Re: Recommended swap for 2.4.x.

[idalton@ferret.phonewave.net]

On Mon, Jan 29, 2001 at 03:23:35PM -0800, alex@foogod.com wrote:
> On Mon, Jan 29, 2001 at 02:57:44PM -0800, Alan Olsen wrote:
> > 
> > What is the recommended amount of swap with the 2.4.x kernels?
> 
> AFAIK, swap requirements for applications running under a 2.4 kernel have not 
> changed significantly from 2.2 kernels (please anyone correct me if I'm wrong),
> so the basic answer is:  About as much as you needed with for a 2.2 kernel.
> 
> > The standard rule is usually memory x 2.  (But that is more a Solaris
> > superstition than anything else.)
> 
> This always struck me as the most stupid rule of thumb I'd ever heard of.  
> With this metric, systems which precisely need swap the most (low-RAM systems) 
> get the least of it, and those that need it the least (those with gigs of RAM) 
> get tons of swap they don't need.  I don't know how this keeps perpetuating, 
> as it should be plainly brain damaged to anybody who thinks about it for a 
> couple of seconds, but somehow it does.

I think this has more to do with the Olden Days when kernel panics would
be written out to swap. Or something. You wanted more swap than ram so
the dump wouldn't overwrite your fs.

> My general recommendation is:
> 1) Take the best guess you can at how much total memory you're ever going to 
>    need at one time.  This can vary with the type of tasks you're doing 
>    (server/desktop/image-editing/etc), the software programs you're using, and 
>    so on.  There is no easy way to figure this out, but I would recommend that 
>    if you come up with anything less than 128MB, you're probably being too 
>    optimistic.
> 2) Subtract the amount of RAM you have (believe it or not, the more RAM you 
>    have, the less swap you need.  Imagine that).
> 3) Round up to a nice breaking point (multiples of 64MB are nice and are easy 
>    to remember), just for convenience.
> 4) Add a little bit of extra just in case (it's better to have too much than 
>    too little, particularly since disk is cheap).  I usually add somewhere 
>    around 64MB.
> 
> For most people, for most systems, this comes out somewhere between 128MB and 
> 256MB of swap needed (in some cases you may need 512MB or more, but if you've 
> got those sorts of memory demands you may want to carefully consider whether 
> more RAM wouldn't be a good investment).  If in doubt, go for the larger 
> number.  After all, with an 8.1GB drive, how much are you going to miss a puny 
> 0.25GB (256MB) chunk of it?

This is pretty close to the rules-of-thumb I use. Some round numbers
that work for me:

XFree86: 48MB
Window Manager: ? (fvwm I give about 16MB; E I hear might want 48MB)
Netscape/Mozilla: 64MB
Base system: 32MB
Basic audio: 16MB (mostly buffer space)
Other applications: ?

Minimum swap: 16MB (I allocate in 16MB chunks)
Maximum swap: 1.5x - 2x physical memory.

-- Ferret
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