mil@mendel.acc.Virginia.EDU (Maria I. Lasaga) (04/08/91)
Given the virtual memory capacities of system 7.0, will there be any need to purchase additional RAM? If not, will it even be advantageous to have more RAM rather than less? ------------------------------------------------------------------------ maria i. lasaga department of psychology gilmer hall university of virginia charlottesville, va 22903 mil@virginia ------------------------------------------------------------------------
dplatt@ntg.uucp (Dave Platt) (04/13/91)
In article <1991Apr8.034446.14836@murdoch.acc.Virginia.EDU> mil@mendel.acc.Virginia.EDU (Maria I. Lasaga) writes: > >Given the virtual memory capacities of system 7.0, will there be any >need to purchase additional RAM? If not, will it even be advantageous >to have more RAM rather than less? Well, think of it this way. When you access real RAM in a random fashion, it has an access time that falls somewhere in the 80-150 nanosecond range. When you access a page of virtual memory that's out on disk, it has an access time on the order of tens of milliseconds. This is many _thousands_ of times slower! Virtual memory is a wonderful feature... within certain limits. Ideally, you want to have enough physical RAM to keep _all_ of your "working set" (the memory you're accessing frequently) in RAM at one time. Whenever your working-set size exceeds the amount of RAM available, you're going to start paging to disk, and this will slow your machine down quite a lot. The rule I learned back in college, lo those many years ago, is "If your working-set size exceeds three times your physical memory allocation, you're doomed to slow death by swapping." If you're running several different applications, and switching between them infrequently, and there's not a lot of heavy processing and memory- accessing being done by the background applications, then you can get by with a modest amount of RAM. You'll probably see a burst of disk activity, and some slugging behavior, when you switch between applications... the application you've brought forwards will be paged in off of the disk, and another application will be paged out. If you're running several large, memory-intensive applications, with frequent switching between them and/or lots of background processing, then you'll really want to have enough RAM to hold most of the applications in memory. Otherwise, your disk will be eternally paging, and you'll spend most of your time waiting for the disk. As far as I know, System 7's VM paging system is entirely synchronous. You cannot have one program using the CPU while VM is paging another process out to disk. This is true even on a IIfx, which is (in principle) capable of using DMA for SCSI data transfers... the Mac OS doesn't take advantage of this capability. Having more RAM is almost always a win. -- Dave Platt VOICE: (415) 813-8917 UUCP: ...apple!ntg!dplatt USNAIL: New Technologies Group Inc. 2468 Embarcardero Way, Palo Alto CA 94303
hoepfner@heawk1.gsfc.nasa.gov (Patrick Hoepfner) (04/15/91)
mil@mendel.acc.Virginia.EDU (Maria I. Lasaga) writes: >Given the virtual memory capacities of system 7.0, will there be any >need to purchase additional RAM? If not, will it even be advantageous >to have more RAM rather than less? I have heard that Apple suggests that so that performance doesn't slow down too much, you should only double your current RAM. That is for those with 4MB, VM will bring you up to 8MB with out too much of a performance hit. The other piece of the Virtual Memory equasion is the number of NuBus or PDS cards installed into your mac. Subtract 1MB fof each card installed. For the user with 4MB RAM and one card (e.g. video card), VM will bring you up to 7MB. Remember that Virtual Memory doesn't solve everything and RAM prices are getting pretty cheap these days.
hoepfner@heawk1.gsfc.nasa.gov (Patrick Hoepfner) (04/20/91)
hoepfner@heawk1.gsfc.nasa.gov (Patrick Hoepfner) writes: >mil@mendel.acc.Virginia.EDU (Maria I. Lasaga) writes: >>Given the virtual memory capacities of system 7.0, will there be any >>need to purchase additional RAM? If not, will it even be advantageous >>to have more RAM rather than less? > I have heard that Apple suggests that so that performance doesn't >slow down too much, you should only double your current RAM. That is >for those with 4MB, VM will bring you up to 8MB with out too much of >a performance hit. I REALLY hate folling up on my screwups but the formula for the Maximum RAM (real and virtual) for a Mac is total memory minus 1MB for each NuBus or PDS card. What this means is... For a non 32bit clean machine, (say a Mac II) with 6 NuBus cards is 16MB - 6MB(1 per board) = 10MB total memory address space available. Sorry about the screw up... hoepfner@heasfs.gsfc.nasa.gov