Worlds greatest kernel
kmacy at fsmware.com
Thu Oct 9 20:20:22 PDT 2003
> I had assumed, perhaps incorrectly, that Xen was like VMWare, in
> that it required a host OS to live on, and then one or more Xenified
> guest OSes could run under Xen.
Xen is like VMWare, however, not Workstation or GSX, but ESX. They
are in effect microkernels with the machine interface as an API. VMWare
ESX virtualizes the hardware perfectly. Unfortunately, on x86 this
requires dynamic translation as a number of instructions don't cause
traps but simply fail silently and shadow page tables because TLB refills
are hardware driven. For activities that require a lot of privilege
transfers this has a great deal of overhead. Xen does a much less complete
job of virtualization, requiring a certain amount of porting. This
"paravirtualization" approoach requires much less overhead. The
only benchmark on which XenoLinux (Linux running under Xen) performed
substantially (8%) slower than native Linux was the PostgreSQL benchmark.
I attribute this to the fact that they currently don't support superpages,
even for kernel KVA. I've been in communication with them in regards to
this and have encouraged them to measure the difference in TLB misses
between native and virtualized. Once they've added support for superpages
I expected the difference to diminish.
I hope this clarifies the details some.
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