[gentoo-user] Re: Near freezes during large emerges

William Kenworthy <billk@×××××××××.au> writes:

> On Mon, 2011-01-17 at 16:23 -0800, Grant wrote:
>> > I think the idea is never use swap if possible, but in a case where
>> > you don't have swap space or run out of swap space I think it's still
>> > possible to lose data.
>>
>> Isn't swap just an extension of system memory?  Isn't adding 4GB of
>> memory just as effective at preventing out-of-memory as dedicating 4GB
>> of HD space to swap?  I can understand enabling swap on a laptop or
>> other system with constrained memory capacity, but doesn't it make
>> sense to disable swap and add memory on a 24GB server?
>>
>> Is swap basically a way to save money on RAM?
>
> No swap contains pages from memory that have not been accessed for
> awhile so they can be stored elsewhere freeing ram for actual active
> pages.  When they need to be accessed, they have to be swapped back in,
> and often something swapped back out to make room for it.

Like BillK said, it's not, whatever gets there must be swapped back in
to be used again.

For example, let's say you have a program that needs to open a really
really big file, an 8GiB file (no FAT jokes, please), and you have 4GiB
of RAM and 6GiB on swapfiles/partitions. 

The system will not be able to put the entire file on the physical
memory (and it may not even be able to do so on virtual memory (memory
as the process sees it), if you're on 32 bits, IIRC).

This means, although the program can access the file data, and from its
own point-of-view, all data is on memory, the system will have to do
some swap-in-swap-out gymnastics every time the program wants a chunk of
the file that's not on RAM.

OTOH if you actually add 6GiB of RAM, you'll probably be able to do it
all from RAM (8GiB for the file, and the other 2GiB for whatever else is
running).

It is better to see it as BillK describes - not a way to extend your
memory, but a way to store not-that-frequently-used pages when you need
to load something else.

It saves money, but it's still expensive - on time.

-- 
Nuno J. Silva
gopher://sdf-eu.org/1/users/njsg