[TAG] using smp kernel, get 100% cpu usage one one cpu without any real load on the system

[René Pfeiffer]

Hello, Jim!

On Oct 08, 2007 at 1421 -0400, jim ruxton appeared and said:
> Hi I'm on a P4 running an smp kernel ie.
> 
> uname -a :
>  
> Linux jims-laptop 2.6.20-16-386 #2 Sun Sep 23 19:47:10 UTC 2007 i686
> GNU/Linux

Well, if you run a SMP kernel the output is usually something like this:

Linux nightfall 2.6.22.9 #1 SMP PREEMPT Tue Oct 2 22:02:38 CEST 2007
x86_64 GNU/Linux

uname shows a "SMP" next to the version. Double check if your kernel has
activated SMP code. Your output of /proc/cpuinfo only shows one
processor. You should have two (indicated by a different ID next to the
field "processor").

It should be noted that although some P4 CPUs have hyperthreading not
all mainboards or BIOS variants support it. I've seen P4s not capable of
hyperthreading simply because of the hardware or the BIOS on the board.

> For some reason occasionaly one of my cpu's runs away and starts showing
> 100% cpu usage, however top only shows the program using the most cpu at
> 7% or less. Where is that 93% going? This is driving me crazy.

Does the system become unresponsive or sluggish?

Can you check if "top" shows any distribution of the load? My top and
procinfo tools also show what the load is like, such as IRQ activity,
waiting for I/O or other things. This is an output of procinfo on my
SMP system (Intel Core 2 Duo):

lynx at nightfall:~$ procinfo 
Linux 2.6.22.9 (lynx at nightfall) (gcc [can't parse]) #???  2CPU [nightfall.luchs.at]

Memory:      Total        Used        Free      Shared     Buffers      
Mem:       1028328      967664       60664           0       16480
Swap:       995988         972      995016

Bootup: Tue Oct  2 22:06:31 2007    Load average: 0.03 0.07 0.08 1/133 27736

user  :       6:15:19.98   2.2%  page in :  4400622  disk 1:      451r     214w
nice  :       0:01:00.35   0.0%  page out:  3204961  disk 2:     6518r    4384w
system:       2:15:43.08   0.8%  page act:   858311  disk 3:   139898r  364672w
IOwait:       0:06:14.76   0.0%  page dea:   479245
hw irq:       0:25:24.70   0.1%  page flt: 34457299
sw irq:       0:18:43.76   0.1%  swap in :       25
idle  :  11d 10:38:31.98  95.6%  swap out:      233
uptime:   5d 23:37:23.38         context :109507629
[...]

There's user, nice, system, IOwait, the IRQ modes and idle time. And
yes, now I see that procinfo also counts the CPUs by printing "2CPU". :)
Using "x86info -v" is yet another way of checking SMP mode and CPU
capabilities.

Best regards,
Ren?.

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