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