Making WordPress Stable on EC2-Micro
EC2 Micro Instance LimitationsEC2 offers a lot of advantages over many web site hosting options. I am a bit of a control freak and like having full control over my web server. This has advantages and disadvantages of course, meaning more work but more flexibility. Running a WordPress blog on a micro instance can be a serious challenge. I have fought with getting my site to have a minimum level of stability, and here are some of my notes on what helped. Amazon offers a free EC2 micro instance for a year to new users, so it is a very attractive option for hosting a web site. The EC2 micro instance is pretty cheap compared to the other system options that Amazon offers, but there are some caveats that may shock you after using it for a while. There are a couple of major problems with using this option for hosting a website:
- CPU Usage restrictions: If you use 100% CPU for more than a few minutes, Amazon will “steal” CPU time from the instance, meaning that they throttle your instance. This can last (from my observations) as long as five minutes, and then you get a few seconds of 100% again, then the restrictions are back. This will cripple your website, making it slow, and even timing-out requests.
- Limited Memory: The instance is limited to 613MB of RAM, and does not have a swap partition. If you run out of memory the system will panic and reboot.
Here is one symptom of CPU throttling from EC2, looking at the CPU usage from the “top” command:
According to the top man page: ”st = steal (time given to other DomU instances)”If you have more than 1000 visitors or so a day, a micro instance probably isn’t worth your time. But for many small sites (like mine) it does make sense. I wasn’t aware of these limitations before setting up my site, and I very quickly ran into site reliability issues. Here are a few of the things that I did to make my site more stable. You can save a lot of money by purchasing a reserved instance for a year, but my advice is to run for a few months before making the leap. If you find that your micro instance doesn’t cut it, you have just thrown away a chunk of cash. So, let’s look at a few of the things you can do to make a WordPress site run reasonably well on a Micro Instance:
- Tune Apache to run the correct number of threads.
- Use the minimum required memory for MySQL.
- Pre-cache your web pages.
- Use a content distribution network (CDN) such as CloudFront.
- Setup a swap partition.
- Reacting to site overload:
- Configure alerting for CPU usage and network traffic.
- Be ready to rent a larger instance if you get a big traffic spike.
- Use a 32 bit operating system.
Proactive Steps:There are a few settings that can have a big impact on site stability. Remember, a micro instance is really limited, so consider carefully if you really want to host a site on it! The examples here use Amazon’s Linux AMI (which appears to be based upon the CentOS distribution.)
Tuning Apache:You should have your site up and running in WordPress before attempting to tune it, otherwise you won’t have realistic numbers. The first step is to tune Apache. This will greatly reduce the memory usage, but does make your site more susceptible to denial of service attacks–not that a micro instance is particularly resistant to a DoS in the first place. At least normal traffic (probably) won’t cause the site to crash. The first step is to figure out how much RAM each prefork thread requires. You can get the memory usage in KB using the ‘ps’ command:
[root@domU tag]# ps xav |grep httpd 1457 ? Ss 0:25 1218 333 393026 40980 6.7 /usr/sbin/httpd -k start 15484 ? S 0:09 199 333 418642 64692 10.5 /usr/sbin/httpd -k start 15485 ? S 0:04 4 333 419330 64808 10.6 /usr/sbin/httpd -k start 15486 ? S 0:05 35 333 419590 64904 10.6 /usr/sbin/httpd -k start 15487 ? S 0:05 7 333 419174 64516 10.5 /usr/sbin/httpd -k start . . .There are two fields we are interested in, the virtual memory and real memory being used by each process (the 7th and 8th fields respectively.) We don’t need to have a real accurate number so just just use one of the values that looks reasonable. So in this case, we we will figure actual memory use for a thread by using the formula:
real * ( 1 - (real / virtual) )So, sampling one of these processes, we get:
65 132 * (1 - (65 132 / 419 842)) = 55 027.7764Or roughly 53MB per process. So, to ensure Apache doesn’t run the system out of memory we will want to limit the threads to a number that is less than the physical memory available. In this case, ten threads should be safe. This means that if we receive more than ten simultaneous requests, the other requests will be queued until a worker thread is available. In order to maximize performance, we will also configure the system to have this number of threads available all of the time. In Apache’s configuration file (/etc/httpd/conf/httpd.conf) we will modify the prefork section to look like the following:
<IfModule prefork.c> StartServers 10 MinSpareServers 10 MaxSpareServers 10 MaxClients 10 MaxRequestsPerChild 4000 </IfModule>Once the modifications are in place, you can restart by typing:
Limiting MySQL’s Memory:Once we setup caching on the site, we will greatly reduce the number of database calls. Not that a small site will have extensive database requirements. It isn’t necessary to perform extensive analysis here, just use the example configuration that is provided with the MySQL RPM package.
[root@domU etc]# cd /etc [root@domU etc]# mv my.cnf my.cnf.backup [root@domU etc]# cp /usr/share/mysql/my-small.cnf ./my.cnfThere is one modification I would recommend to this file, which is to limit connections to the localhost address by adding the following line to the [mysqld] section of the configuration:
bind-address=127.0.0.1Then restart the service.
[root@domU etc]# service mysqld restart
Pre-cache the pages:Most of the work a WordPress site does is just serving pages. There really is no reason to dynamically recreate the page from the database every time that a visitor requests an article. The easiest option is to use a WordPress plugin to create static pages. The tool I chose for this is WP Super Cache. I won’t tell you how to set it up, as the task is really easy.
Use a CDN:Not serving images and files directly from the website has a few benefits: less connections, and regional distribution. Since you are already hosted on Amazon’s services it makes sense to use CloudFront for this, and setup is super easy. Once you have the CloudFront setup and working, configure the CDN options in WP Super Cache to serve up the files.
Setup a Swap Partition:Whether you want to take this step is really up to you. Once your system starts swapping it will probably run extremely slow. But a panic and reboot can be even worse if it causes database corruption. Another downside is that if the system does start swapping it will use gobs of bandwidth because you are using a network-based disk for swap, and this isn’t free on EC2. There are good reasons not to use swap, but I personally use one just to act as a safety net. Here I create a 1GB file, format it as swap, activate it, and configure it to be mounted at boot time:
[root@domU etc]# dd if=/dev/zero of=/var/swap bs=`expr 1024 \* 1024` count=1024 [root@domU etc]# mkswap /var/swap [root@domU etc]# swapon /var/swap [root@domU etc]# echo “/var/swap none swap sw 0 0” >> /etc/fstab