Optimising Web Servers

I was lucky enough to attend PyCon-AU recently and one talk in particular highlighted the process of web server optimisation.

Graham Dumpleton’s add-in talk Web Server Bottlenecks And Performance Tuning available on YouTube (with the majority of PyCon-AU talks)

The first big note at the beginning is that the majority of the delay in user’s perception of a website is caused by the browser rendering the page. Though not covered in the talk for those that haven’t used the tool YSlow (for Firefox and Chrome) or Google’s Developer Tools (ctrl-alt-I in Chrome), both tools will give you pretty much identical recommendations as to how to configure the application page generated and server caching /compression settings to maximise the ease at which a web browser will render the page. These recommendations also will also minimise the second most dominate effect in web pages displayed, network latency and bandwidth. Once you have completed this the process of making web pages faster on the web server begins to take a measurable effect to the end user.

The majority of the talk however continues talking about web server configuration. The issues you will find at the web server are the memory, CPU and I/O are the constraints that you may hit depending on your application.

Measuring memory usage by considering an applications use of memory multiplies by how many concurrently running processes will give you an idea of how much memory is needed. Remember always that spare memory is disk cache for Linux based systems and this is significant in reducing I/O read time for things like static content serving. Memory reduction can be helped by front-end proxying as described by the question at offset 19:40 and relating it to the earlier description of threads and processes.  In short the buffering provided by Nginx in a single process on the input ensures that the application code isn’t running until a large amount of input is ready and that output is buffered in Nginx such that the process can end quicker while Nginx trickles the web page out to the client depending on the network speed. This reduction in the running time of the application enables the server to support better concurrency and hence better memory usage. This is why we at Open Query like to Nginx as the web server for the larger websites of our clients.

Database is effectively an I/O constraint from the web server perspective as it should be on a different server if you run something more than a simple blog or an application where database utilisation is very low.  A database query that requires input from the web request that takes a long time to run will add to the time taken in rendering the page in fairly significant terms. Taking note of which queries are slow, like enabling the slow query log is the first step to identifying problem. Significant gains can usually be made by using indexes and using the database rather than the application to do joins, iterations and sorting. Of course much more optimisation of server and queries is possible and Open Query is happy to help.

Thanks again to PyCon speakers, organisers, sponsors and delegates. I had a great time.

3 Responses to “Optimising Web Servers”

  1. The bit I forgot to mention was http://wiki.nginx.org/X-accel / X-Sendfile . These configurations allow a backend application to tell the web server to serve up the content specified by the x-accel-redirect HTTP header in Nginx or X-Sendfile HTTP header in lighttpy and Apache’s HTTPd external mod_xsendfile https://tn123.org/mod_xsendfile/. This is ideal for large files that need to be served up but require some authentication or logging before this occurs. Using these options, the backend finishes sooner, hence lower concurrency and memory efficiency, and the web servers do what they are good at – serving static content.

  2. Can’t wait to see the slides (I’m allergic to videos), sounds like a good talk!

    I gave a somewhat similar presentation recently at the eZ Summercamp in Croatia. It focises on the eZ Publish CMS, but has a lot of generic advice as well.

    Slides available at http://www.slideshare.net/gggeek/ez-performance-measurement

  3. great. Slides are the same as what he did in US: http://www.slideshare.net/GrahamDumpleton/pycon-us-2012-web-server-bottlenecks-and-performance-tuning

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