What are all these rcuos processes running on my server?

Question

I just started my first home server running Ubuntu 14.04.2 Server Edition.

I installed OpenSSH and that's it; I haven't even opened the port yet to make it accessible to the outside world.

I ran top just to see what was running and I got this:

What are all these rcuos/ processes?

Answer 1

As answered here, these processes belong to "Reduce OS Jitter" which is turned on by the following kernel flags

CONFIG_RCU_NOCB_CPU
CONFIG_RCU_NOCB_CPU_ALL
CONFIG_RCU_STALL_COMMON
CONFIG_RCU_USER_QS

Since 13.10 (Saucy) they are enabled by default.

See:

• http://ubuntuforums.org/showthread.php?t=2225218
• https://www.kernel.org/doc/Documentation/kernel-per-CPU-kthreads.txt

Answer 2

This is part of the kernel's internal read/copy/update system. They're not real processes, just kernel scheduling entities. If you look at them with top, they'll show in brackets. They're used as needed to clean up stale information once it's no longer being accessed, dispatching the work to avoid spikes in kernel latency.

You can find more information here if you really want the ugly details.