How do you hotplug-enable new CPU and RAM in a virtual machine?

Question

I am running Ubuntu in a virtual machine and I'd like to add CPU and RAM without rebooting the device. How can I hotplug this new virtual hardware?

Solution 1

These can be enabled through the use of the /sys filesystem using root credentials.

For the CPU, you change the 0 to a 1 in to the appropriate file of the pattern: /sys/devices/system/cpu/cpu*/online.

For the RAM, you find the state in the files /sys/devices/system/memory/memory*/state and change the contents from offline to online.

The script below will turn all CPU and RAM online for you.

#!/bin/bash
# Based on script by William Lam - http://engineering.ucsb.edu/~duonglt/vmware/

# Bring CPUs online
for CPU_DIR in /sys/devices/system/cpu/cpu[0-9]*
do
    CPU=${CPU_DIR##*/}
    echo "Found cpu: '${CPU_DIR}' ..."
    CPU_STATE_FILE="${CPU_DIR}/online"
    if [ -f "${CPU_STATE_FILE}" ]; then
        if grep -qx 1 "${CPU_STATE_FILE}"; then
            echo -e "\t${CPU} already online"
        else
            echo -e "\t${CPU} is new cpu, onlining cpu ..."
            echo 1 > "${CPU_STATE_FILE}"
        fi
    else 
        echo -e "\t${CPU} already configured prior to hot-add"
    fi
done

# Bring all new Memory online
for RAM in $(grep line /sys/devices/system/memory/*/state)
do
    echo "Found ram: ${RAM} ..."
    if [[ "${RAM}" == *":offline" ]]; then
        echo "Bringing online"
        echo $RAM | sed "s/:offline$//"|sed "s/^/echo online > /"|source /dev/stdin
    else
        echo "Already online"
    fi
done

Solution 2

Instead of operating the kernel parameters, you can automatically enable hotplugged CPU or memory by using udev rules:

/etc/udev/rules.d/94-hotplug-cpu-mem.rules:

ACTION=="add", SUBSYSTEM=="cpu", ATTR{online}=="0", ATTR{online}="1"

ACTION=="add", SUBSYSTEM=="memory", ATTR{state}=="offline", ATTR{state}="online"

Tested on CentOS 6/7, Ubuntu 14.

Debian 7 crashed for unknown reason. Further testing would be required.

Comments

[flickerfly]

Yes, that script was proven necessary, and working.

[Napster_X]

I was also expecting Hotplug to work out of the box, but seems like it doesn't ... this is what you need to do, didn't test the script, but manually took care of it, the logic should work. Super Thanx

[Fabiano Tarlao]

It worked like a charm! Thanks. I'm just curios about how do vmware host and guest Linux OS react to this change in number of cores, at the level of NUMA (I know that vmware is quite good to communicate the right NUMA structure at the guest, but I like to know if this change si "acquired". How does this work exactly? Regards

[flickerfly]

@FabianoTarlao Are you wanting to know how the NUMA structure results in /sys/devices/system/* occurring? I've got no idea. Sounds like something you could find digging in the kernel code or openvmtools If you find out, let us know. Interesting question. github.com/vmware/open-vm-tools

[Fabiano Tarlao]

I have written it in my todo list .. for the free-time :-D