Permission to Disable Cores?

[Neo Zhou]

Hi, 

My research group is in the process of submitting an artifact for the SOSP conference. The server machines we used for our original specification used 4 cores, but the smallest number of cores for cloudlab hardware seems to be 8. We tried using XenVMs, to limit the number of cores used for the server machines, but my collaborators found severely reduced performance. Instead, we were hoping to manually disable cpu cores by editing the /sys directory via a command like echo 0 > /sys/devices/system/cpu/cpu0/online. I tried this and got a permission denied. Is there any way that we can enable this for ourselves, and our reviewers?

Thanks for your time,
Neo

[mma...@wisc.edu]

If you are on a bare-metal machine, you should be able to do that as root.

echo 0 | sudo tee /sys/devices/system/cpu/cpu0/online

You may get errors due to the kernel not being able to hot-unplug a core. In that case, you can look for more info in the dmesg log or else go look at the kernel code to see what the error might be.

[Neo Zhou]

Weird, when I run the command above on my experiment with bare metal machines (zhouaea-104241), I get a "permission denied" message. Attached is what I see. 

[Neo Zhou]

I had to change my experiment to zhouaea-104274 for another reason, but I tried the command above again. I still got a permission denied message.

[mma...@wisc.edu]

Does it still happen if you try to disable other cores?

Also, is this a VM or bare metal?

[Neo Zhou]

Oh, I see. I tried to disable cpu1 and it worked correctly. It seems like cpu0 is off limits. All of my machines are bare metal. Thanks for your help!