Linux kernel does supports cpu-hotplug mechanism. You can enable or disable CPU or CPU core 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. [donotprint]
| Tutorial details | |
|---|---|
| Difficulty level | Intermediate |
| Root privileges | Yes |
| Requirements | None |
| Est. reading time | 3 minutes |
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List all current cpus and cores 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
I’ve total 8 core cpu logically started from cpu0 to cpu7. To get more human readable format, try:
# lscpu
Sample outputs:
Architecture: x86_64 CPU op-mode(s): 32-bit, 64-bit Byte Order: Little Endian CPU(s): 32 On-line CPU(s) list: 0-31 Thread(s) per core: 2 Core(s) per socket: 8 Socket(s): 2 NUMA node(s): 2 Vendor ID: GenuineIntel CPU family: 6 Model: 45 Stepping: 7 CPU MHz: 2000.209 BogoMIPS: 4001.65 Virtualization: VT-x L1d cache: 32K L1i cache: 32K L2 cache: 256K L3 cache: 20480K NUMA node0 CPU(s): 0-7,16-23 NUMA node1 CPU(s): 8-15,24-31
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 ?
Warning: It is not possible to disable CPU0 on Linux systems i.e do not try to take cpu0 offline. Some architectures may have some special dependency on a certain CPU. For e.g in IA64 platforms we have ability to sent platform interrupts to the OS. a.k.a Corrected Platform Error Interrupts (CPEI). In current ACPI specifications, we didn’t have a way to change the target CPU. Hence if the current ACPI version doesn’t support such re-direction, we disable that CPU by making it not-removable. In such cases you will also notice that the online file is missing under cpu0.
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
Sample session:
How do I verify cpu is online and offline?
Type the following cat command to see a list of cpus which are online:
# cat /sys/devices/system/cpu/online
To see a list of all offline cpus, run:
# cat /sys/devices/system/cpu/offline











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
You can know online core number by this command:
lscpu | grep ^On
Does anyone know if this actually de-energises the CPU? In other words, does the disabled core/processor still use power?
See this doc for more on this feature. https://www.kernel.org/doc/Documentation/cpu-hotplug.txt
Q: Why can’t i remove CPU0 on some systems?
A: Some architectures may have some special dependency on a certain CPU.
For e.g in IA64 platforms we have ability to sent platform interrupts to the
OS. a.k.a Corrected Platform Error Interrupts (CPEI). In current ACPI
specifications, we didn’t have a way to change the target CPU. Hence if the
current ACPI version doesn’t support such re-direction, we disable that CPU
by making it not-removable.
In such cases you will also notice that the online file is missing under cpu0.
Christian: changing a state on a CPU core does not remove it from Linux. Thus, the device would still be polled and would require power. One way to disable a core would be outside of the OS. If supported, one would change the number of CPU cores within the bios. This of course would require an outage.
If my server has 2 physical processor and I want to disable and remove only one processor which has 2 cores if I make offline the CPU cores as per above given commands to remove one physical processor but I dont want to get stop/kill/terminate any job which is handeling by that CPU which i want to remove.
Is there any possibility to migrate all running process from one CPU/Cores to another CPU/Cores or stop handelling any new request to any specific CPU/cores.
I am able to disable the CPU By this process but it get enabled again after every reboot.
It is safe to make a cpu offline and bring it back online if the CPU is stuck with softlock error with CPU showing 100% utilization ? Will it affect the applications which were being served before the CPU went into a softlock ?