I want to measure performance of the apache2 and nginx servers.
As a result I expect charts like on this website.
I can measure request per second using following tools: httperf, ab, JMeter or Gatling but how to measure the memory consumed by these servers?
I found this site but I don't know if it is correct.
The ab.c open-source program does it (per-process, per-thread, etc.), see an example of report here, a comparison between two servers there (the point of your question) showing how much memory usage can differ depending on servers and configuration options.