I've noticed that sometimes swap is used even when there's plenty of memory. For example just now I've had 2 out of 4GB ram used and additional 800mb of swap used. I had to swap off, because the performance drop caused by this is unbearable. Shouldn't the swap only be used when out of ram? I don't know maybe I approached the limit, but I highly doubt it. It would have to be at least 4.8GB of used memory to make sense. I watched a movie and then noticed the performance drop, but I doubt the movie would occupy 2GB of ram.
Even if there's a situation when I run out of ram and swap is used, shouldn't the contents of swap go back to ram automatically when some ram is freed? It just doesn't make any sense to have 2GB of ram available and use 800mb of swap. It isn't the first time I had to swap off because of system being super slow.

No - see swappiness:
http://en.wikipedia.org/wiki/Swappiness

No, the data will stay in swap until it's needed again.

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Thanks