Best use of multiple cores?

J

Jay Levitt

Member
The Penryn 8-way (dual quad-core Xeon) Mac Pros have been released, and I've been promising myself one ever since the rumors started. That means I'll be switching from a dual-core Windows XP desktop to a Mac Pro with Parallels to run my Windows XP apps.

I know that Parallels (unlike VM Fusion) can't create a dual-core virtual machine yet. If I set up multiple VMs, though, can they each run on different cores? Is that the best/only way to take advantage of the multiple cores?

I intend to configure a boatload of RAM on this box, so that shouldn't be an issue.
 
I've got multiple virtual machines running on my MacPro at the same time (a couple XP machines for IE6 and IE7, maybe a joomla or a drupal CMS) and they all run in separate Parallels applications, so it's up to the Mac OS to schedule them to whatever cores are available.

From my experience the Mac OS does a fine job of moving threads around to keep all the cores busy.
 
I am making the exact same switch. I am a heavy PC user, and now that I can get a Mac Pro, with 2xQuad-Core, and 8GB of RAM, I am willing to make the upgrade:)

So, if I understand it correctly; I cannot dedicate CPU power to the Vista VM, or the coherence session, but I can dictate how much RAM is assigned, and this is ring-fenced?

Also, does Parallels take advantage of the 4 cores in each CPU?

KB
 
Killerbob said:
So, if I understand it correctly; I cannot dedicate CPU power to the Vista VM, or the coherence session,
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No - apart from a very crude choice in the VM of whether to optimize VM performance or host OS performance
but I can dictate how much RAM is assigned, and this is ring-fenced?
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Well, the windows machine will always "see" that amount of RAM - not sure what priority that gets in the OSX virtual memory system (I suspect it gets locked in physical RAM if poss). Have plenty of RAM if you're planning to use multiple VMS while also running stuff in OS X.
Also, does Parallels take advantage of the 4 cores in each CPU?
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Each VM only "sees" a single CPU - but on the OS X side, the Parallels app itself, which is doing lots of I/O and other services for windows, is multi-threaded and hence can potentially benefit from more cores. Multi-cores should also help if you're running more than 1 VM at a time.
 
Well multi-cores help even with a single VM, but more than dual-core only helps with a single VM if you have a lot going on with the OS X side. One core for OS X and one for Windows actually works out pretty nicely.
 
But we are not talking about single or double cores here. In the new MP htere are two CPUs, each with 4 cores. So, how will Parallels deal with that? Say I am running Vista Ult. in a VM, with dedicated 2GB RAM. How many CPUs, and Cores, will I see in that Vista "machine"?
 
With Parallels Desktop, in the current form, Vista will see a single core. But, the other cores will be allocated to OS X, which, as pointed out above, can help VM performance as Parallels Desktop is a multi-threaded application that supports multiple cores. There is a certain amount of processing that takes place in OS X and a certain amount that takes place in the VM, so technically it can leach off the other cores a little.

Forthcoming Parallels Server is supposed to support multiple cores. Yummy.
 
...which, and please help a newbie, will mean if I run two, three, or four VMs of Vista Ult. they each will have the full run of one core each, still leaving one CPU, with four cores, to OS X?
 
Let the total cores on your computer = n
You can have n-1 virtual machines each with a core assigned to them.

OS X can access everything that your VM is not using at that point in time.
 
Killerbob said:
But we are not talking about single or double cores here. In the new MP htere are two CPUs, each with 4 cores. So, how will Parallels deal with that? Say I am running Vista Ult. in a VM, with dedicated 2GB RAM. How many CPUs, and Cores, will I see in that Vista "machine"?
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One. Sorry.

Having 2 cores might make I/O, Coherence etc. and anything you're doing on Mac OS work more smoothly, but I doubt that you'll see much difference with 8.

If you have a 4- or 8-core Mac Pro then VMWare Fusion's ability to make multi-core VMs might be a deciding factor. For the majority of 2-core Mac users - and those using Windows for office software and other light tasks - its probably not a big issue.

Quite frankly, 4 or 8 cores is an overkill for most people, anyway - its most useful for people using high-end graphics and video software that has been optimised for multiprocessor systems (which, of course, is the target market for Mac Pro) - and if you're trying to run that sort of software under Windows then you'd be better off with a BootCamp install.
 
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