>> >> 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).
That all makes perfect sense. So the reason a swap larger than maybe 1GB is not usually implemented is because idle processes don't normally have more than a few hundred MB of pages in memory? Wouldn't a sufficiently large swap (100GB for example) completely prevent out of memory conditions and the oom-killer? - Grant
