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How can you Reduce Disk Swapping in Linux?
A member of the voiceferous Clan Anonymous Coward asks: "I am finding that on my 64meg box that Linux is using the swap file even though it looks like there is plenty of RAM, but it is used by the disk(?) cache. For example, 4.8M of swap is used when the disk cache is 20M. What can be tuned to reduce the disk cache size? Sample /proc/meminfo info follows:
total:64569344 used:57200640 free:7368704 shared:32337920 buffers:3235840 cached:20496384 used-swap:4837376"
Linux has a very interesting memory management system that isn't intuitive at first. It enlists swap space even when you have plenty of physical RAM available. The degree to which you enlist swap space depends highly on how much disk activity you're performing and less on whether your program fits in memory. So every time you access the disk the kernel makes a decision: swap out physical RAM to speed up the disk or overwrite existing cache data to speed up the program. That decision has little to do with how far your resident set has exceeded physical RAM but mainly depends on how recently you accessed your resident set.
In fact as you approach 3/4 of your physical RAM the kernel becomes very reluctant to keep programs in memory. It's virtually impossible to use 100% of physical RAM because by that time the kernel is swapping out your resident sets with every disk access. Just try recording some uncompressed video. You'll see the video recording software fits perfectly well in RAM but after writing a few gigabytes of data the kernel has swapped out everything it can to increase disk caching. In fact your first fwrite will be very slow, as the kernel is writing physical RAM to disk to free up cache and only then writing your actual data.
Now this scheme can get very problematic if you really need the programs that the kernel swaps out. There are parameters in the kernel source which determine the swapping threshold but the problem is more what to set those parameters to than where they are.