Thank you all for your useful explications,I will take them into consideration
This week everything became normal as before, the swap is not used (I don't know why)
To answer to your questions, suddenly moodle became slow (pages take a long time to load), it's true that at that time moodle was very solicited, so, I made some changes in apache configuration according to moodle doc apache performance and also this page helped me a lot and than moodle went back to normal and the RAM usage increased. In the server we have LAMP plus two moodle instances but one of them is not very used. I said that it's not that dramatic according to what I have read in the page you posted.
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