Path: utzoo!utgpu!news-server.csri.toronto.edu!rutgers!cs.utexas.edu!uunet!fed!arccs2!m1phm02 From: m1phm02@fed.frb.gov (Patrick H. McAllister) Newsgroups: comp.unix.aix Subject: Re: RS/6000 memory and virtual memory Message-ID: Date: 6 Nov 90 10:58:07 GMT References: Sender: news@fed.FRB.GOV Distribution: usa Organization: Federal Reserve Board Lines: 38 In-reply-to: mccalpin@perelandra.cms.udel.edu's message of 5 Nov 90 14:43:29 GMT In article mccalpin@perelandra.cms.udel.edu (John D. McCalpin) writes: ------------------------------------------------- COMMENT for System Configurers who are NEW to AIX ------------------------------------------------- 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