[Vm-dev] BFS and CFS and Cogs, Oh My

Casey Ransberger casey.obrien.r at gmail.com
Fri Apr 19 15:02:59 UTC 2013

Well hi, Alex:) Nice to meet you!

I agree wrt to real workloads, microbenches mean very little. Apache might
be a good candidate for benching. That's off the top of my head, see below
about how I'd love to hear what you'd like instrumented.

My main problem with BFS is that the name -- under the rather restrictive
conditions I had to agree to in order to join the Pi Foundation's message
boards -- cannot even be discussed. That and it might upset some parents,
not to mention possibly confusing some kids.

Let's not doubt that kids will get their hands in the kernel. That would be
a poor assumption, if any of them are a bit like we were.

The other reason for suggesting a fork is that the original author has
stated that he hasn't got intentions around supporting the work on a broad
scale, and I think something which could end up empowering Pi users ought
to have someone backing it up. I'm considering being that person. This is
the part where my own self interest says "shut up and go home" and I fail
almost entirely to listen.

Here's what Con and I talked about: I would fork, change the name, track
his work, and then contribute back anything of value. Downstream forkiness,
basically. I shield him from support randomization, and that makes this
thing supportable.

What I'm getting at: it's a matter of branding. Sorry, I don't use the
marketing-department hat often, at least not in public, but here we are!

Also: it's worth noting that I'm a bit of a culture-jammer. Changing the
name might have some very funny positive effects. At Apple, when no one
wanted to hear about Smalltalk anymore, some very clever people "invented"
Squeak. Which of course they'd already invented as Smalltalk, etcetera. But
the radar hadn't learned about Squeak yet, and so the balloon sailed away
underneath it one more time. Or that's the version of the story that I

I'll take your advice and start experimenting (was going to anyway.) The
output of my experimentation, assuming I don't get run over by a bus in the
meantime, will be some macro benches. Then we can start talking turkey, no?
;) Anyway I'd like to come with facts and numbers to a discussion like
that, rather than conjecture.

Thanks for your thoughtful reply!

I'd like to ask a favour: can you name off some not-Squeak applications
(maybe Python based stuff or something?) that you'd like to see some
numbers around between the two schedulers? Feel free to reply to me
directly, as I imagine this veers away from the focus of vm-dev a bit.


On Fri, Apr 19, 2013 at 6:16 AM, Alex Bradbury <asb at asbradbury.org> wrote:

> On 19 April 2013 08:38, Casey Ransberger <casey.obrien.r at gmail.com> wrote:
> >
> > I had a brief chat with Con Kolivas, who did BFS (which implements
> kernel stuff that will make Cog happier under Linux on machines with
> sub-supercomputing quantities of CPUs) tonight.
> >
> > It sounds like there are actually two reasons it hasn't made it into the
> mainline kernel:
> >
> > a) he doesn't have time to support it, and
> > b) the other kernel folks don't want it.
> >
> > Oh well. Since right now I'm focused on Raspbian, I sent a message
> explaining what it was, why I want it, etc on their web board. If I do get
> it in, support would have to fall to me. Yikes, right? ;)
> Yes, for political reasons it seems unlikely anything like BFS would
> get in to the upstream kernel. If someone can do work to actually show
> noticeable performance gains then that would make us (the Raspberry Pi
> Foundation) interested in exploring further. Real workloads that
> perform much better with an alternative scheduler would be much more
> interesting than microbenchmarks. Of course the next step after that
> wouldn't be dumping the upstream scheduler and switching to BFS, but
> it would certainly justify taking a closer look.
> I'm not entirely sure why you want to fork BFS - as far as I can see
> Con Kolivas is keeping the BFS and his larger -ck patchset up to date
> with upstream releases.
> In conclusion (from a Raspberry Pi perspective): please do play with
> BFS on the pi, do something useful with it (if it solves the recently
> discussed issues with heartbeat+cogvm then swell), then let's think
> about where to go from there.
> Regards,
> Alex
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