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How does one exit the X server?

I would like to shutdown the graphical interface and every related graphical application on my Ubuntu 11.10 (with 4 GiB memory and 4 GiB swap) temporarily in order to free up memory. How can I achieve this?

I try to reduce memory consumption because I am doing very memory intensive computations (SAT solving with 1.25 million clauses) which easily consumes 4 GiB of memory and more. I want to prevent swapping for as long as possible because this will slow down the computations considerably.