Free up inactive memory in OS X

Let me preface this by stating that normally you should not have to do this, and it is typically best to let the OS manage the RAM usage. I suggest you use this method only when necessary rather than as a standard practice. I'm also making an assumption that your familiar with the Terminal app, primarily in the situation where you want to schedule this. If you are not, you may need to do some research on Terminal outside of this post.

If you're like me, you've had an issue on an OS X server and when you check it out there is little to no free RAM available but you have a bunch stuck in the state known as inactive memory. This typically indicates one of two things. Either you need more RAM to run your server applications effectively, or something is causing a memory leak. In my case it is a little of both. However, running out and picking up Apple Server memory on a whim isn't always an option. Tracking down a faulting program isn't always easy, or quick either. If you're struggling to keep an OS X server accessible while waiting for an upgrade window or to give yourself time to troubleshoot, or if you're having problems with the personal Mac's memory allocation, this may be a temporary workaround you can use.

There is a command named "purge" that will free up the inactive memory. You can read more about it on the purge man page. You can simply open Terminal and issue the command, then press Enter. You may need to use sudo purge, but nonetheless you can invoke purge and free up some RAM immediately.

However, if this is a server or continues happening, you may want to script this command and call it on a schedule. While waiting for a shipment of RAM for a couple OS X servers having this issue, I used this script provided by Daniel Payne on stackoverflow.com in this article.

#!/bin/bash
free=`vm_stat | grep free | awk '{print $3}'`
freer=${free%%.*}
if [ "$freer" -lt "18000" ]
then
    nice purge
fi

I opened Terminal and used vi to create the script file, but you should be able to use Textedit or any text editor you'd like. Just make sure to name it with the extension of .sh at the end, so your file should be named something like freeRAM.sh. What this script does is run the purge command if there are less than 18000 free memory pages available. You will want to modify that value to match the minimum amount of memory you will accept before running the command. This eliminates unnecessary running of the purge command. Remember that this is the value in memory pages, not B/KB/MB/GB. If you don't know page size your system is using, you can run vm_stat within Terminal and it will tell you in the first line. For my server I was using the default page size of 4096 bytes. This means that if I wanted to run the purge command if there was less than 200MB of free memory, I would need to substitue the 18000 in the script above with

200MB * 1024KB/1MB * 1024B/KB * 1PG/4096/KB = 51200 (instead of 18000)

Once you have your shell script saved you need to add it to the schedule. You can use cron, but it appears to be deprecated in newer version of OS X so instead we'll use launchctrl. I used the guide found here to create my .plist file and get it into the launch daemons schedule.

You can also create your script using Automator and then schedule it using iCal. However, I wanted to run the script multiple times a day, and it appears that using iCal allows once a day as the most frequent option.