MacBook – The free vs. inactive memory ratio on a Mac with 16 GB RAM has me concerned

I have found, on my machine, that Mac OS X is a little slow to page the right things out.

Whenever doing anything particularly memory hungry, I will often quit a few applications to help it out. Safari is a huge memory hog if it has been running for a while and is top of the list of applications to quit. In my case this was when I tried to run VMWare Fusion without quite enough actual RAM.

You probably know this but here are a couple of links to Apple documentation which describe memory usage and virtual memory on OS X:

http://support.apple.com/kb/ht1342

http://developer.apple.com/library/mac/#documentation/Performance/Conceptual/ManagingMemory/Articles/AboutMemory.html

Here is the explanation that is given: The inactive list contains pages that are currently resident in physical memory but have not been accessed recently. It's hard to get an exact definition of what this means but is seems to include a file system cache as well as actual inactive memory.

Inactive memory is probably a difficult thing to handle on an interactive GUI driven OS where a user may choose to switch to any running application at any time and providing responsiveness in this sort of situation is important.... but the flipside of that is that in a situation like yours the OS doesn't quite know what the right thing to do is.

It is possible to purge (some of) the inactive memory by using the purge command. This may need to be installed by installing the CHUD tools but you may already have that installed. I haven't tried using the purge command so YMMV...

I've rambled on a bit and may or may not have answered your question. After writing all this out, I found another question on this site which much of the same info that might also help.

1. If you run Parallels off a disk image, you can expand that partition to as much disk space as you have available. All it requires is shutting down the VM. As far as I know you can't shrink a disk image.

   • If you're running Parallels off a Boot Camp partition, it's possible in some cases to expand that partition later on, using third party tools. This depends on how the other partitions on the disk are set up (typically a partition can be expanded to fill free space after it, but you can't change the starting point on the disk).

2. How much RAM you allocate can be changed using Parallels. You simply shut down the VM, change the setting in Parallels and restart. This doesn't change whether you're using a disk image or a Boot Camp partition, but it's worth pointing out that if you boot from the Boot Camp partition (not in Parallels), that's running Windows directly and it will use all the RAM available to the system, like any other laptop.

   • The RAM allocated to your VM is only "used" when the VM is running. If you shut down Parallels, OS X can use the full amount in your system.

3. How you allocate disk space and RAM depends on what you're doing with your VM, and what you expect to be doing with OS X while that VM is running. Think of it as configuring a whole new computer - you need a quantity of hard drive space and RAM sufficient for the tasks at hand. The easy part is that you can (mostly) change your mind later on if you guess wrong with a VM.