Messages in this thread | | | Date | Wed, 26 Mar 2008 08:26:01 +0100 (CET) | From | Michael Meyer <> | Subject | Re: performance differences: "maxcpus=1" vs. "echo 0 > /sys/devices/system/cpu/cpu1/online" |
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--- 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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