[other_os] Why is swap space used while RAM is available?

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http://ubuntuforums.org – Here's a problem I've been facing for a while: normally, as I understand it, anything that gets cached in RAM should not effect anything that actually needs RAM. This video (1 - swap.ogv) shows me using dd to copy a file to /dev/null, which also causes it to be cached as this happens. After a while, swap starts being used. What's even stranger is that if swap contained the rest of the file I was copying, it would have used a lot more than the 11MB it did, so I'm not sure what those 11MB are. In the video, once the dd command is done, I run './mem'. mem is a simple program which allocates 1 (Hardware)