Symptoms

Memory you have hot added in Linux guest virtual machines is not registered using the free or top commands.

Cause

This issue occurs when Linux requires manual intervention to expose the newly added memory in the guest. The process is called setting the memory to online.

Resolution

To resolve this issue, set the memory online in RHEL/CentOS, and Ubuntu/Debian.

Verify if the operating system actually sees the newly added memory:

grep -i MemTotal /proc/meminfo

Look for memory that appears offline (to tools like free and top):

grep line /sys/devices/system/memory/*/state
BASH TIP: You can copy/paste multi-line bash/shell Linux scripts directly from websites like this one and run them without syntax errors. You never had to create a .sh file first and then execute it.
Type ( and then paste the contents of your clipboard, and then type ) and press enter.

Use the following shell script to set all memory files to 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

You can run the script twice and you should see similar output:

Found ram: /sys/devices/system/memory/memory0/state:online ...
Already online
Found ram: /sys/devices/system/memory/memory10/state:online ...
Already online
Found ram: /sys/devices/system/memory/memory11/state:online ...
Already online
Found ram: /sys/devices/system/memory/memory12/state:online ...
Already online
Found ram: /sys/devices/system/memory/memory13/state:online ...
Already online
Found ram: /sys/devices/system/memory/memory14/state:online ...
Already online
Found ram: /sys/devices/system/memory/memory15/state:online ...
Already online
Found ram: /sys/devices/system/memory/memory1/state:online ...
Already online
Found ram: /sys/devices/system/memory/memory2/state:online ...
Already online
Found ram: /sys/devices/system/memory/memory3/state:online ...
Already online
Found ram: /sys/devices/system/memory/memory4/state:online ...
Already online
Found ram: /sys/devices/system/memory/memory5/state:online ...
Already online
Found ram: /sys/devices/system/memory/memory6/state:online ...
Already online
Found ram: /sys/devices/system/memory/memory7/state:online ...
Already online
Found ram: /sys/devices/system/memory/memory8/state:online ...
Already online
Found ram: /sys/devices/system/memory/memory9/state:online ...
Already online

Or you can manually update each file when memory appears offline:

echo online >/sys/devices/system/memory/memory[number]/state

Verify you can see the extra memory by running this command:

free -m

Turn On all Available Guest vCPU and Memory

The script below will turn on all available vCPU and memory on your VM:

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

# Bring CPUs online
for CPU in $(ls /sys/devices/system/cpu/ |grep -E '(cpu[0-9])')
do
        CPU_DIR="/sys/devices/system/cpu/${CPU}"
        echo "Found cpu: \"${CPU_DIR}\" ..."
        CPU_STATE_FILE="${CPU_DIR}/online"
        if [ -f "${CPU_STATE_FILE}" ]; then
                STATE=$(cat "${CPU_STATE_FILE}" | grep 1)
                if [ "${STATE}" == "1" ]; 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

 

Source & credit: http://www.supermaru.com/2016/10/hot-add-cpu-memory-ubuntu-guest-vmware/

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