Comments on: Linux Hotplug a CPU

By: Alex

Alex — Thu, 23 Jun 2011 01:46:54 +0000

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.

By: Martin Dimitrov

Martin Dimitrov — Mon, 07 Feb 2011 22:04:52 +0000

I also get the same problem: “echo: write error: Invalid argument”
Did anyone find a resolution for this?

Thank you

By: VA_DOS

VA_DOS — Tue, 21 Dec 2010 20:53:07 +0000

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

By: Callan Bryant

Callan Bryant — Mon, 09 Aug 2010 15:17:55 +0000

sudo does not support the > operator
Run the command using sudo -i to get an interactive prompt first

By: Anonymous

Anonymous — Tue, 15 Jun 2010 12:16:06 +0000

Could you please tell us how you managed, I get the same error “access denied” when run as root

By: Fabian Kempers

Fabian Kempers — Tue, 08 Sep 2009 11:15:29 +0000

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?

By: mangoo

mangoo — Wed, 01 Jul 2009 09:42:18 +0000

Not only high-end servers – CPU/memory hotplugging is often used in virtualized systems.

By: Stephan

Stephan — Mon, 20 Apr 2009 11:03:48 +0000

Ok, I’v managed.
Thx.
S.

By: Stephan

Stephan — Mon, 20 Apr 2009 07:34:09 +0000

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.

By: Vivek Gite

Vivek Gite — Sun, 19 Apr 2009 16:03:37 +0000

You must be root user to hotplug a CPU.

By: Stephan

Stephan — Sun, 19 Apr 2009 15:57:36 +0000

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

By: jrush

jrush — Sun, 19 Apr 2009 13:09:31 +0000

It’s really works !
On my desktop Atom 330 (dual core + hyperthreading = 4 CPU), i can disable 3 CPUs of 4 :)

By: Vivek Gite

Vivek Gite — Fri, 03 Apr 2009 15:01:22 +0000

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

By: Miker

Miker — Fri, 03 Apr 2009 14:32:13 +0000

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?

By: Vivek Gite

Vivek Gite — Fri, 03 Apr 2009 14:29:01 +0000

Just unmount it and if it is hotplug just replace it.

By: Akshay Sulakhe

Akshay Sulakhe — Fri, 03 Apr 2009 13:57:22 +0000

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???