I am using ubuntu precise 12.04 and I am booting a paravirtualized ubuntu 12.04 domU, all being 64 bits. The VM has memory=1024 assigned and xentop at the host correctly shows 1024MB, but htop in the domU only reports 989MB.

Where are the other 35MB going?

I do not see such a difference with ubuntu hardy 12.04 domUs...


It is normal for the booted kernel to see slightly less memory than is available. It happens on bare metal also.

The reason why you probably see the "correct" amount in hardy (I assume you mean 8.04, not 12.04) is that this probably used a Xen-patched kernel as opposed to a pvops mainline kernel like in 12.04.

The old Xen patched kernels did provide exactly as much RAM as configured, whereas when the patch went upstream it ended up behaving the same as normal Linux.

Every single pvops kernel I have seen has slightly less memory available than is installed (bare metal) or configured (for a VM).

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