out of swap space??

[Robert Withrow]

In article <1991Apr26.142301.6486 at virtech.uucp> cpcahil at virtech.uucp (Conor P. Cahill) writes:
|100MB of disk sounds like a lot to waste,
... and
|You do not want to run out of swap space.  If that means you over-configure
|a bit, so be it.

Disk space is wasted only if it really *is* wasted, i.e. unused.  If
you need 100 MB of swap space then using 100MB is not wastefull.

On the on the other hand, you must recogonize that the required size
of the swap space is a function of many variables, only one of which
is the size of memory.  This I would not blindly follow any rule that
says that ``swap space should be 2-3 times the size of memory''.
There are a number of implicit assumptions in this sort of rule that
are ulikely to be true in many systems.  The same can be said for the
sizes of various kernel parameters.  

Things to consider:

  1) the true ``minimum size'':  e.g. in SVR4 the minimum size of swap
space == size of memory, due to the swapping algorithms.

  2) memory requirements of your workload.

  3) the number of (simultaneous) users.

  4) the criticality of the workload (i.e. what harm would be caused
by running out of swap space). In single user configurations, the harm
caused is often not more than annoyance and some wasted time.  In
multiuser production environments it would likely result in expensive
loss of productivity.

  5) The use that could be made of the disk space if it were not used
for swapping.

As a data point, I have found that with 16MB of memory, in a
workstation (e.g. single user with uucp, network, and program
development workload [with X windows]) environment, 1.5 x memory is
perfectly adequate for the swap space (using SVR3.2 and SVR4).  In
fact, I think the swap space could be made even smaller and I would
still not have any problems.

The bottom line is: there is no simple formula to refer to.

-- 
---
 Robert Withrow, R.W. Withrow Associates, Swampscott MA 01907 USA
 Tel: +1 617 598 4480, Fax: +1 617 598 4430, Net: witr at rwwa.COM