How many CPUs & RAM for VM?

B

Bruce Carillon

Member
My MacBook Pro has 8 available CPUs and 8 GB of RAM, how many CPUs and GBs of RAM should I designate to the Windows 7 guest OS in the VM? I'm not concerned with battery life since the majority of my time is spent connect to AC power.

Thanks!

Mac OS X Lion 10.7.2
 
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I suggest 1 CPU and 1.5-2 GB RAM max. to the VM. It depends on what you want to do with the VM of course. Don't assign more than 50% of your Mac's resources to the VM; however, you won't see much of a difference between 1 CPU and 4, or between 1.5 and 4 Gigs, on your VM - so you might as well save them for stuff you run on your Mac.
 
Thanks for the reply... I'm really trying to get performance of the VM optimal since I use an application, Dragon Naturally Speaking, for dictation and it seems really sluggish. Is it possible that too many CPUs and too much RAM to the VM will actually hinder VM performance?

Thanks again!
 
Yes. My experience is that assigning just one core to the VM keeps all the VM's processes assigned to that core, and the Mac can use the other core(s) to do whatever it needs to do (including keeping the VM happy). As far as I understand, monitoring and maintaining the various threads and processes that are concurrently running on the multiple cores becomes the harder the more cores are available. So you can help your Mac by limiting the number of cores made available to whatever is running in the VM. Limiting memory is also likely to help as your Mac is then less likely to run out of memory (which means more disk swapping).
Parallels, it must be said, does a good job of memory management, using only the memory that the VM really needs - plus of course the overhead. So if you only have Windows and, say, Word open, it will not reserve all the specified memory for itself, but only maybe a Gig or so. The memory that you determine in the VM setting is more like the upper limit of what the VM will claim for itself.
 
Thanks again, very good info to know. I'm going to try this configuration and see how it works for me.
 

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Thanks this is really good information. I had the same question as '105437' and I can confirm what you are saying in that under similar hardware I had 1 CPU and 2 GB RAM allocated to the virtual machine, then upped it to 4 CPU and 4 GB Ram, hoping to increase performance on the Virtual Machine, which was actually fine, I'm not sure what I was thinking. As expected the Mac got slower, but the Virtual Machine (64 bit Windows 7) got EVEN SLOWER than before. In fact, it almost came to a grinding halt. Wow. I would not have guessed that. But your explanation makes sense and it makes me wonder why that info is not more readily available from the panel where those setting are made. Although, when I set RAM to 4GB, I foolishly failed to notice that I was just inside the 'red' zone.

One slightly tangential comment: Power Hungry applications, like Adobe Premiere Pro for instance, should JUST NOT BE RUN while Parallels Desktop is running any virtual machine, IMHO. I plan on upgrading my main hard drive on my MacBook Pro to an SSD drive and I'm betting this will be less of a problem. Anyhow, THANK YOU Shaddam IV for the great information.

-Michael
 
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