RS/6000 memory and virtual memory

[Patrick H. McAllister]

In article <MCCALPIN.90Nov5094329 at pereland.cms.udel.edu> mccalpin at perelandra.cms.udel.edu (John D. McCalpin) writes:

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   COMMENT for System Configurers who are NEW to AIX
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   The AIX documentation recommends that you have at least twice as much
   paging space as real memory.  Unlike many other systems that make the
   same recommendation, this is *very important* for AIX.

   For reasons that are not clear to me, AIX makes a copy of every
   running process out in the swap space even if there is plenty of main
   memory available.  As a consequence, you cannot use more physical
   memory than you have swap space for!

   I am working with a 3rd-party manufacturer who is gearing up to
   produce SIMM's for the RS/6000 machines.  I installed 8 4-MB SIMM's in
   my Model 320 on Friday for a test drive and found that I could only
   create a 24 MB job since I only had 36 MB of swap space of which the
   O/S uses 12 MB in my configuration.

   Those of you with single 120MB disks will want to take note of this if
   you plan to increase your installed physical memory....

I believe -- I'm sure someone will correct me if I'm wrong -- that the fact
you discuss follows from the RS/6000 using the same sort of single-level
storage architecture as the old RT (and some other IBM machines). The only
'storage' that the machine formally addresses is disk, that is all machine
addresses are actually addresses for disk storage. RAM is just a large disk
cache, in the sense that it has no separate address space. If this is true,
it need not be quite true that there would be 'a copy of every running
process out in the swap space' in that permanent, read-only data structures
(such as, potentially, executable code) can be directly mapped into the 
correct pages in the file system.

Is what I have said correct, anyone? And if so, what is the reason to use
this architecture? (Just idle curiosity . . .)

Pat