Compare usage of memory by window managers

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http://unix.stackexchange.com – I'd like to test a few window managers to see which of them takes less memory and CPU in certain time period (so is better choice for older computers). I'd like to make the tests myself (not to read a review). Could you please advise me: Which application for testing would you use to have precise results How would you use it (in case it isn't intuitive, e.g. choice of switches or settings) (HowTos)