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


--- Wander Winkelhorst <w.winkelhorst@xxxxxxxxx>
schrieb:

On Tue, Mar 25, 2008 at 6:23 PM, Michael Meyer
<mike65134@xxxxxxxx> wrote:

--- Andi Kleen <andi@xxxxxxxxxxxxxx> schrieb:


> Luciano Rocha <luciano@xxxxxxxxxxx> 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

maxcpus=2 and
echo 0 > /sys/devices/system/cpu/cpu1/online:
real 0m3.757s
user 0m3.632s
sys 0m0.112s



What kind of CPU are you using? Some Intel CPU's do
"funny stuff",
like dynamically overclocking itself when working on
a single thread,
or using all of the 2nd level cache instead of
sharing it with the
second core.

Regards,
Wander Winkelhorst.


Intel Core 2 Duo E6600 (2.4 Ghz). I do not think that
it is capable of dynamically overclocking.

$ cat /proc/cpuinfo
processor : 0
vendor_id : GenuineIntel
cpu family : 6
model : 15
model name : Intel(R) Core(TM)2 CPU 6600 @
2.40GHz
stepping : 6
cpu MHz : 1600.000
cache size : 4096 KB
physical id : 0
siblings : 2
core id : 0
cpu cores : 2
fdiv_bug : no
hlt_bug : no
f00f_bug : no
coma_bug : no
fpu : yes
fpu_exception : yes
cpuid level : 10
wp : yes
flags : fpu vme de pse tsc msr pae mce cx8 apic sep
mtrr pge mca cmov pat pse36 clflush dts acpi mmx fxsr
sse sse2 ss ht tm pbe nx lm constant_tsc arch_perfmon
pebs bts pni monitor ds_cpl vmx est tm2 ssse3 cx16
xtpr lahf_lm
bogomips : 4791.87
clflush size : 64

processor : 1
vendor_id : GenuineIntel
cpu family : 6
model : 15
model name : Intel(R) Core(TM)2 CPU 6600 @
2.40GHz
stepping : 6
cpu MHz : 1600.000
cache size : 4096 KB
physical id : 0
siblings : 2
core id : 1
cpu cores : 2
fdiv_bug : no
hlt_bug : no
f00f_bug : no
coma_bug : no
fpu : yes
fpu_exception : yes
cpuid level : 10
wp : yes
flags : fpu vme de pse tsc msr pae mce cx8 apic sep
mtrr pge mca cmov pat pse36 clflush dts acpi mmx fxsr
sse sse2 ss ht tm pbe nx lm constant_tsc arch_perfmon
pebs bts pni monitor ds_cpl vmx est tm2 ssse3 cx16
xtpr lahf_lm
bogomips : 4788.35
clflush size : 64





E-Mails jetzt auf Ihrem Handy.
www.yahoo.de/go

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/

Relevant Pages