High cache usage of free memory

Question

I dont know if this is normal but my fresh archlinux x86_64 install with Xfce uses round 300Mb of 4Gb ram space after boot but then after i start and close some programs i get to 3 Gb used of 3.87 Gb free memory when all applycation are closed. Now i understand that kernel caches some data in memory but i cant clear it all with commands:
sync
echo 1 > /proc/sys/vm/drop_caches
echo 2 > /proc/sys/vm/drop_caches
echo 3 > /proc/sys/vm/drop_caches
After i do that system reports 540 Mb used compared to 300Mb from boot. Is there a way to limit the ammount of cache usage so i dont end up having all free memory used by cache?
Last edited by Anril (2009-12-29 13:19:52)

Answer 1

Ram that is unused is wasted ram, the linux kernel manages it very well so no worries there.
If you have swap usage it's because most probably you have things stored in the ram that are not being used often but cannot be discarded too so things being used more often take precedence.
If you think you have enough ram for all your needs then why not disable swap? I've read that there might be a speed penalty for running without swap but I've been running my system without swap (arch64 4G ram) and I've never had any bad surprises.
There are a few things that might prompt swap usage (don't quote me on that though, it just seems to me to be the case), if you copy many files from one place to the other the kernel will use all the ram available to try to cache them, if you copy many GB then all the ram will get filled (and maybe other things will get evicted to swap).
It's not a bug, free ram is there to be used, things that have been used/needed recently get to stay in the ram, other things can go to the swap, if you can cache a file that has been recently used and need to use it again shortly after then it's much faster if it is in the ram instead of having to read it again from the hard disk. The logic behind it is good, it's just that some corner cases may make behave badly.