By default, unless there's a compatibility issue, Linux will use all your available processors. If we would like to modify that policy we can use a tool called taskset. Let's install it on our system:
$ sudo apt-get install taskset
Now let's see how many CPUs we have (you should already know tho :)):
$ cat /proc/cpuinfo
or, execute the command top and then press 1. This will break down the CPU status:
top - 17:52:13 up 9:00, 21 users, load average: 1.75, 1.77, 1.71
Tasks: 210 total, 3 running, 207 sleeping, 0 stopped, 0 zombie
%Cpu0 : 33.1 us, 7.8 sy, 0.0 ni, 58.1 id, 0.7 wa, 0.0 hi, 0.3 si, 0.0 st
%Cpu1 : 13.1 us, 16.4 sy, 0.0 ni, 68.1 id, 2.3 wa, 0.0 hi, 0.0 si, 0.0 st
%Cpu2 : 15.7 us, 6.8 sy, 0.0 ni, 77.5 id, 0.0 wa, 0.0 hi, 0.0 si, 0.0 st
%Cpu3 : 9.4 us, 26.2 sy, 0.0 ni, 62.1 id, 2.0 wa, 0.0 hi, 0.3 si, 0.0 st
KiB Mem: 8188992 total, 7601696 used, 587296 free, 64868 buffers
KiB Swap: 15624188 total, 1320 used, 15622868 free, 2751376 cached
In our example we have 4 CPUs. Let's run the kaffeine media player on the two first ones:
$ taskset 03 kaffeine
The hex mask works as the man page details:
0x00000001 (01) for the #1
0x00000002 (02) for the #2
0x00000003 (03) for #1 and #2
0x00000004 (04) for #3
and so on. An 'f' mask would mean system managed on all the processors.
We can also specify the processor instead of the mask:
$ taskset -c 3 <command to execute>
Let's check what affinity the process have:
$ taskset -p 700
pid 700's current affinity mask: 3
We can modify it:
$ taskset -p 03 700
If we want to assign a processor range:
$ taskset -pc 3-4 700
pid 700's current affinity list: 0,1
pid 700's new affinity list: 2,3
Are we using all the CPUs for the system? an easy way to do it is checking the /proc/interrupts file:
$ cat /proc/interrupts
CPU0 CPU1 CPU2 CPU3
0: 77 34 19 22 IO-APIC-edge timer
1: 8 7 11 9 IO-APIC-edge i8042
8: 0 0 0 1 IO-APIC-edge rtc0
9: 10052 9947 10066 9977 IO-APIC-fasteoi acpi
10: 64476 64276 64381 64603 IO-APIC-edge ite-cir
12: 348 303 347 327 IO-APIC-edge i8042
16: 33 37 36 37 IO-APIC-fasteoi mmc0, ehci_hcd:usb1
17: 0 0 0 0 IO-APIC-fasteoi brcmsmac
18: 0 0 0 0 IO-APIC-fasteoi ips
19: 117 152 120 130 IO-APIC-fasteoi firewire_ohci
23: 8770094 8769570 8768178 8770493 IO-APIC-fasteoi ehci_hcd:usb2
41: 300133 300414 300139 300056 PCI-MSI-edge ahci
42: 56 58 57 59 PCI-MSI-edge snd_hda_intel
43: 25 26 24 25 PCI-MSI-edge snd_hda_intel
44: 1093397 1093774 1094078 1092726 PCI-MSI-edge eth0
45: 463403 463582 464719 463726 PCI-MSI-edge fglrx[0]@PCI:2:0:0
NMI: 34 22 16 14 Non-maskable interrupts
LOC: 48777039 50875209 50309525 47323147 Local timer interrupts
SPU: 0 0 0 0 Spurious interrupts
PMI: 34 22 16 14 Performance monitoring interrupts
IWI: 0 0 0 0 IRQ work interrupts
RES: 15846948 16275344 8478731 8848893 Rescheduling interrupts
CAL: 59602 55581 89923 81541 Function call interrupts
TLB: 180524 182431 106818 105437 TLB shootdowns
TRM: 0 0 0 0 Thermal event interrupts
THR: 0 0 0 0 Threshold APIC interrupts
MCE: 0 0 0 0 Machine check exceptions
MCP: 112 112 112 112 Machine check polls
ERR: 0
If any of the CPU columns would be plenty of 0's, that would mean trouble. Otherwise all our CPUs are plenty of work.
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