I've seen sysadmins playing with /proc/sys/vm/drop_caches (echo 3 into this file from a cron) in order to free caches. Is there a real interrest in doing this? Aren't the clean caches reclaimed when needed?
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In normal operation it's not necessary. The normal reason for doing it is to be able to test the performance of some program when it is running with a cold cache.