Dale wrote:
Grant wrote:
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.
- Grant
To confuse you even more, there is a swappiness setting as well. On
my old x86 rig, I have 2Gbs of ram. My hard drive is really slow
since it is IDE. I set swappiness to 20. That tells the kernel that
I have swap space but don't use it unless you must. For what I use
the rig for, 2Gbs is plenty of ram. The lower the swappiness setting,
the less the kernel will try to use *SWAP* . The higher the setting,
the more it will try to use swap.
I have a new rig that is amd64 and has SATA drives which are pretty
fast. I still have swappiness set to 20. Why do I have it set to 20
when the drives are faster you ask? I have it set to 20 because I
have 16Gbs of ram here. Even if I have portage's work directory on
tmpfs and am compiling OOo, it should not need swap then either.
By the way, my swap partition is 1Gb on both systems. Why have it
this way since one machine has 2Gbs and one has 16Gbs? As it has been
said, you want a little swap and even using a little swap is OK. You
just don't want it to be using swap and actually swapping data all the
time. On my old rig, it started out with 512Mbs. I use KDE and it
got to the point where it was using enough ram that it was not just
using swap and letting things sit, it was actively swapping data from
swap and doing so a lot. It would only be using a 100Mbs sometimes
200Mbs. The point is, it was slowing the system down because of the
swapping process. I bought a stick of ram and all was well again. It
would still use a 100Mbs of swap at times but it would not be actively
swapping the data back and forth so it wasn't a big deal.
I think the point is this, it is good to have a little swap. It is
even OK for it to use a little swap when it is mostly sitting there.
When you notice it using swap and it is actively swapping and moving
things back and forth, you need more memory. Having the swap may can
save you from a crash but is can also give you a "time to add more
ram" hint too. If Linux starts using swap a good bit, you need more ram.
I do like that attic analogy tho. You may not mind going up in the
attic and dragging the tree down once a year but you may not want to
go to the attic to get a glass of water. That would put a lot of wear
on the stairs and it would also get old after a while to.
Dale
:-) :-)
P. S. Corrected a disconnect between brain and keyboard. It is in
*bold* in the first paragraph.
