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I have Windows 7 32-bit professional as host OS. Ubuntu 12.04 32-bit runs really fast as a guest, but Ubuntu 12.04 64-bit is very slow. I did apply all the patches, installed guest addons and enabled VT-x/AMD-V and using the latest VirtualBox (4.2.18).

Is it the expected behavior or am I missing something?

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Do both VMs have the same resources allocated? Are you running the two at the same time? What are the HW specs of your host machine? – sgtbeano Nov 29 '13 at 11:12
I am not running them simultaneously. 64-bit guest was slow with the same resources initially, so I bumped the resources to 64-bit.But 64-bit still runs slow than the 32-bit which has less resources.I am using a 2nd Gen Ci5/8GB/500GB. It has 8GB, but the 32-bit Windows can address 4GB. – Praveen Sripati Nov 29 '13 at 11:33
This question seems to shed some more light ->… - I know that relates to different VM software but the principle is the same. – sgtbeano Nov 29 '13 at 11:36
the m/c in query is 32-bit, but I am using a Ci5 which is 64-bit processor. it's completely different. – Praveen Sripati Nov 29 '13 at 11:43
Not really, if you look at the answers - you don't have an issue with your processor being supported, but you are mixing 32-bit host with 64-bit guest. Given that you have a difference in performance between the 32-bit and 64-bit guest I would guess that some sort of translation is having happen which is causing your performance drop. – sgtbeano Nov 29 '13 at 11:50

My understanding is :

When you use VirtuaBox as a virtualization software, you are basically running the guest VM over the virtualization layer (i.e. VirtualBox) which itself runs on Windows 32 bit. So indirectly whatever restrictions that the host OS puts on Virtualbox will cause performance issues to its guest. One such limitation is memory.

However if you had used a true barebone virtualization software like Hyper V or VmWare ESXi, then the lowest layer is the virtualization layer and then comes the host followed by the guest. In this case the limitations of the hosts do not apply on the guest.

So my suggestion is either upgrade the host to x64 or switch to Hyper V /VMWare ESXi or XenApp Server

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