On Friday 29 September 2006 21:37, Luca wrote:
> So my question is simple, is it normal that linux uses all the ram
> memory and not swap partition?
> (Think I should order another 1G DDR memory card...)
Yes, it is.
In addition to the usual use of RAM for storing there programs being
executed and the memory they allocate with malloc, realloc, etc, Linux
also stores there a disk cache.
When a program accesses the disk the kernel a) reads the data required
from the disk, b) stores it in the memory. When another (or the same)
process requests the same data Linux reads it from the memory rather
than the disk, that greatly decreses the data access time. This "disk"
data in memory is called "disk cache".
But when no more memory (ram) is available the kernel can decide
between the two actions to do. 1) begin to use the swap, 2) flush the
disk cache to the disk and use the "freed" memory. Since the disk cache
has anyway to be written to the disk (since the cache is "transparent"
for programs, they don't know about it, thinking the data they write to
the disk actually goes to the disk) the kernel selects the latter case.
As you can see, if using the technique described ram will be ~100% full
almost all the time (because the kernel uses the memory not allocated
for any process as the disk cache).
Swap is used only when there is no enough "actually free" memory. For
example, if some of your programs tries to eat all the memory in the
system then at some point the kernel will begin using swap.
So, you will have to order another memory card if either you swap is
often used or you want to increase the performance of your system by
increasing the disk cache size. But note that in the latter case you
will still see that the memory is ~100% full while the swap is 100%
free.
P.S. If you wonder, I have 512M RAM on my desktop, 503M being used.
When I lanch the program "freeing" the disk cache memory, only
160M is actually used when I'm in KDE, and only 30M is used when I
am in console with apache 2.0 in use.
Also note that I don't use the swap at all since 512M is really
sufficient for me.
--
Nothing but perfection
pv
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