Swapping (was Re: SCSI and 386/ix)

Tom Neff tneff at bfmny0.UU.NET
Wed Feb 7 01:23:09 AEST 1990

In article <PCG.90Feb5191855 at rupert.cs.aber.ac.uk> pcg at rupert.cs.aber.ac.uk (Piercarlo Grandi) writes:
>Well, in most timesharing applications your system _should_ swap.
>A very rough rule of thumb is that if a resource is less than 30%
>used it is wasted (and if over 60% used it is insufficient, because
>queues will form on a resource that is more than 60% committed on average).
>Given that the majority of processes in a timesharing system is
>inactive, and possibly for fiarly long times, swapping/paging is
>healthy. If it does not occur, than you have too much memory for
>your application mix. If you don't want it to occur because it is
>too slow, this means that the swap bandwidth is undersized.

These principles apply to large multi-user timesharing systems but not
to supermicro workstations, which are mostly where UNIX/386 runs.

On a 386/486 workstation you should shoot for very little swapping.
Memory is fairly cheap and performance already has enough bottlenecks
to contend with.  AT386 bus and disk controller performance will definitely
exact a penalty if you start swapping significantly.

Since swap space is typically just one configurably-sized partition among
several on a disk drive, there's no obligation to "force" swapping in order
to avoid wasting hardware.  A 5MB partition is peanuts.

While it's nice to know that the swapping capability is there when you
need it occasionally, the 'sar -w' listing should have plenty of zeroes
on average.

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