How to strip down Amazon Linux EC2 to maximize available RAM

May 29, 2016 by Hayden James, in Blog Linux

Firstly, this article mainly applies to low memory Amazon Linux EC2 instance types such as t2.nano and t2.micro. The instructions below are also specific to Amazon Linux AMI, which like CentOS, is based on RHEL (Red Hat Enterprise Linux). Note: This article is from 2016 and applies to Amazon Linux Version 1 only.

Table of Contents

Update all packages – yum package manager

First, login to your new Amazon Linux EC2 instance and update all packages:

sudo yum update

Then if you are not going to use MTA (mail transfer agent) you can disable sendmail service:

sudo chkconfig sendmail off
sudo service sendmail stop

Reduce the number of getty services

Edit /etc/sysconfig/init and replace:

ACTIVE_CONSOLES=/dev/tty[1-6]

with…

ACTIVE_CONSOLES=/dev/tty[1-1]

Replace with agetty with mingetty

Update! – AWS now installs mingetty by default. In which case if the output of the install command is “already installed”, then simply make the line change to the /etc/init/serial.conf file as described below.

Since agetty is heavier on RAM, lets replace with mingetty.

Install mingetty first:

sudo yum install mingetty

then edit /etc/init/serial.conf and replace:

exec /sbin/agetty /dev/$DEV $SPEED vt100-nav

with

exec /sbin/mingetty /dev/$DEV $SPEED vt100-nav

Disable yum-updatesd and replace it with a simple cron job

Update! – AWS no longer installs yum-updatesd by default. You can still setup the cron if you’d like. Check to see if yum-updatesd is installed using this command to list: chkconfig

This will save resident memory.

sudo chkconfig yum-updatesd off

sudo yum remove yum-updatesd

Next, create yum update cron instead. Add a new file /etc/cron.daily/yum.cron with contents:

#!/bin/sh 
/usr/bin/yum -R 120 -e 0 -d 0 -y update yum 
/usr/bin/yum -R 10 -e 0 -d 0 -y update

Followed by:

sudo chmod +x /etc/cron.daily/yum.cron

Disable IPv6 support

sudo chkconfig ip6tables off

Disable Network Time Protocol (NTP) daemom

Next, disable ntpd. Run “top” and press shift + M to sort by memory usage, you’ll notice that ntpd is close to the top of the list. Its used to keep your server clock in sync. You can replace with weekly cron so you can disable the service and further reduce memory usage.

sudo service ntpd stop
sudo chkconfig ntpd off
sudo chkconfog ntpdate off

Now add a new file named ntpdate-sync to the /etc/cron.weekly directory with the contents:

#! /bin/sh
/usr/sbin/ntpdate pool.ntp.org

Followed by:

sudo chmod +x /etc/cron.weekly/ntpdate-sync

To test run:

sudo /etc/cron.weekly/ntpdate-sync

Output should be something like:

11 Oct 22:57:49 ntpdate[1174]: adjust time server 97.107.134.213 offset -0.017816 sec

These are some basic steps to lower memory consumption of first boot. More noticeable on the smaller Amazon Linux EC2 instances.

Tuning Amazon Linux EC2 swappiness and cache pressure

Another method of squeezing the most from your Amazon Linux EC2’s limited RAM, is to tune the system’s swappiness (tendency to swap) and cache pressure (tendency to reclaim cache).

swappiness (Recommended value 10 to 60. 0 if you don’t have swap added) – This control is used to define how aggressive the kernel will swap memory pages. Higher values will increase aggressiveness, lower values decrease the amount of swap. (default = 60)

vfs_cache_pressure (Recommend value 50 to 200) – Controls the tendency of the kernel to reclaim the memory which is used for caching of directory and inode objects. (default = 100)

Add these lines to the end of the /etc/sysctl.conf file.

vm.swappiness=10
vm.vfs_cache_pressure=200

Increasing the cache pressure may be somewhat counter productive since caching is good for performance. However, swapping too often can also reduce your server’s overall performance. Use this for example if free -m shows say more than 60% of RAM being used by cache/buffers …remember this NOT a bad thing! However, depending on what is being cached you may reduce memory usage/contention and as a result gain performance by adjusting.

To check current values using these commands:

sudo cat /proc/sys/vm/swappiness
sudo cat /proc/sys/vm/vfs_cache_pressure

To enable these settings without rebooting use the following commands:

sudo sysctl -w vm.swappiness=10
sudo sysctl -w vm.vfs_cache_pressure=200

How To Add Swap on Amazon Linux EC2

Of course, on low-memory instances swap is wise. To add a 1GB swap file for example, from command line you’ll type:

sudo dd if=/dev/zero of=/swapfile bs=1024 count=1048576

Now setup the swap file with the command:

sudo mkswap /swapfile
sudo chmod 600 /swapfile

Now enable the swap:

sudo swapon /swapfile

If you use the top command, you should now see the 1gb swap added. So now lets make swap persistent so it’s not dropped when you reboot. Edit /etc/fstab file and add this line as the last line:

/swapfile swap swap defaults 0 0

When you reboot, use the free -h or df -h command to check for swap.

Remember, adding swap can help save your server from running out of memory but if it’s already using a big chunk of swap (aka swapping), that is never good for performance. A lot can be expanded upon with regards to swap and paging/swapping. However, the point today is that stripping/tuning the AMI.

Note: this article was originally published on Nov 21, 2013. It has been updated to ensure that the suggested changes are still compatible.

Also see: Strip Down Apache to Improve Performance & Memory Efficiency