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The mistery of swappiness

Nov 11, 2009, 13:17 (4 Talkback[s])


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"If all available memory is exhausted (application memory, buffers and filesystem cache) and any memory allocation is requested the kernel needs to free a few pages of memory. It can either swap out application memory or drop some filesystem cache. The "swappiness" knob affects the probability which one is chosen.

"This means that at a swappiness of 0 the kernel will try to never swap out a process, and at 100 it will try to always swap out processes and keep the filesystem cache intact. So with the default, if you use more than ca. 40% of your memory for applications and the rest is used as filesystem cache it will already start swapping a bit. The hilarious result is that you may up swapping a lot with lots of memory left - think of a machine with 64GB RAM! If you try to use 32G memory you'll be in swap hell."

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