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Znik
Asked: 2014-04-05 02:38:14 +0800 CST

Linux makes cache for swap. what for?

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It looks like joke: swapfile on "ramdysk" in 64bits system?

I look into file: /proc/meminfo and made some investigation. Look for line: SwapCached: nnnnn kB . One is what I don't understand. What for linux kernel caches swap? What for caching this? Isn't better idea leaving page as active?

Another, what is difference between swapping and paging? I never found exactly explanation about it. Where can I see counters?

Thanks for any explanation.

linux

3 Answers

  1. I don't think SwapCached means what you think it does. From the documentation

    SwapCache: Memory that once was swapped out, is swapped back in but still also is in the swapfile (if memory is needed it doesn't need to be swapped out AGAIN because it is already in the swapfile. This saves I/O)

    Also note that SwapCached doesn't count as swap even though it's in the swapfile.

    Cached: Memory in the pagecache (diskcache) minus SwapCache

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  2. Best Answer

    Red Hat explains it like this:

    "SwapCache: Memory that once was swapped out, is swapped back in but still also is in the swapfile (if memory is needed it doesn't need to be swapped out AGAIN because it is already in the swapfile. This saves I/O)"

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  3. I see better article than redhat's one: http://www.westnet.com/~gsmith/content/linux-pdflush.htm Thank you neutrinus and lain for swap cache explanation. It is sense. Another sense is, last swapped out page has the highest chance reverting to active state (saving I/O).

    I don't thank for minus signer, who doesn't explain me difference between paging and swapping. I didn't find clear documentation for this.

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