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When I open the Activity Monitor, I get something looking like this: http://support.apple.com/kb/HT1342.

But why the heck is "VM size" around 100-200 GB??

Here is the capture from the apple support:

Activity Monitor

See the "VM size: 172.23 GB".

Is that a bug, or can you explain to me where are those GB?

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up vote 10 down vote accepted

They are virtual and cost nothing to make. The VM hands out space to each program that asks for any but leaves tons of room to grow between each real allocation. These virtual addresses get mapped into the real RAM space as needed and mapped out when they are freed or otherwise inactive. They can even get paged to your storage if needed.

You can reboot to convince yourself it starts back at zero. This is expected that it grows over time and not a bug. The real query is what apps are requesting all that data and if you don't mind looking at files in the terminal sudo sysdiagnose is a great tool to dive deeper into virtual and real memory allocation.

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OK thanks for the info. That's weird I know the process of virtual memory, but 210 GB and I started my computer 3 hours ago... It seems crazy to me. But thanks –  Matthieu Napoli Mar 3 '12 at 16:01
    
Run sudo sysdiagnose - it will map all memory and show you the tools to break down vm allocation. There's a ton of other info - but it will account for the memory in more detail. –  bmike Mar 3 '12 at 17:05
    
do you install sysdiagnose with MacPorts? It says command not found. –  Matthieu Napoli Mar 4 '12 at 10:25
    
It's lion only, but it's a shell script that calls (among other things) /usr/bin/allmemory, /usr/bin/heap, /usr/bin/vmmap –  bmike Mar 4 '12 at 12:49
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