Hi 

        Just to clean things out, as i see many times people saying that
they dont need swap in modern machines... and this "talk" might push
more people in to thinking about that (not that you are really saying
this)

On Wed, 2 Nov 2011 20:44:43 -0400, Timothy J Massey <tmassey AT obscorp DOT com>
wrote:
> >   Swap might keep the process from failing, 
> war, but for the most part the days of swap are over.  I don't care how 
> much RAM you have, if you have a swap file of 1GB residing on a single 
> SATA spindle and you're actually using all 1GB, your system will be 
> unusable *anyway*, so who cares if it crashed a little sooner?  (The


        Dont forget that swap is also "used" to extend the total 
virtual memory of the machine... people that use java know this already,
but other apps also do it... many apps reserve memory for future use, 
but arent really using it. That memory is wasted without swap, and
create "out of memory" errors when the machine just have a few MBs of
RAM used.

        Apps that require a lot of virtual memory, even if they only
use a few MBs, usually need swap files... sometime very big ones!

        Say you configure a java to use a max RAM of 4GB... it will
require that memory to exist, even it its just using a few hundred 
MB. A 2GB of ram +2GB of swap will start the java, but a 2GB of
ram + 100MB of swap will not. the java app will not use really the
swap until it demands about 2GB. this is a stupid example, but 
instead of one app put many apps requesting RAM and the example
and result is the same.

With a very small swap, you are "eating" real memory that will not
be used, but with swap, that "unallocated memory" is mapped to
swap. This way you are "using" swap and dont even put a real
byte on the disk. you dont lose performance, only gain as you
have more free RAM and can support more simultaneous process.

        many apps over-commit ram... almost all languages with 
virtual  machines do this, mysql can be configure to reserve more 
memory, apache, loading many modules and external process also 
allocate a lot more ram that its really using. hey, i have 
several chrome threads requesting  +- 800MB of ram, but only 
using about 150... my firefox with MANY tabs open is requesting 
3GB of ram and using about 2GB

        So some swap usage is normal for long live process, even
with many RAM, but even if the swap usage is zero, having some 
swap already have the potential to free that same size of
memory on the machine.

        Swap sizing depends on what the machine will do, but
at least the same size as RAM or at least 2GB (whatever is
bigger) should be considered the minimum. A machine with
4GB of ram and 100MB of swap is wasting ram to save a 
few GB of a (usually) giant, multi GB or TB HD

        Linux hides a little all this, but in solaris you
clearly see 2 swap usages in parallel, the real swap usage 
and virtual memory swap usage.

        so... please dont shrink your swap too much, even
if you have enough RAM and dont see swap usage... it is
still useful!

higuita
-- 
Naturally the common people don't want war... but after all it is the
leaders of a country who determine the policy, and it is always a 
simple matter to drag the people along, whether it is a democracy, or
a fascist dictatorship, or a parliament, or a communist dictatorship.
Voice or no voice, the people can always be brought to the bidding of
the leaders. That is easy. All you have to do is tell them they are 
being attacked, and denounce the pacifists for lack of patriotism and
exposing the country to danger.  It works the same in every country.
           -- Hermann Goering, Nazi and war criminal, 1883-1946

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