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Often Overlooked Linux OS Tweaks

Kirk Kirkconnell of Couchbase Published

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There are two simple linux OS level settings that people seem to be overlooking setting correctly on their production systems I have seen. These are documented elsewhere, but they keep coming up and seems like they need some quick review here. It is not like these are some super secret setting or magic bullet performance fixing items necessarily, but they are things that in a production Couchbase DB should be set correctly as below and incorporated into whatever system/process you use to bootstrap the nodes you use for Couchbase. They help with memcached performance and rebalance performance and in some cases stability issues.

Please make sure you test these out in a test environment first before moving to production with them obviously.

Swappiness should to be turned off

This one is pretty straightforward if you know about the Linux virtual memory system. Swappiness levels tell the virtual memory subsystem how much it should try and swap to disk. The thing is, the system will try to swap out items in memory even when there is plenty of RAM available to the system. The OS default is usually 60, which is a little aggressive IMO. You can see what value your system is set to by running the following command:

cat /proc/sys/vm/swappiness

Since Couchbase is tuned to really operate in memory as much as possible. You can gain or at minimum not lose performance by just changing the swappiness value to 0. In non-tech talk, this tells the virtual memory subsystem of the OS to not swap items from RAM to disk unless it really really has to, which if you have sized your nodes correctly, swapping should not be needed. To set this, perform the following process use sudo or just become root if you ride in the wild west.

# Set the value for the running system
sudo echo 0 > /proc/sys/vm/swappiness

# Backup sysctl.conf
sudo cp -p /etc/sysctl.conf /etc/sysctl.conf.`date +%Y%m%d-%H:%M`

# Set the value in /etc/sysctl.conf so it stays after reboot.
sudo echo '' >> /etc/sysctl.conf
sudo echo '#Set swappiness to 0 to avoid swapping' >> /etc/sysctl.conf
sudo echo 'vm.swappiness = 0' >> /etc/sysctl.conf

Make sure that you either have or modify your process that builds your OSs to do this. This is especially critical for public/private clouds where it is so easy to bring up new instances. You need to make this part of your build process for a Couchbase node.

Disable Transparent Huge Pages (THP)

Starting in Red Hat Enterprise Linux (RHEL) version 6, so this includes CentOS 6 and 7 too, a new default method of managing huge pages was implemented in the OS. It combines smaller memory pages into Huge Pages without the running processes knowing. The idea is to reduce the number of lookups on TLB required and therefor increase performance. It brings in abstraction for automatation and management of huge pages basically.  Couchbase Engineering has determined that under some conditions, Couchbase Server can be negatively impacted by severe page allocation delays when THP is enabled. Couchbase therefore recommends that THP be disabled on all Couchbase Server nodes

To disable this on a running system temporarily, run the following commands:

# Disable THP on a running system
sudo echo never > /sys/kernel/mm/transparent_hugepage/enabled
sudo echo never > /sys/kernel/mm/transparent_hugepage/defrag

To disable this permanently, do the following:

# Backup rc.local
sudo cp -p /etc/rc.local /etc/rc.local.`date +%Y%m%d-%H:%M`

Then copy the following into the bottom of /etc/rc.local.

if test -f /sys/kernel/mm/transparent_hugepage/enabled; then
  echo never > /sys/kernel/mm/transparent_hugepage/enabled

if test -f /sys/kernel/mm/transparent_hugepage/defrag; then
   echo never > /sys/kernel/mm/transparent_hugepage/defrag


If you do not want to reboot just yet, just use the echo commands above and know that the next time you reboot this will also be in place. Some people like to reboot if there is some change to such a file. Some do not.

Note: There is a different way to do this that you will find elsewhere and edits /etc/grub.conf. My problem with it is that it would get blown out with each and every kernel update in the future. What I propose is easier to manage in the long run and easy to put into something like Puppet module or Chef recipe to append to the end of rc.local when you boot strap a node.

THP is a great feature for some things, but causes problems with applications like Couchbase. It is not alone in this. If you go search the Internet for transparent huge pages, there are multiple documented issues from other DB and application vendors about this. Until something has been found to work with this, it is just best to turn THP off.

Just remember, THP only effects v6.x and newer versions of CentOS and it seems Redhat on 6.0 to 6.3. As of v6.4, Redhat looks to have implemented a memory profile system that has THP off by default, but I cannot find a lot of information on this yet. More as I know it.

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