Jay Taylor's notes

Transparent Huge Pages (THP) is a Linux memory management system that reduces the overhead of Translation Lookaside Buffer (TLB) lookups on machines with large amounts of memory by using larger memory pages.

However, database workloads often perform poorly with THP, because they tend to have sparse rather than contiguous memory access patterns. You should disable THP on Linux machines to ensure best performance with MongoDB.

Init Script¶

Important:

If you are using tuned or ktune (for example, if you are running Red Hat or CentOS 6+), you must additionally configure them so that THP is not re-enabled. See Using tuned and ktune.

Create the `init.d` script.¶

Create the following file at /etc/init.d/disable-transparent-hugepages:

#!/bin/sh
### BEGIN INIT INFO
# Provides:          disable-transparent-hugepages
# Required-Start:    $local_fs
# Required-Stop:
# X-Start-Before:    mongod mongodb-mms-automation-agent
# Default-Start:     2 3 4 5
# Default-Stop:      0 1 6
# Short-Description: Disable Linux transparent huge pages
# Description:       Disable Linux transparent huge pages, to improve
#                    database 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

    unset thp_path
    ;;
esac

Make it executable.¶

Run the following command to ensure that the init script can be used:

sudo chmod 755 /etc/init.d/disable-transparent-hugepages

Configure your operating system to run it on boot.¶

Use the appropriate command to configure the new init script on your Linux distribution.

Distribution	Command
Ubuntu and Debian	sudo update-rc.d disable-transparent-hugepages defaults
SUSE	sudo insserv /etc/init.d/disable-transparent-hugepages
Red Hat, CentOS, Amazon Linux, and derivatives	sudo chkconfig --add disable-transparent-hugepages

Override tuned and ktune, if applicable¶

If you are using tuned or ktune (for example, if you are running Red Hat or CentOS 6+) you must now configure them to preserve the above settings.

Using `tuned` and `ktune`¶

Important:

If using tuned or ktune, you must perform this step in addition to installing the init script.

tuned and ktune are dynamic kernel tuning tools available on Red Hat and CentOS that can disable transparent huge pages.

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

Red Hat/CentOS 6¶

Create a new profile.¶

Create a new profile from an existing default profile by copying the relevant directory. In the example we use the default profile as the base and call our new profile no-thp.

sudo cp -r /etc/tune-profiles/default /etc/tune-profiles/no-thp

Edit `ktune.sh`.¶

Edit /etc/tune-profiles/no-thp/ktune.sh and add the following:

set_transparent_hugepages never

to the start() block of the file, before the return 0 statement.

Enable the new profile.¶

Finally, enable the new profile by issuing:

sudo tuned-adm profile no-thp

Red Hat/CentOS 7¶

Create a new profile.¶

Create a new tuned profile directory:

sudo mkdir /etc/tuned/no-thp

Edit `tuned.conf`.¶

Create and edit /etc/tuned/no-thp/tuned.conf so that it contains the following:

[main]
include=virtual-guest

[vm]
transparent_hugepages=never

Enable the new profile.¶

Finally, enable the new profile by issuing:

sudo tuned-adm profile no-thp

Test Your Changes¶

You can check the status of THP support by issuing the following commands:

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

On Red Hat Enterprise Linux, CentOS, and potentially other Red Hat-based derivatives, you may instead need to use the following:

cat /sys/kernel/mm/redhat_transparent_hugepage/enabled
cat /sys/kernel/mm/redhat_transparent_hugepage/defrag

For both files, the correct output resembles:

always madvise [never]

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Jay Taylor's notes