[clue-tech] 2.6 kernel compile options make Intel CPU slow

[Jim Ockers]

Hi CLUEbies,

I have a curious problem.  We compiled a custom 2.6 Linux kernel for an 
embedded platform.  The target hardware is Intel P4 or AMD.  The 
hardware used to run a 2.4 Linux kernel.  Here is the fast/slowness matrix:

       Intel      AMD
2.4    fast       fast
2.6    slow       fast

Of course you want to know what "slow" means.  We have a test which we 
use to expose thermal throttling of the Intel CPU, since without 
"special" software there is no easy way to tell if the CPU is undergoing 
thermal throttling.  Here is the test:

[root]# time dd if=/dev/zero of=/dev/null bs=1024k count=100000
100000+0 records in
100000+0 records out
104857600000 bytes (105 GB) copied, 6.60495 seconds, 15.9 GB/s

On a "fast" system (per the matrix above) that command returns in about 
2-2.5 seconds.  On a "slow" system (even without any thermal throttling) 
that command returns in 7-9 seconds.  With thermal throttling it is much 
worse, anywhere from 10-180 seconds depending on the level of thermal 
throttling.

Here is the cpuinfo for the affected processor:

[root]# cat /proc/cpuinfo
processor       : 0
vendor_id       : GenuineIntel
cpu family      : 15
model           : 4
model name      : Intel(R) Pentium(R) 4 CPU 3.00GHz
stepping        : 1
cpu MHz         : 2992.760
cache size      : 1024 KB
physical id     : 0
siblings        : 2
core id         : 0
cpu cores       : 1
fdiv_bug        : no
hlt_bug         : no
f00f_bug        : no
coma_bug        : no
fpu             : yes
fpu_exception   : yes
cpuid level     : 5
wp              : yes
flags           : fpu vme de pse tsc msr pae mce cx8 apic mtrr pge mca 
cmov pat pse36 clflush dts acpi mmx fxsr sse sse2 ss ht tm pbe 
constant_tsc pni monitor ds_cpl cid xtpr
bogomips        : 5987.38


I could attach or send the .config file from which the 2.6 kernel is 
compiled but it is big and noisy.  Does anyone have suggestions about 
what CPU options we should try to change in the kernel configuration to 
make it "fast" with a 2.6 kernel on this Intel P4 CPU? There is a 
dizzying number of config categories that might be related, in 
categories like these:

# Processor type and features
# ACPI (Advanced Configuration and Power Interface) Support
# CPU Frequency scaling
# CPUFreq processor drivers

We need to fix this because it is causing a lot of our systems to be 
unnecessarily slow.  Any ideas, anyone?  What other information might 
you need about this?

Thanks,
Jim

-- 
Jim Ockers, P.Eng. (ockers at ockers.net)
Contact info: http://www.ockers.ca/pason.html


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