Straylight Run

Apache Bench is either the first or second most useful PHP tool (with Xdebug being the other). I described the basic theory of Apache Bench in an earlier post. That’s a short post, so I won’t repeat it. This will be another short post, with a small note on how I use it day-to-day. If you are changing something in the system, a piece of code, a database setting, an OS setting… anything! for performance reasons, and you want to see if it makes any difference, use Apache Bench. Fire up a quick test before the change, and after the change. ab runs very quickly (on the order of a few minutes on a slow machine), so you can run 1000 requests and not have to worry about your sample size. I even run it on my laptop. Even though my laptop introduces a lot of noise, it still gives relative results. I usually run it two ways before the change, and two ways after.

% ab -n 1000 -c 1 http://www.whatever.com

That usually gets me a good idea of improving performance.

% ab -c 100 -t 60 http://www.whatever.com

That usually gets me a good idea of scaling under load.

UPDATE: There have been reports that Apache Bench is not reliable.

When we talk about performance, it is the response time of a single request, web page, SQL query, etc. It is the actual execution time for something in the absence of load. To illustrate, suppose you wanted to test the performance of a web page using Apache Bench. You should run something like:

% ab -n 1000 -c 1 http://www.whatever.com

The -n is the number of requests and the -c is the number of concurrent requests. Since we’re interested in end-to-end response time, we only need one concurrent request. Scalability is usually about throughput, or the number of concurrent requests within a certain period of time. Using the example above, the Apache Bench command should be something like:

% ab -c 100 -t 60 http://www.whatever.com

The -t is the amount of time to run the test. We can vary -c until individual response times begin to grow, at which point something in the system has reached its maximum capacity.