On Thursday, July 21 at 20:07 (-0700), Grant said:
> >> Then why not have a really big swap file? If swap is useful as a > >> second layer of caching behind RAM, why doesn't everyone with some > >> extra hard drive space have a 100GB swap file? > >> > > You've not understood what I said, I think. Swap is not useful as > > filesystem cache. Swap is as efficient (probably a little less) > than > > the files on the disk. It's RAM that's efficient as filesystem > cache. > > > > Where swap comes in is the kernel can swap out pages from "stale" > > processes, and reclaim the RAM as filesystem cache. > > That all makes perfect sense, but if a small swap is good and a large > swap is not any better, I'm missing something. Maybe the pages from > stale processes never total more than a small amount? I don't see how > that could be Because you're (likely) never going to be using 100GB of memory at one time for all your processes, let alone "idle" processes, so what's the point of allocating all that swap? Continuing the analogy, it's like getting a stadium-sized attic that's 100x bigger than the house your building it on just to store a Christmas tree and a few other items. Here's another way of looking at it. The kernel wants to use *all* your RAM. RAM is fast (compared to disk). But it wants to use the RAM for stuff that's actually needed most at the present time. So say you have 4G RAM. You're only using maybe 1.5G memory for applications. So the kernel is going to try to use the remaining 2.5G for cache when/if it needs to. But let's say you're hitting the disk a lot because you're compiling something, then the kernel might decide it would like to cache more files than the 2.5G. So it sees you have 300M of paged in process memory that hasn't been used in a long while. A better use of RAM may be to swap out those 300M and use it for more filesystem cache, causing your compilation to run faster. But if you have a 100G swap file and only 300M of "idle" pages then all that extra swap isn't going to be of any use. Similarly, you don't want to swap out all of the 1.5G RAM because some of it is actually being actively used (e.g. by the compiler).
