Why it is better to use only 1 processor core in virtual machine?

hajime

macrumors 603
Original poster
Jul 23, 2007
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Based on experience and advice from others, it seems that even with a 4-core CPU, it is still better to assign only 1 processor core to the virtual machine in Fusion. Anybody knows why?
 

Gav2k

macrumors G3
Jul 24, 2009
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Don't quote me but I think it's due to vm requiring a core to be free to perform its duty. So if you assign all cores it'll spend more time waiting for the cores to be free.
 

marzer

macrumors 65816
Nov 14, 2009
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Colorado
Based on experience and advice from others, it seems that even with a 4-core CPU, it is still better to assign only 1 processor core to the virtual machine in Fusion. Anybody knows why?
That's not my experience. I've been running Virtualbox on the i7 CPU for several years and assign at least 2 processors to each VM, whether Windows 7 or Ubuntu. I find the VMs are more responsive with little to no impact to the host.
 

hajime

macrumors 603
Original poster
Jul 23, 2007
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That's not my experience. I've been running Virtualbox on the i7 CPU for several years and assign at least 2 processors to each VM, whether Windows 7 or Ubuntu. I find the VMs are more responsive with little to no impact to the host.
My rMBP has an i7 2.6GHz CPU with 4-core. So, I should assign 2 processor cores to the virtual machine in Fusion?

I assigned 8 out of 16GB RAM to the virtual machine. Is this an optimal setting?
 

marzer

macrumors 65816
Nov 14, 2009
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Colorado
My rMBP has an i7 2.6GHz CPU with 4-core. So, I should assign 2 processor cores to the virtual machine in Fusion?

I assigned 8 out of 16GB RAM to the virtual machine. Is this an optimal setting?
Why not? There's no risk in playing around with VM settings. Tweak processor count and memory to find out what gives you the best balance between your VM and host machine performance. Try various settings to see which you like best.

There are no optimal settings in general. It comes down to what you do with your VM, how responsive you would like it to be, how responsive you would like you host machine to perform while the VM is operating.

For instance, if all you do is dink around with VM and do no serious work on it, then you may as well config it with minimal resources to lessen the impact to the host machine.
 

jdelgado

macrumors regular
Oct 25, 2009
153
7
Chile
My guess is that the optimal settings depend on what you want to do. For my particular case, they are 1 core and 4GB of RAM. I use Windows 7, 32bit (no point in assigning more than 4GB) and the programs I use do not care for more cores.
I run VMs in a mid-2012 i7 MBP with 16GB of RAM and a 500 GB SSD. FWIW, when I installed the SSD and increased the RAM to 16GB, VM performance improved significantly; much more noticeable that any settings change.