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OS X does not behave exactly in this manner, but is indeed confusing.

Linux never needs a 'purge' or equivalent. Doing a 'drop_cache' only slows the system slightly because nothing is cached. 'sudo purge' can prevent OS X from using swap – i.e. running out of RAM even though there is RAM it could reclaim. This behavior has been tweaked but not fixed in Mavericks.

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Actually, purge is completely unrelated to swap. Purge forces disk cache to be flushed to disk, and can be used to approximate a cold disk buffer cache scenario for performance analysis.

Some small amount of swap usage (compared to total RAM) is perfectly normal, by the way. More cache, for instance, is a better use of RAM than avoiding swap usage by keeping never-accessed pages present.

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