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So I was wondering if anybody knew if there was any performance issues with virtualizing a 32 bit os, in my case 32 bit windows 7, on a 64 bit os, mac osX lion. I remember reading that running 32 bit processes in a 64 bit system can cause performance robbing situations with paging memory. Does this apply to virtualizing or am I misinformed?

I fully understand that the performance of the guest also depends on the amount or memory and cores you devote to your vm, but I was wondering more on the mechanics and the effects of mixing 32 bits with 64 bits.

Thanks everyone.

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migrated from stackoverflow.com Feb 19 '12 at 1:41

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16-bit applications running in the 32-bit VM are going to require emulation when the host has put the processor into 64-bit mode. –  Ben Voigt Feb 19 '12 at 1:30
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You are misinformed. There is no penalty with running 32-bit guests or applications on a 64-bit host OS that isn't more than made up for by the performance benefits of a 64-bit host OS. That is, a 32-bit application or guest will almost always be as fast or faster on a 64-bit host than it would be on an otherwise equivalent 32-bit host. –  David Schwartz Feb 19 '12 at 1:56

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in theory it shouldnt be a problem. We could debate the semantics but If you are running any 32 bit programs on your 64 bit computer then you are running those programs in a virtual environment (which you likely are running 32 bit processes). But specifically speaking, when you are running windows on mac, your biggest problems are the challenges that your hypervisor will have in translating windows drivers to interact with the devices on your system. Usually this means the hypervisor works extra hard in keeping everything organized and translated appropriately. Bottom line, your biggest performance drop will have to do with how much power you pull from the processor constantly pegged out. If you are used to 6 hour mac battery life, say hello to 2hr battery life or less.

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That makes a lot of sense. Battery life definitely drops, but that is definitely expected if you making the cpu work harder. –  Ben Cannon Feb 19 '12 at 1:41

From what I understand, the host would just translate the addresses to a 32-bit address space. Personally, I've never had any noticeable performance issues and I've never heard of such.

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Running the guest inside a reasonbly good VM should not cause any problems. VMWare (my favourite) has never given me any reason to think about this, much less worry.

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I'm running VMWare too and I'm pretty pleased. I was more just wondering if anybody had a better insight to effects of mixing the operatings, if there was any. –  Ben Cannon Feb 19 '12 at 1:39

It depends on the virtual machine software you're using, but with the main ones (such as VMware, VirtualBox and Microsoft VirtualPC/Hypervisor) you won't see any downside to running a 32-bit guest in a 64 bit host on x86/x64. This is because the virtual machine will actually switch the CPU to 32-bit mode while the guest is running.

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