swap space utilization under 4.2

[Mike Caplinger]

Our experience has been that we run out of swap space much sooner under
4.2 than we did under 4.1, with swap areas of the same size.  This has
been "explained" by the fact that the 4.2 swap area allocation scheme
causes increased fragmentation.

However, careful examination of 4.1 and 4.2 source reveals that the VM
code has changed only little between the two versions.  In particular,
the parameters DMMIN and DMMAX, constants in 4.1, have become variables
in 4.2.  But they are immediately set to what seem to be the same
values.  (I'm on shaky ground here, since the only 4.1 source I could
find may have had these parameters' values changed.)  I find no sign of
a modification that might cause more fragmentation under 4.2.

Does anybody know what difference between 4.1 and 4.2 might have caused
more fragmentation?  I am assuming that the move to 4K blocks isn't
responsible, and doesn't affect the treatment of the swap device.  If
I'm wrong, then is there some easy way (short of changing DMMAX, etc.)
to reduce the fragmentation?

By the way, experiments indicate that with a 16.7 meg swap area, the
system can't create a 0.5 meg process when there is slightly more than
10 meg in existing processes.

	- Mike

p.s.
Why do I ask?  We have several VAXen with single RA81s and its
standard, 16.7 meg swap area.  Last semester the students using one of
the machines found they could only run 3 Modula compiles before running
out of swap area, so they evolved a scheme of passing three Coke cans
around the terminal room; only people with one of these cans was
allowed (via peer pressure) to do a compile.  Needless to say, I would
prefer to have this kludge go away.