Xen and KVM kernel compile slowdown [closed]

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In this slide deck on Xen vs KVM, the benchmarks indicate that CPU and disk is nearly as fast under virtualization (~10% slowdown). Yet virtualization slows down a开发者_如何学JAVA kernel compile by more than a factor of 2. What causes this?

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A Linux kernel compile is a process heavy operation (lots of short-lived processes). Process creation/destruction involves setting up and tearing down MMU state. MMU virtualization is expensive (especially in software). There is a VMware whitepaper that shows the improvement in compile-time benchmarks when hardware MMU virtualization is used. I don't think the hardware used in the referenced slide provides hardware MMU virtualization. I don't know if either KVM or Xen support it (but I suspect so).