Memory consumption

(this post will also appear in our virtual private server blog)

Nowadays, CPU performance is something that is pretty easy to achieve with comparably little investment. Intel’s L/X34xx, E/L/X56xx, or the latest E3 12xx(L) CPUs as well as AMD’s new Opteron 41xx series will cover most requirements one could come up with for a startup site, even with a considerable number of users online at the same time.

A fact, however, that is often neglected, is memory consumption. Memory is key and crucial to your website performing well and supporting a large number of users online at the same time.

Let us consider a typical LAMP environment, i.e. you are running Linux, Apache, MySQL, and PHP. The operating system will not need much overhead, typically a couple hundred MB should be enough, MySQL will need what you give to it (large buffers, high number of connections, etc. increase memory usage, naturally), and PHP is a double-edged sword in many cases anyway. One of the largest memory hog in moderately busy machines is the webserver itself – most often Apache. Even if carefully tuned, and going to extreme setups with either prefork or worker, you can end up anywhere between 5 to 15 MB per connection. Depending on the number and sort of modules you need, this will vary a lot – lightweight servers will be closer to the lower end, but an Apache having everything enabled is more likely to end up with 15MB of RAM per connection – there are alternatives to Apache, and most control panels using Apache will go for the prefork MPM, and unless you want to recompile and configure on your own, this is basically what you have to live with.

Let’s assume 10MB per connection, an average value. Imagine you want to be able to serve 500 simultaneous connections – with 10MB per connection, you are going to need 5GB of RAM for the webserver alone! If a lot of these connections are database intensive, then you might be coming close to 8GB of RAM usage already: so in order to have some safety margin, you should be going for 12GB or 16GB of RAM in your dedicated server (depending on CPU architecture and memory channels) or virtual private server.

Even on a site that is not really busy, with maybe 50 connections at the same time, you should be aware of overall memory consumption: 50 x 10MB, plus OS overhead, MySQL, and PHP will most likely add up to 2GB of memory as well, so here you should be going for 4GB already to have some growth potential.

What happens if the machine runs out of connections and/or memory, and eventually also out of swap space?

  • your site will respond more slowly;
  • your site may stop processing requests altogether;
  • your site may throw errors;
  • your site might become inconsistent: nothing is worse than an partially processed request that gets through to the database backend at random only;
  • your site may become unreachable;

A dedicated server will be able to handle such problems somewhat better than a virtual private server – due to its architecture and swap space available. The latter often does not have any swap space (there are virtualisation architectures where this is not true, however, such as KVM, for example) and will be hit full swing instead of being able to mitigate the crisis arising from low memory.

You want to avoid such scenarios – for the sake of your business and your customers. Do not save on memory – better have some spare than have none left.

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