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A Question About Virtual Memory
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Aug 16, 2001, 06:59 PM
 
Is there an advangage to leaving virtual memory on but only for one meg. Are you restricted from anyting if you have virtual memory turned of. If not, why is it turned on to such an insignificant amount of ram as default. Maybe this question is dumb, but an answer would be much appreciated.
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Aug 16, 2001, 09:07 PM
 
Under the virtual memory implementation in Mac OS 7 to 9, applications require less RAM with VM on. Do a "Get Info" on an application and you'll see something like "This application will require 2345K more RAM if virtual memory is disabled". Also, having VM on gives you limited memory protection; sometimes crashes that would take down the whole system with VM off instead only crash the offending app.

This all goes away with Mac OS X; it uses the superior Mach VM system which is always on, doesn't require configuration, and doesn't require applications to specify their memory sizes.
     
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Aug 16, 2001, 09:19 PM
 
Hmm, this is hardly an iMac question, but that's a problem for the mods, not me.

Anyway, the reason that some people suggest setting the VM to 1 Meg, even if you don't really want the disadvantages of VM under OS 9 is because this allows the operating system to take advantage of one aspect of VM which can save you a bit of memory.

Notice that when you turn on VM, the memory needed to run applications decreases? Check the memory allocation part of the Get Info box for an app. The reason is that when you have not VM and you launch an app, the entire app must be loaded into your memeory. Even the 5 million features of Word that you'll never in a million years use, like 3d rendered text.

However, when you turn VM on, the system is able to more dynamically load the apps into memory. At first the main text of the program will be loaded, then other less common pieced of code can be loaded if you request to use them. So, this will decrease your memory usage and will speed up load times of the applications (a little). The only downside is the system may need to access the disk when you need to use some code that isn't paged in.
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Aug 17, 2001, 03:21 AM
 
     
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Aug 17, 2001, 11:35 AM
 
Maybe I'm different but I can't set my VM to less than 561 MB (I have 576 MB orf real RAM installed). How does one set VM to 1 MB. I have OS 9.1.
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Aug 17, 2001, 02:13 PM
 
I always thought the suggested VM setting was 1 MB OVER physical RAM so in your case it would be 577. With that much real RAM though, I don't think you really need to use VM, unless it has to be on.
     
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Aug 17, 2001, 03:42 PM
 
as always, lots of real ram helps.
i run 9.1, with 320meg ram (okay, not all that much, but not a little...)
apps need more memory with VM off, but (for me), things run a little faster, better.

some audio apps (for example) insist on running without VM.

if you do use it, as said above, run it 1meg over your installed ram (unless you've got very little; ie 64meg...)

experiment!
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Aug 17, 2001, 03:44 PM
 
We've discussed the pros and cons of using Virtual Memory many times. I think the general consensus is that the more physical memory you have, the less necessary VM is in your system. Apple sells computers with relatively small amounts of physical memory installed, I think that's why they recommend using VM. However, with the way memory chips have dropped in price, you can put massive amounts of memory in your Mac, fairly cheaply. The general rule of thumb is that with 256MB or more physical, try VM off.
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Aug 17, 2001, 07:47 PM
 
Originally posted by Fredo:
<STRONG>We've discussed the pros and cons of using Virtual Memory many times. I think the general consensus is that the more physical memory you have, the less necessary VM is in your system. Apple sells computers with relatively small amounts of physical memory installed, I think that's why they recommend using VM. However, with the way memory chips have dropped in price, you can put massive amounts of memory in your Mac, fairly cheaply. The general rule of thumb is that with 256MB or more physical, try VM off. </STRONG>
Another good reason to use Virtual Memory is that it has better memory management. If you've used Mac OS without VM, you'll notice that some of your available memory 'disappears,' even when you have no applications running. This is due to memory fragmentation. When MacOS applications need memory (ie, starting up) it will only grab free memory from the largest contiguous block of free memory. The more computer work you do, the more fragmented you memory may become as applications fail to release blocks of memory, or dynamicly loaded libraries remain with memory (such as Open Transport if you check the 'Load only when needed' box). If memory becomes fragmanted enough, you may not be able to launch the smallest of programs, no matter how much RAM you have since the system cannot find a large enough block of contiguous free memory.. I have 768 MB of RAM. Just as an experiment, I ran without VM for a whole day. Normal I have 500 MB of free RAM (I have a very large RAM Disk). Well, after about 5 hours of hard use (Internet stuff, development stuff, playing games which were stable without VM), I tried to start up UT which I manual configured to use 250MB. No go. I had 160 MB 'free'.

With VM turned on, the system manages memory a lot more efficiently. I can run all day, or all week, with VM on and fragmented memory never becomes an issue. I also run it becomes many of the games I play run faster with it on. Some games, like Diablo II, require VM to be on due to the tremendous amount of dynamic memory allocation.

One interesting note about VM, is that those applications which require less memory when running, are actually increasing the size of your memory. When VM is on, certain parts of an application's code does not need to be loaded into memory. Instead, the actual physical address of the block of code is mapped to your memory. Ie, say a web browser will run with 4 MB less RAM when VM is on and you have 65 MB of RAM (64 MB + 1MB with VM set to 1 MB above physical RAM). Your system memory will increase to 69 MB, but the System is designed to show end users a set memory size. That is why HD usage may increase when VM is turned on (aside from HD space being used as regular RAM.
     
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Aug 17, 2001, 08:22 PM
 
I have 256 megs of ram. Should i leave virtual memory at one meg, or should i increase it.
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Aug 17, 2001, 09:46 PM
 
I also have my own theory that MacOS memory management will be less fussy if you do one thing- load all of your small, simple apps first and load your big, memory-hogging apps last. Also, apps that gradually expand their memory allocation as you use them (Netscape, Internet Explorer, VPC, Photoshop?, audio processing apps, etc.) should be loaded last. Once you have loaded the big, memory-growth apps, avoid loading any other apps (or if you do, load them, do what you planned to do, and then quit them ASAP). Once I do this, I find the memory fragmentation issue to be pretty much worry-free. Ideally, you would have only 1 big app as your last load. That way it is free to expand its memory space is free to expand as needed w/o running into the memory space of another big app. If you must run multiple big apps, make sure they have a healthy memory allocation so they get enough elbow room from the start with room to grow.
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Aug 18, 2001, 03:57 PM
 
gto47, try this: Put VM on, set at 257. Run it for a day or two, then take VM off. See which you like better.
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Aug 23, 2001, 05:04 AM
 
I turned off VM in my Indigo iBook with 320 MB real RAM, and Word and Excel (I know, I know, it's not like I have a choice) would just freeze after running for about 10 minutes...I didn't have to be working in them, they'd just freeze. I'd walk away for 10 minutes, and when I came back, Bam! seizure. Running by themselves, running together, no other apps running, didn't make any difference. No idea why...so, I just leave it on. It sets itself automatically to 321 MB. Regards to you all. Ken
     
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Aug 23, 2001, 08:52 PM
 
With VM off, you have to manage the memory that goes to your applications.
Keep VM off.
Try giving the app triple the suggested amount and see how it works. Also, close out applications before you open new ones.
Run your utilities regularly, and you'll have a smooth running, snappy, fast system.
Good luck.
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