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My PC currently only has 1GB of RAM, and I am currently using 86% of that. With 2GB of RAM, would I be using 43% right now?

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No, quite the reverse. Under the same load, the system should need about the same amount of free memory to handle it. With twice as much memory, the same amount free would mean 93% in use. (What would be the point of adding another GB of RAM just to have it not be used?! If the system was that stupid, why bother adding RAM? It won't run any faster, it'd just waste more memory.) Modern operating systems only need free memory to handle certain specific unusual cases. The rest of the memory, they are able to use. –  David Schwartz Apr 8 '12 at 8:06
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@David Schwartz I posted this 3 years ago, now I can tell you right now with everything running and 12gb of RAM my system does use a lot more resources, I generally run around 80-95% in use, A huge chunk of that seems to be 1 of Google Chromes extensions which I think is the Flash extension as it just grows untilI run out of RAM, I then close Flash and my RAM free's up almost 50% of it...seems to be a HUGE memory leak in Chrome/Flash –  jasondavis Apr 9 '12 at 13:09
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8 Answers

up vote 9 down vote accepted

Yes, your math is correct, to some extent.

However, note that most modern operating systems use more memory for caching. In particular, Vista and Windows 7 are really aggressive with this. This article does a good job of explaining how memory management in Vista works--basically, it tries to use up your free (idle) memory for its cache, and when needed by another program, that memory is released again.

So the OS might appear to be taking up more memory, but it is only for caching purposes and can be freed at any time.

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thanks, I'm using windows 7 so it's probably similar to vista –  jasondavis Sep 2 '09 at 21:56
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Even XP and earlier versions of windows used free memory as disk cache; this isn't new to Vista. –  Captain Segfault Sep 3 '09 at 0:56
    
XP and earlier versions of Windows were based on the design assumption that memory was scarce and couldn't use significant amounts of memory as a disk cache. Vista and later take the more modern approach of trying to keep as much memory in use as possible. –  David Schwartz Apr 9 '12 at 4:52
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Not necessarily, most operating systems use extra memory as a buffer.

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vista used to occupy about 50% of my RAM when i had 2GB of RAM,idle

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My company recently upgraded their XP laptops from 1 GB to 2 GB and my machine is currently using 1 GB of RAM, so XP probably uses more, too.

It's not like your memory is only RAM. There is also the pagefile, which is basically used for the same data. Just think of RAM as the faster part of your system memory. If the OS notices there is more RAM available, it will use more of it to store data that would normally reside in the pagefile. But if needed, the memory can be released quickly again, so that's nothing to worry about.

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This is really totally incorrect. Windows will use very, very little pagefile unless it is under memory pressure. It the OS notices there is more RAM available, it will use more of it to store data that would it would normally read from the filesystems. –  David Schwartz Apr 9 '12 at 4:54
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No you would most probably not be using 43% of your RAM if you upgrade to 2GB. Some programs utilize RAM in a logarithmic manner - more RAM detected, they will attempt to use more then.

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No.

Having recently upgraded my RAM, I know that XP will use more for buffering and caching purposes when it has more to mess with. With 768mb RAM simple services in Windows like spoolsv.exe (printer related) would use about 1 to 1.5mb of RAM. With an upgrade to 2.5gb of RAM spoolsv.exe is now hogging 4mb, as are many other services that previously used around 1mb.

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It is important to understand that memory usage in Windows is highly dynamic. Memory usage is dependent on system workload and how much memory is available. When more memory is available the system will enlarge the system cache and process working sets. When memory is short then working sets will be trimmed back. Physical memory usage is controlled almost entirely by the system with applications having only limited influence.

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No, you'd probably be using less than 86% of that. It's like the way a family lives in a house. If you have 10% of the house empty and you move into a house twice the size, you eventually wind up with the new house about 90% full as well. Managing space is an active process, and more space means you can keep more stuff. There's no point in having more space just to keep more of it empty.

Free memory is just a cushion, a necessary evil in case the system needs space faster than it can free it. Otherwise, the system does its best to use all available memory. That tends to result in the least disk I/O and the most responsive system.

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