An Introduction to Full Virtualization With Xen (Linux.com)

Posted Oct 25, 2012 10:55 UTC (Thu) by rwmj (subscriber, #5474)
In reply to: An Introduction to Full Virtualization With Xen (Linux.com) by Tobu
Parent article: An Introduction to Full Virtualization With Xen (Linux.com)

I love what Anil is doing with Mirage, but it's not true that this is an argument for Xen over KVM.

In the KVM world, you can already write a virtual machine that is entirely self-contained and requires no operating system. It's called .. erm .. a *process*, and Linux has had them for rather a long time.

KVM virtual machines are just regular processes, and you can run ordinary processes alongside them.

In a realworld case, say that your virtualized Apache server isn't getting the performance you need running under KVM. Well, just run an Apache server on the host instead.

Rich.

An Introduction to Full Virtualization With Xen (Linux.com)

Posted Oct 25, 2012 12:08 UTC (Thu) by Tobu (subscriber, #24111) [Link] (3 responses)

You mean a process with a kernel, a scheduler, a page allocator, etc, underneath? That's not the same level of safety and implementation control at all.

An Introduction to Full Virtualization With Xen (Linux.com)

Posted Oct 25, 2012 13:00 UTC (Thu) by rwmj (subscriber, #5474) [Link] (2 responses)

No, I mean what I said: a process.

An Introduction to Full Virtualization With Xen (Linux.com)

Posted Oct 25, 2012 13:37 UTC (Thu) by Tobu (subscriber, #24111) [Link] (1 responses)

You did, but Apache doesn't illustrate very well. It targets POSIX, and just about every operation it does (network, memory, storage) goes through the syscall boundary. It also has multiple processes, which implies another large chunk of ipc, scheduling, and resource management is done outside of it. These abstractions are rigid boundaries that it cannot cross.

An Introduction to Full Virtualization With Xen (Linux.com)

Posted Oct 25, 2012 14:00 UTC (Thu) by rwmj (subscriber, #5474) [Link]

OK, but you can use mmap, O_DIRECT etc with a Linux kernel.

If you mean that it's better to program directly against the Xen hypervisor or some other exokernel, instead of using Linux at all, then Mirage is certainly an argument for doing that. (Also loving it myself because it's written largely in OCaml ...)

But the at some point I just know that my program is going to want to write to a file or ask the user a question, and then having Linux around and improving its support for low-level ops starts to look like a better long term option.