to deepest idle state using
    > http://lkml.org/lkml/2009/5/22/431
    >
    > [3] cpuset: add new API to change cpuset top group's cpus
    > http://lkml.org/lkml/2009/5/19/54
    >
    > For getting stuff off a certain CPU, cpu-hotplug framework seems to do
    > the right thing. Identifying bottlenecks in the framework can
    > significantly help other use cases.
    >
    > --Vaidy
    >


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