WP Super Cache reduces this strain by storing a static HTML version of the current pages and posts. This In turn means that MySQL is called fewer times thus reducing the strain. When the page changes, such as when someone comments or the post is altered, this HTML cache is simply updated.

I’ve also included benchmarks so you can compare the different options it has.

Lets get started!

Off Mode

When this option is enabled WP Super Cache is completely disabled, really only useful if you’re having problems with it.

I’m benchmarking this site with the Apache HTTP server benchmarking tool from another VPS in the same data center, running the benchmark from the same server would only give inaccurate results. I’ve also left the number of requests low otherwise the benchmark would take too long to run. I probably should mention the command I’m using  so here you are:

ab -c 5 -n 1000 http://benscobie.com/

The Requests per second is what we’re mainly looking at:

Concurrency Level:      5
Time taken for tests:   62.744147 seconds
Complete requests:      1000
Failed requests:        87
(Connect: 0, Length: 87, Exceptions: 0)
Write errors:           0
Total transferred:      4959906 bytes
HTML transferred:       4674906 bytes
Requests per second:    15.94 [#/sec] (mean)
Time per request:       313.721 [ms] (mean)
Time per request:       62.744 [ms] (mean, across all concurrent requests)
Transfer rate:          77.19 [Kbytes/sec] received

Half-on Mode

When this option is enabled static HTML files are generated so the database doesn’t need to be called, however the PHP engine still needs to be loaded to serve the cached files. Your blog however will still load a lot faster.

I’ve chosen to go with 10000 requests this time to get a better average:

Concurrency Level:      5
Time taken for tests:   34.855934 seconds
Complete requests:      10000
Failed requests:        0
Write errors:           0
Total transferred:      51030000 bytes
HTML transferred:       47980000 bytes
Requests per second:    286.90 [#/sec] (mean)
Time per request:       17.428 [ms] (mean)
Time per request:       3.486 [ms] (mean, across all concurrent requests)
Transfer rate:          1429.68 [Kbytes/sec] received

As you can see with just the Half-mode turned on the site can handle nearly 18x more requests than without it.

On Mode

This option is like the Half-On mode above but it gets around having to call the PHP engine, which in turn will help speed up your site even more. It also adds a few more features such as having the option to preload the cache. You can also enable Super Cache Compression which will compress your HTML files on the first page request and serve these to your visitors. This is a much better option to use because turning on something such as zlib compression will waste CPU cycles and will be served a tad slower, this is because the HTML file has to be compressed on every page view.

When I first ran this benchmark using the On mode a few days ago I didn’t get amazing results like this, maybe I had Super Cache Compression turned off, but here you go(This was with Super Cache Compression enabled):

Concurrency Level: 5
Time taken for tests: 4.835772 seconds
Complete requests: 10000
Failed requests: 0
Write errors: 0
Total transferred: 51870000 bytes
HTML transferred: 48180000 bytes
Requests per second: 2067.92 [#/sec] (mean)
Time per request: 2.418 [ms] (mean)
Time per request: 0.484 [ms] (mean, across all concurrent requests)
Transfer rate: 10474.85 [Kbytes/sec] received

That’s a staggering 7x more requests than the Half-on mode and nearly 130x more requests than with the plugin disabled. Now if your site got featured on the first page of Digg your server should be able to cope with the amount of requests you’ll receive!

Another thing that came to mind was that I could setup Nginx behind Apache to serve the HTML files and let Apache do everything else, I should be able to squeeze a lot more requests out of that. I haven’t learnt how to do something like that just yet but when I do I’ll post the results. Also if anybody has any good tutorials on how to do this could you please post them below too! :-)