Disabling Transparent Huge Pages (THP)

concept

Transparent huge pages (THP) is a memory management system that is enabled by default in most Linux operating systems.

You must disable THP in order for Enterprise Analytics to function on Linux, since THP enabled can worsen performance and increasing the risk of out-of-memory (OOM) conditions.

In Linux operating systems, THP is a feature that provides a way for the CPU and OS to create pre-allocated contiguous memory space to improve application performance. THP automates the creation of contiguous memory space and conceals much of the complexity of using actual huge pages on systems with large amounts of memory.

THP is enabled by default in most Linux operating systems, and functions well for most applications and processes. However, THP is detrimental to Couchbase’s performance.

Since you tend to have more random, sparse data access, you allocate pages that can remain mostly empty. This leads to memory fragmentation as portions of memory are not used but still accounted for in the RSS. As a result, the data stored may be smaller while RSS can be more, increasing the risk of out-of-memory (OOM) conditions. Therefore, you must disable THP on Linux systems so that Enterprise Analytics performs optimally.

Using Init Script

Create the init script:

vi /etc/init.d/disable-thp

Add the following contents:

#!/bin/bash
### BEGIN INIT INFO
# Provides:          disable-thp
# Required-Start:    $local_fs
# Required-Stop:
# X-Start-Before:    enterprise-analytics
# Default-Start:     2 3 4 5
# Default-Stop:      0 1 6
# Short-Description: Disable THP
# Description:       Disables transparent huge pages (THP) on boot, to improve
#                    Couchbase performance.
### END INIT INFO

case $1 in
  start)
    if [ -d /sys/kernel/mm/transparent_hugepage ]; then
      thp_path=/sys/kernel/mm/transparent_hugepage
    elif [ -d /sys/kernel/mm/redhat_transparent_hugepage ]; then
      thp_path=/sys/kernel/mm/redhat_transparent_hugepage
    else
      return 0
    fi

    echo 'never' > ${thp_path}/enabled
    echo 'never' > ${thp_path}/defrag

    re='^[0-1]+$'
    if [[ $(cat ${thp_path}/khugepaged/defrag) =~ $re ]]
    then
      # RHEL 7
      echo 0  > ${thp_path}/khugepaged/defrag
    else
      # RHEL 6
      echo 'no' > ${thp_path}/khugepaged/defrag
    fi

    unset re
    unset thp_path
    ;;
esac

Save and close your editor.

Make the script executable:
```
sudo chmod 755 /etc/init.d/disable-thp
```

Configure the OS to run the script on boot:

Red Hat
Ubuntu and Debian

sudo chkconfig --add disable-thp

sudo update-rc.d disable-thp defaults

Override tuned and ktune, if necessary:

If you use tuned or ktune (for example, if you run Red Hat 7+) you must also configure them to preserve the above settings after reboot.
Reboot the system and verify that THP is disabled:

If Using `tuned` and `ktune`

tuned and ktune are system monitoring and tuning tools available on Red Hat. When they’re in use on a system, you can use them to enable and disable THP.

To disable THP in tuned and ktune, you need to edit or create a new profile that sets THP to never.

Red Hat 7

Create a new tuned directory for the new profile:
```
sudo mkdir /etc/tuned/no-thp
```

Create and edit tuned.conf:

vi /etc/tuned/no-thp/tuned.conf

Add the following contents:

[main]
include=virtual-guest

[vm]
transparent_hugepages=never

Save and close your editor.

Enable the new profile:
```
sudo tuned-adm profile no-thp
```

Verify THP Status

You can check the THP status by issuing the following commands.

Red Hat

cat /sys/kernel/mm/transparent_hugepage/enabled

cat /sys/kernel/mm/transparent_hugepage/defrag

Other Linux Variants

cat /sys/kernel/mm/transparent_hugepage/enabled

cat /sys/kernel/mm/transparent_hugepage/defrag

If THP is properly disabled, the output of both commands should be the following:

always madvise [never]

Using a THP Service

Create a service file:

vi /etc/systemd/system/disable-thp.service

Add the service configuration details to the file and then save it:

[Unit]
Description=Disable Transparent Huge Pages (THP)
DefaultDependencies=no
After=sysinit.target local-fs.target
Before=enterprise-analytics.service

[Service]
Type=oneshot
ExecStart=/bin/sh -c 'echo never | tee /sys/kernel/mm/transparent_hugepage/enabled > /dev/null'
ExecStart=/bin/sh -c 'echo never | tee /sys/kernel/mm/transparent_hugepage/defrag > /dev/null'

[Install]
WantedBy=basic.target

Reload the systemctl files:
```
sudo systemctl daemon-reload
```
Start the service:
```
sudo systemctl start disable-thp
```
Make sure that the service starts whenever you reboot it.
```
sudo systemctl enable disable-thp
```

Verify THP is Disabled

Execute the following commands to make sure the service has disabled the THP.

cat /sys/kernel/mm/transparent_hugepage/enabled
cat /sys/kernel/mm/transparent_hugepage/defrag