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"Boyd Stephen Smith Jr." <bss03@××××××××××.net> posted |
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200701111413.31743.bss03@××××××××××.net, excerpted below, on Thu, 11 Jan |
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2007 14:13:19 -0600: |
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|
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> [I'm a little fuzzy here as to weather a read-only |
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> page is ever written to actual swap space, the kernel may just remember |
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> where (on disk) the original is keep and release the real memory page.] |
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>From all I've read, Linux never rewrites to swap a clean page held |
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elsewhere on disk. Swap is for "anonymous" memory only, that is, memory |
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not backed by files elsewhere on the system. We have COW (copy on write) |
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here once again. If you are editing a file, until that little "modified" |
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indicator comes on, Linux considers the memory disk-backed by the file |
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itself. As soon as the copy in memory is modified, until it is saved, it |
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becomes "anonymous" memory, NOT backed by that file on disk, and thus |
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subject to being written to swap, until such time as the save is done, the |
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memory marked clean once again, the swap space it took if it had been |
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swapped out marked free, and the memory is mapped once again to the file |
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as it exists on disk, until the next modification yet again... |
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That's for local disk, anyway. It's very possibly (and reasonably so) |
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different with NFS and other remote file systems, where local swap may be |
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rather faster and more dependable than access to the remote file system. |
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I'd assume files on those are treated as anonymous memory for |
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reliability and performance reasons, with memory mapped files being |
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written to swap accordingly. Of course, FUSE will operate by rather |
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different (and unknown to me) rules as well, given it operates in |
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userspace. As such, I'd guess the kernel treats them as unreliable and |
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thus files mapped off of them as anonymous (read to-swap) memory, avoiding |
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the complications sure to follow if it is indeed found unreliable. |
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-- |
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Duncan - List replies preferred. No HTML msgs. |
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"Every nonfree program has a lord, a master -- |
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and if you use the program, he is your master." Richard Stallman |
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-- |
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