Ubuntu – How to run a program with only one CPU core

The accepted answer may be good advice, but it certainly isn't an answer to your question. Your question doesn't indicate to me that you aren't qualified to hear the answer either, in fact it sounds like a good example of a situation in which you'd want to specifically indicate which processors should do what.

Getting to the point, what you want are CPU Sets. They are supported in 11.10 of Ubuntu, since it uses a recent enough Linux Kernel. I believe Linux 2.6 is required.

You can read the man page for cpuset here: http://www.kernel.org/doc/man-pages/online/pages/man7/cpuset.7.html

Here is another page for general reading: http://www.bullopensource.org/cpuset/

Linux may be "great" at deciding how to allocate CPU time, but it isn't perfect.

You can make use of CPU hotplug abilities to achieve your objective. You can boot CPUs 0-13 and then add the others (CPUs 15-27, and 29-31) afterwards.

All Xeon processors have hyper threading, so I assume you mean 16 cores at 2 threads per core, for a total of 32 CPUs. This answer is written, and tested, for a 4 core, 2 threads per core, processor, where core 2 is the bad one.

First, as sudo, edit /etc/default/grub and add the maximum boot time CPUs, maxcpus=, to your GRUB_CMDLINE_LINUX_DEFAULT line. Example for my system:

Was:

GRUB_CMDLINE_LINUX_DEFAULT="ipv6.disable=1 consoleblank=300 cpuidle_sysfs_switch cpuidle.governor=teo"

Now:

GRUB_CMDLINE_LINUX_DEFAULT="ipv6.disable=1 consoleblank=300 cpuidle_sysfs_switch cpuidle.governor=teo maxcpus=2"

Where I used maxcpus=2 you would use maxcpus=14.

Save a copy of grub first, and run sudo update-grub after. Thus, the system will boot only using cores 0 and 1, and in CPUs 0,1 being on-line:

doug@s15:~$ grep . /sys/devices/system/cpu/cpu*/online
/sys/devices/system/cpu/cpu1/online:1
/sys/devices/system/cpu/cpu2/online:0
/sys/devices/system/cpu/cpu3/online:0
/sys/devices/system/cpu/cpu4/online:0
/sys/devices/system/cpu/cpu5/online:0
/sys/devices/system/cpu/cpu6/online:0
/sys/devices/system/cpu/cpu7/online:0

Note: For the default Ubuntu kernel configurations CPU 0 is always online, and there is no such thing as:

doug@s15:~$ grep . /sys/devices/system/cpu/cpu0/online
grep: /sys/devices/system/cpu/cpu0/online: No such file or directory

O.K. so now, bring the other desired cores and CPUs on-line:

doug@s15:~$ echo 1 | sudo tee /sys/devices/system/cpu/cpu3/online
1
doug@s15:~$ echo 1 | sudo tee /sys/devices/system/cpu/cpu4/online
1
doug@s15:~$ echo 1 | sudo tee /sys/devices/system/cpu/cpu5/online
1
doug@s15:~$ echo 1 | sudo tee /sys/devices/system/cpu/cpu7/online

And check:

doug@s15:~$ grep . /sys/devices/system/cpu/cpu*/online
/sys/devices/system/cpu/cpu1/online:1
/sys/devices/system/cpu/cpu2/online:0
/sys/devices/system/cpu/cpu3/online:1
/sys/devices/system/cpu/cpu4/online:1
/sys/devices/system/cpu/cpu5/online:1
/sys/devices/system/cpu/cpu6/online:0
/sys/devices/system/cpu/cpu7/online:1

So now, I have cores 0,1,3 on-line and core 2 offline and 6 CPUs available. Note that core 0 = cpus 0 and 4, core 1 = cpus 1 and 5, ...

EDIT 1: For 32 CPUs, perhaps you have multiple nodes (processors), so the core to CPU mapping might be different.

EDIT 2: It may be that the CPUs that are brought on-line after boot default to using the performance governor in the intel_pstate CPU frequency scaling driver, which is the kernel configuration default (which gets changed to powersave 1 minute after boot, for the boot enabled CPUs). You might want to check and set all CPU governors to your preference, typically the powersave governor. To check do:

grep . /sys/devices/system/cpu/cpu*/cpufreq/scaling_driver
grep . /sys/devices/system/cpu/cpu*/cpufreq/scaling_governor

To change governors do, for example (notice as root):

# for file in /sys/devices/system/cpu/cpu*/cpufreq/scaling_governor; do echo "powersave" > $file; done

Once you have things working the way you want you can automate the additional on-line after boot step (see other questions and answers for "how to").

Note: It seems to me that you should be able to achieve your objective in one boot step via "cpu_possible_mask" manipulation via "possible_cpus=n", but I couldn't get it to work. Someone else might know.