How do I hotplug a CPU on a running Linux system? I would like to dynamically enable or disable a CPU on a running system?
Linux kernel does supports cpu-hotplug mechanism. You can enable or disable CPU without a system reboot. CPU hotplug is not just useful to replace defective components it can also be applied in other contexts to increase the productivity of a system. For example on a single system running multiple Linux partitions, as the workloads change it would be extremely useful to be able to move CPUs from one partition to the next as required without rebooting or interrupting the workloads. This is known as dynamic partitioning. Other applications include Instant Capacity on Demand where extra CPUs are present in a system but aren't activated. This is useful for customers that predict growth and therefore the need for more computing power but do not have at the time of purchase the means to afford.
List all current cpus in the system
Type the following command:
# cd /sys/devices/system/cpu
# ls -l
Sample output:
total 0 drwxr-xr-x 4 root root 0 Apr 2 12:03 cpu0 drwxr-xr-x 4 root root 0 Feb 15 07:06 cpu1 drwxr-xr-x 4 root root 0 Feb 15 07:06 cpu2 drwxr-xr-x 4 root root 0 Feb 15 07:06 cpu3 drwxr-xr-x 4 root root 0 Feb 15 07:06 cpu4 drwxr-xr-x 4 root root 0 Feb 15 07:06 cpu5 drwxr-xr-x 4 root root 0 Feb 15 07:06 cpu6 drwxr-xr-x 4 root root 0 Feb 15 07:06 cpu7 -rw-r--r-- 1 root root 4096 Apr 2 12:03 sched_mc_power_savings
Under each directory you would find an "online" file which is the control file to logically online/offline a processor.
How do I logically turn off (offline) cpu#6 ?
Type the following command:
# echo 0 > /sys/devices/system/cpu/cpu6/online
# grep "processor" /proc/cpuinfo
How do I logically turn on (online) cpu#6 ?
Type the following command:
# echo 1 > /sys/devices/system/cpu/cpu6/online
# grep "processor" /proc/cpuinfo
Once done, you can can actually remove CPU if your BIOS and vendor supports such operation.
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{ 18 comments… read them below or add one }
How to do such a similar thing for a hard disk…Juts unmount it or anything else is necessaey to remove and then add other???
Just unmount it and if it is hotplug just replace it.
I’m really curious about this. Can anyone comment if they’ve actually tried it? I’m not sure if any of the x86_(64) systems I’ve ever worked with support this capability – anyone know of systems that do?
You need high end servers such as IBM Power6 series to actually add or remove CPU / memory without rebooting the server. This is called Capacity Upgrade on Demand (CUoD). Other vendor such as Sun and HP also provides the same stuff. Your average server only support hard disk hot-plug. Another example is HP ProLiant ML570 G2 business server powered by Intel Xeon CPUs. Ask, your vendor for CUoD aware server and you will get the list. Be prepared to pay extra for all these fancy features ;)
HTH
It’s really works !
On my desktop Atom 330 (dual core + hyperthreading = 4 CPU), i can disable 3 CPUs of 4 :)
I tried to offline a core in my i386 (kernel 2.6), but I was notified a permission denied.
stefano@stefano-laptop:~$ echo 1 > /sys/devices/system/cpu/cpu1/online
bash: /sys/devices/system/cpu/cpu1/online: Permission denied
stefano@stefano-laptop:~$ sudo echo 1 > /sys/devices/system/cpu/cpu1/online
bash: /sys/devices/system/cpu/cpu1/online: Permission denied
Can anybody tell me y? Is it by default enabled in the latest versions?
Thx
S.
You must be root user to hotplug a CPU.
I also tried with sudo, as I reported in the post, but didn’t work.
Moreover, in visudo I’ve set:
stefano ALL=(ALL) ALL
Thx.
S.
sudo does not support the > operator
Run the command using sudo -i to get an interactive prompt first
Ok, I’v managed.
Thx.
S.
Could you please tell us how you managed, I get the same error “access denied” when run as root
Not only high-end servers – CPU/memory hotplugging is often used in virtualized systems.
I have put cpu 1 to 3 offline.
When I online a cpu I get the following message:
(root @ rhel5)
# echo 1 > /sys/devices/system/cpu/cpu1/online
-bash: echo: write error: Invalid argument
Anyone else got the same message and a solution to this problem?
It works. But system will crash if you unplugging CPU or core which currently busy by some process.
I’m looking at taskset utility to resolve this problem
I also get the same problem: “echo: write error: Invalid argument”
Did anyone find a resolution for this?
Thank you
For the people confused by “echo: write error: Invalid argument”: Is the CPU already enabled (if trying to enable it) or disabled (if trying to disable it)? AFAICT, the kernel produces an error when you try to put the CPU in the state it is already in, instead of just ignoring the request.
For the people getting permission errors:
Use “echo 0 | sudo tee /sys/devices/system/cpu/cpu6/online” instead of “sudo echo 0 > /sys/devices/system/cpu/cpu6/online”.
As Vivek points out, you need to be root to hotplug a CPU. You might think “But I used sudo”. However, when you use “>”, bash (or your shell, if you use something else) is responsible for opening the file that you’re trying to write to, and then setting that opened file as the destination for stdout (file descriptor 1). bash will then execute “sudo echo 0″, which will cause sudo to do its magic and run “echo 0″ as root. Unfortunately, bash isn’t running as root, so it will fail to open /sys/devices/system/cpu/cpu6/online.
By instead piping the output of the echo to sudo tee, tee becomes responsible for opening the file and writing to it. Since tee is run under sudo, it can actually succeed.
I only have a file
/sys/devices/system/cpu/online that contains the id of cores like that 0-15. How can I disable one of cores? How can I know online cores number by shell script commands?
Minxia
Does anyone know if this actually de-energises the CPU? In other words, does the disabled core/processor still use power?