Re: 2.4 VM & swap question

[Jakob Østergaard]

On Sun, Jun 17, 2001 at 10:48:36AM -0700, Tom Rini wrote:
> 'lo all.  I've got a question about swap and RAM requirements in 2.4.  Now,
> when 2.4.0 was kicked out, the fact that you need swap=2xRAM was mentioned.
> But what I'm wondering is what exactly are the limits on this.  Right now
> I've got an x86 box w/ 128ram and currently 256swap.  When I had 128, I'd get
> low on ram/swap after some time in X, and doing this seems to 'fix' it, in
> 2.4.4.  However, I've also got 2 PPC boxes, both with 256:256 in 2.4.  One
> of which never has X up, but lots of other activity, and swap usage seems
> to be about the same as 2.2.x (right now 'free' says i'm ~40MB into swap,
> 18day+ uptime).  The other box is a laptop and has X up when it's awake and
> that too doesn't seem to have any problem.  So what exactly is the real
> minium swap ammount?

It completely totally and absolutely depends on the kind of workloads you put
your system under.

I have a database server with 1G phys and 1G swap. It uses 950+ MB for cache,
as it should, and doesn't even *touch* swap.  This is 2.4.5.

I have another box with 384MB phys and 1G swap, and it's usually a few hundred
megs into swap.  That's what long-running memory hogs and big compilers do.

There is no simple answer.  swap = 2*phys may be reasonable for some desktop
uses, I don't know.  But there *is* *no* *simple* *answer*.

With the amount of work that's gone into just *understanding* why the VM
behaves as it does (even after the VM rewrite that was done exactly in order to
come up with a VM we could *understand*), it's beyond me how anyone can even
begin to think that one can define a set of simple and exact rules for minimum
or "optimal" (whatever that means) values for swap.

(if I sound pissed, don't worry, I'm not. I'm frustrated, that's different  ;)
-- 
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:   [e-mail]   : And I see the elder races,         :
:.........................: putrid forms of man                :
:   Jakob Østergaard      : See him rise and claim the earth,  :
:        OZ9ABN           : his downfall is at hand.           :
:.........................:............{Konkhra}...............: