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Re: [K12OSN] swap size



Hmmm.  I seem to recall in the somewhat distant past (oh, say around
SunOS 3.5/4.0 days) that you had to have at least as much swap as
you had memory, since the VM system was implemented in such a way
that it required enough swap to mirror main memory.  Sometime after
that, the VM system was changed (probably with SVR4/SunOS 5.x) such
that swap _adds_ to available memory, and you could have less swap
than main memory if you wanted.

I could, of course, be quite mistaken, but this sticks in my head
all the way back from my college days as a sysadmin on our Sun2
and Sun3 servers (with ASCII terminals for access.)

As others have said, none of this matters much for modern systems
like Linux, where you really just want swap + main memory to be
more than the total memory required by all the apps you want to
run simultaneously.

I do question the assertion made here that too much swap can degrade
performance.  Maybe I am overestimating the intelligence of the
VM system in Linux (which I have not studied), but why would too
much VM cause a slowdown?  Does the VM system dump unused pages
to swap sooner if there is more swap?

-Don

Kirk Rheinlander wrote:
OK, so this "rule of thumb" was specified by Windoze some 30+ years ago, or some 15+ year before Windoze existed. Interesting view of history....I'm sure that Bell Labs would get a kick out of this interpretation. Revisionist history at its' finest!

With Win2k at least, you use most of swap space, even if physical RAM is not even close to being allocated. Swap space on 'Nix's are exactly what John indicated they were, and the swap=mem is just a rule of thumb, and the subject of more than a few performance tuning scenarios.

Kirk

At 12:17 PM 4/29/2002, you wrote:

> I'd love to know where that rule of thumb came from. It's been
> around for 20 years or more. In my opinion, it's completely wrong.

It comes from interpretation of swap space for Windoze servers. Basically NT
or 2k suggest swap space to the same size as your available RAM in order to
be able to do disk dumps in case of system crashes.


Regards
Anthon Walter

-- Don Christensen Senior Software Development Engineer djc cisco com Cisco Systems, Santa Cruz, CA "It was a new day yesterday, but it's an old day now."





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