Re: [Xen-users] Openmirage, erlang on xen, etc

[Antti Kantee <pooka@xxxxxx>]

On 20.8.2013 9:15, Darren Shepherd wrote:
> I'm really interested in openmirage, erlang on xen, mini-os, rump kernels (just 
> saw that today), or anything that runs directly on xen.  I think there's a huge 
> potential and use case with these technologies.
>
> I have a random question though.  I've only seen these type of things running 
> on Xen.  Is there any specific reason you don't see much on KVM?  Maybe they 
> exist and I just haven't seen them.  I was just curious if there was something 
> specific to Xen's architecture that made these framework easier to develop.
Hi,

I can only speak for work I've been involved with, so the following is 
about rump kernels.
Let's start by putting the capability of running rump kernels on top of 
Xen into perspective.  Support was a handful of lines of code and a few 
days of work.  This should first of all be contrasted to the 6+ years of 
work on rump kernels.  Second, given that rump kernels are about 
bringing drivers which have been "trapped" inside a monolithic kernel to 
other environments without having to modify the drivers, there's really 
30+ years of code history at play with rump kernels.  The fact that it 
was quick and easy to make work suggests both that it is easy to plug 
non-traditional OS's on top of Xen (or on top of mini-os at any rate), 
and that the anykernel (which is the kernel architecture which makes 
rump kernels possible) is a working idea.  Now, the good thing about 
doing work in small pieces, such as the one in question here, is that 
you don't have to think/evaluate too much before starting -- if it turns 
out to be a bad idea, just toss it in the bucket and try to get another 
small piece right.
My underlying motivation for this work was to get rump kernels running 
on some environment without preemptively scheduled threads; long story 
short: it will help gain a better understanding of optimal scheduling 
for high-performance rump kernels in userspace.  At some point I'll 
translate the lessons learned from this into sched_setparam()s and 
sched_yield()s -- unless Xen proves to be a better platform to high 
performance "userspace" drivers.  Fundamentally, here we're talking 
about using rump kernels as exokernels.  However, "exokernel" as a term 
probably has too much historical ballast, so I think I'll use "jinba 
ittai" to describe that runtime model from now on.
Finally, getting to your actual question, I picked Xen because 1) I'd 
had discussions without various folks about running rump kernels on top 
of Xen 2) the Linux distro I used to do the work (Void Linux) has great 
Xen support.  To be honest, I didn't even consider KVM before starting 
the work.  For all I know, it might have been better.  But thinking 
about it now, a rump kernel is all about paravirtualization, so KVM does 
sound like a tool that's optimizing for the case that I'm not interested in.
  - antti

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