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In defence of swap: common misconceptions (chrisdown.name)

submitted 4 months ago by [email protected] to c/[email protected]

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To be clear, I don't blame the poster of this comment at all for the content of their post – this is accepted as "common knowledge" by a lot of Linux sysadmins and is probably one of the most likely things that you will hear from one if you ask them to talk about swap. It is unfortunately also, however, a misunderstanding of the purpose and use of swap, especially on modern systems.

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[–] [email protected] 0 points 4 months ago (1 children)

I have 20GB in my current setup and it was never full. If anything gets swapped in this situation it means it needlessly slows me down.

Not necessarily. Your memory also contains file backed pages (i.e. "file system cache"). These pages are typically not counted when determining "memory usage", because they can always be discarded.

It is often advantageous to keep frequently use files in cache in favor of unfrequently used memory pages.

[–] [email protected] 0 points 4 months ago (1 children)

so you think it's faster to keep cache for files on a disk, almost like where the files already are, instead of the 14 GB of actually free RAM that the "free" command shows? if that's your opinion, okay, but i don't agree at all. (btw. that command also shows cache and i think that's included.)

[–] [email protected] 1 points 4 months ago* (last edited 4 months ago)

You are misunderstanding.

The file cache is never written out to the swapfile, because files are already on disk, like you say. The file cache is kept in memory and the kernel may decide it's more advantageous to swap out unused anonymous memory pages to disk than flushing a file from the cache. You can use the vm.swappiness parameter to finetune this behavior to your liking btw. Lower values favor keeping more anonymous memory pages in memory, higher values favor file backed pages.

To give an extreme example of where this is useful: I have a use case where I process a number of large video files (each 2GiB-10Gib in size). The job I'm doing involves doing several passes over the same file sequentially. You can bet your ass that caching them in memory speeds things up dramatically: the first pass, where it has to read the file on disk is at 200x speed (relative to the video's duration), the second pass at 15000x speed.

Even in less extreme circumstances it also helps by keeping frequently accessed files in your home directory in memory, for example your browser profile. Your browser and desktop environment would be much more sluggish if it had to reach out to disk every time for every file it touched.

And you are free to disagree, but it's not my opinion but the opinion of kernel developers of just about every operating system built the past 4 decades. So I'd say: take up the argument with them and see how far you get.