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    SubjectRe: performance differences: "maxcpus=1" vs. "echo 0 > /sys/devices/system/cpu/cpu1/online"

    --- Len Brown <lenb@kernel.org> schrieb:

    > On Tuesday 25 March 2008, Michael Meyer wrote:
    > >
    > > --- Andi Kleen <andi@firstfloor.org> schrieb:
    > >
    > > > Luciano Rocha <luciano@eurotux.com> writes:
    > > >
    > > > > On Tue, Mar 25, 2008 at 02:47:50PM +0100, Michael
    > > > Meyer wrote:
    > > > > > Hi,
    > > > > >
    > > > > > what is the difference between booting a dual
    > > > core
    > > > > > machine with "maxcpus=1" or by deactivating the
    > > > second
    > > > > > core at run time with "echo 0 >
    > > > > > /sys/devices/system/cpu/cpu1/online"?
    > > > >
    > > > > maxcpus=1 should turn off the SMP alternative and
    > > > switch to UP only,
    > > > > optimising some locks and instructions.
    > > >
    > > > CPU hot unplug will do the same. But it is unlikely
    > > > it accounts
    > > > for that much performance difference.
    > > >
    > > > If he used maxcpus=0 it would make sense. maxcpus=0
    > > > disables
    > > > the IO-APIC which likely makes a large difference.
    > > > But it should
    > > > be actually slower.
    > > >
    > > > There should be actually no difference in theory
    > > > between max_cpus=1
    > > > and hot unplug to one CPU. Might be some bug.
    > >
    > > I had the following time values:
    > >
    > > maxcpus=1:
    > > real 0m1.642s
    > > user 0m1.528s
    > > sys 0m0.068s
    > >
    > > maxcpus=2 and
    > > echo 1 > /sys/devices/system/cpu/cpu1/online:
    > > real 0m2.579s
    > > user 0m4.096s
    > > sys 0m0.160s
    >
    > this above is the baseline, yes?

    Yes, it is.

    > it is same as if you used no boot param
    > and did not touch the online file, yes?

    Yes. I just repeated it - once without the commands and once with the
    same commands stated above. Same result. So this is the default.

    >
    > > maxcpus=2 and
    > > echo 0 > /sys/devices/system/cpu/cpu1/online:
    > > real 0m3.757s
    > > user 0m3.632s
    > > sys 0m0.112s
    >
    > Please post the contents of
    > # grep . /sys/devices/system/cpu/cpu*/cpufreq/*

    # grep . /sys/devices/system/cpu/cpu*/cpufreq/*
    /sys/devices/system/cpu/cpu0/cpufreq/affected_cpus:0
    /sys/devices/system/cpu/cpu0/cpufreq/cpuinfo_max_freq:2400000
    /sys/devices/system/cpu/cpu0/cpufreq/cpuinfo_min_freq:1600000
    /sys/devices/system/cpu/cpu0/cpufreq/scaling_available_frequencies:2400000
    1600000
    /sys/devices/system/cpu/cpu0/cpufreq/scaling_available_governors:ondemand
    userspace conservative powersave performance
    /sys/devices/system/cpu/cpu0/cpufreq/scaling_cur_freq:1600000
    /sys/devices/system/cpu/cpu0/cpufreq/scaling_driver:acpi-cpufreq
    /sys/devices/system/cpu/cpu0/cpufreq/scaling_governor:ondemand
    /sys/devices/system/cpu/cpu0/cpufreq/scaling_max_freq:2400000
    /sys/devices/system/cpu/cpu0/cpufreq/scaling_min_freq:1600000
    /sys/devices/system/cpu/cpu1/cpufreq/affected_cpus:1
    /sys/devices/system/cpu/cpu1/cpufreq/cpuinfo_max_freq:2400000
    /sys/devices/system/cpu/cpu1/cpufreq/cpuinfo_min_freq:1600000
    /sys/devices/system/cpu/cpu1/cpufreq/scaling_available_frequencies:2400000
    1600000
    /sys/devices/system/cpu/cpu1/cpufreq/scaling_available_governors:ondemand
    userspace conservative powersave performance
    /sys/devices/system/cpu/cpu1/cpufreq/scaling_cur_freq:1600000
    /sys/devices/system/cpu/cpu1/cpufreq/scaling_driver:acpi-cpufreq
    /sys/devices/system/cpu/cpu1/cpufreq/scaling_governor:ondemand
    /sys/devices/system/cpu/cpu1/cpufreq/scaling_max_freq:2400000
    /sys/devices/system/cpu/cpu1/cpufreq/scaling_min_freq:1600000


    > and also
    > grep . /proc/acpi/processor/*/power

    # grep . /proc/acpi/processor/*/power
    /proc/acpi/processor/CPU0/power:active state: C0
    /proc/acpi/processor/CPU0/power:max_cstate: C8
    /proc/acpi/processor/CPU0/power:bus master activity: 00000000
    /proc/acpi/processor/CPU0/power:maximum allowed latency: 8000 usec
    /proc/acpi/processor/CPU0/power:states:
    /proc/acpi/processor/CPU0/power: C1: type[C1]
    promotion[--] demotion[--] latency[000] usage[00000000]
    duration[00000000000000000000]
    /proc/acpi/processor/CPU1/power:active state: C0
    /proc/acpi/processor/CPU1/power:max_cstate: C8
    /proc/acpi/processor/CPU1/power:bus master activity: 00000000
    /proc/acpi/processor/CPU1/power:maximum allowed latency: 8000 usec
    /proc/acpi/processor/CPU1/power:states:
    /proc/acpi/processor/CPU1/power: C1: type[C1]
    promotion[--] demotion[--] latency[000] usage[00000000]
    duration[00000000000000000000]


    >
    > My guess that the maxcpus=1 case benefits from turbo mode, aka EIDA.
    > That benefit, however, is subject to this bug:
    > http://bugzilla.kernel.org/show_bug.cgi?id=5471
    > because for a single thread to run faster than the marketing MHz,
    > the other thread must be in deep-idle, which is prevented
    > by the bug above.
    >
    > If your scaling_available_frequencies includes 2401000
    > then you probably have a turbo-mode enabled processor.

    It does not include 2401000. The processor is an Intel Core 2 Duo E6600
    (2.4GHZ) bought at the beginning of 2007. I do not think that that kind
    of freqency scaling was available back than.

    >
    > one way to verify this would be to disable turbo mode
    > by pegging the MHz like so:
    >
    > # echo 2400000 >
    > /sys/devices/system/cpu/cpu0/cpufreq/cpuinfo_max_freq
    > # echo 2400000 >
    > /sys/devices/system/cpu/cpu1/cpufreq/cpuinfo_max_freq
    >
    > -Len
    >

    This does not work, as both are read-only.


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