Top, ps and cpu usage

One thing that mystify unix newbies is the result difference between top and ps commands about CPU usage:

# ps -o %cpu 21162

but a top -p 21162 give us 18%. There is something wrong right ?
Nope. But the confusion rise from the fact that ps and top don’t have the same definition of what constitute CPU usage.

top give you an average value of CPU consumption per core on a short period of time (by default 3 seconds). A value of 200% means that the process “hogged” in average two core during the last 3 seconds.

On the other hand ps calculate it value on the process lifetime, and don’t take into account how many core have been hog. So a for example a value of 15.3% means that since the process is running 15.3% of it lifetime it has been bugging on the CPU. The other 84.7% of the time the process was doing nothing, probably waiting for some input to append.

So as you can see this two commands have a very different definition of what CPU usage is, and both value are relevant in their own.

[Varnish] Separate cache for mobile device

Let’s say you have a website with two version, one intended for ‘desktop’ browser the other for ‘mobile’ device. Both use the same domain name. How can you get two separate caches ?

The wrong answer

Add the value of the User-Agent field into your vcl_hash function. That simple enough, right ? Wrong ! Very wrong. The User-Agent field is a total mess and it has so many possible values that this setting will make Varnish completely useless. Never do that !

The good answer

To ensure two separate cache you must add into the vcl_hash an header with only two possible value: one for all the ‘desktop’ User-Agent, the other for all the ‘mobile’ ones. That means add a custom header which value will depend on a whole bunch of regex, parsing and sanitizing the horrendous User-Agent value.

Fortunately for you, this annoying task has already been done. You can find a VCL for this specific task here. Import it into your main varnish configuration file and then adjust your vcl_hash for non-static content like this:

sub vcl_hash {
  if (req.url !~ "(?i)\.png|gif|jpeg|jpg|ico|gz|tgz|bz2|tbz|mp3|ogg|zip|rar|otf|ttf|eot|woff|svg|pdf)$") {