Re: [RFC] Unify KVM kernel-space and user-space code into a single project

Ingo Molnar
2010-03-18T22:44:58Z

* Zachary Amsden <zamsden@redhat.com> wrote:

> On 03/18/2010 11:15 AM, Ingo Molnar wrote:
> >* Zachary Amsden<zamsden@redhat.com> wrote:
> >
> >>On 03/18/2010 12:50 AM, Ingo Molnar wrote:
> >>>* Avi Kivity<avi@redhat.com> wrote:
> >>>
> >>>>>The moment any change (be it as trivial as fixing a GUI detail or as
> >>>>>complex as a new feature) involves two or more packages, development speed
> >>>>>slows down to a crawl - while the complexity of the change might be very
> >>>>>low!
> >>>>Why is that?
> >>>It's very simple: because the contribution latencies and overhead compound,
> >>>almost inevitably.
> >>>
> >>>If you ever tried to implement a combo GCC+glibc+kernel feature you'll know
> >>>...
> >>>
> >>>Even with the best-run projects in existence it takes forever and is very
> >>>painful - and here i talk about first hand experience over many years.
> >>Ingo, what you miss is that this is not a bad thing. Fact of the
> >>matter is, it's not just painful, it downright sucks.
> >Our experience is the opposite, and we tried both variants and report about
> >our experience with both models honestly.
> >
> >You only have experience about one variant - the one you advocate.
> >
> >See the assymetry?
> >
> >>This is actually a Good Thing (tm). It means you have to get your
> >>feature and its interfaces well defined and able to version forwards
> >>and backwards independently from each other. And that introduces
> >>some complexity and time and testing, but in the end it's what you
> >>want. You don't introduce a requirement to have the feature, but
> >>take advantage of it if it is there.
> >>
> >>It may take everyone else a couple years to upgrade the compilers,
> >>tools, libraries and kernel, and by that time any bugs introduced by
> >>interacting with this feature will have been ironed out and their
> >>patterns well known.
> >Sorry, but this is pain not true. The 2.4->2.6 kernel cycle debacle has taught
> >us that waiting long to 'iron out' the details has the following effects:
> >
> > - developer pain
> > - user pain
> > - distro pain
> > - disconnect
> > - loss of developers, testers and users
> > - grave bugs discovered months (years ...) down the line
> > - untested features
> > - developer exhaustion
> >
> >It didnt work, trust me - and i've been around long enough to have suffered
> >through the whole 2.5.x misery. Some of our worst ABIs come from that cycle as
> >well.
>
> You're talking about a single project and comparing it to my argument about
> multiple independent projects. In that case, I see no point in the
> discussion. If you want to win the argument by strawman, you are welcome to
> do so.

The kernel is a very complex project with many ABI issues, so all those
arguments apply to it as well. The description you gave:

| This is actually a Good Thing (tm). It means you have to get your feature
| and its interfaces well defined and able to version forwards and backwards
| independently from each other. And that introduces some complexity and
| time and testing, but in the end it's what you want. You don't introduce a
| requirement to have the feature, but take advantage of it if it is there.

matches the kernel too. We have many such situations. (Furthermore, the
tools/perf/ situation, which relates to ABIs and user-space/kernel-space
interactions is similar as well.)

Do you still think i'm making a straw-man argument?

> > Sorry, but i really think you are really trying to rationalize a
> > disadvantage here ...
>
> This could very well be true, but until someone comes forward with
> compelling numbers (as in, developers committed to working on the project,
> number of patches and total amount of code contribution), there is no point
> in having an argument, there really isn't anything to discuss other than
> opinion. My opinion is you need a really strong justification to have a
> successful fork and I don't see that justification.

I can give you rough numbers for tools/perf - if that counts for you.

For the first four months of its existence, when it was a separate project, i
had a single external contributor IIRC.

The moment it went into the kernel repo the number of contributors and
contributions skyrocketed and basically all contributions were top-notch. We
are at 60+ separate contributors now (after about 8 months upstream) - which
is still small compared to the kernel or to Qemu, but huge for a relatively
isolated project like instrumentation.

So in my estimation tools/kvm/ would certainly be popular. Whether it would be
more popular than current Qemu is hard to tell - it would be pure speculation.

Any reliable numbers for the other aspect, whether a split project creates a
more fragile and less developed ABI would be extremely hard to get. I believe
it to be true, but that's my opinion based on my experience with other
projects, extrapolated to KVM/Qemu.

Anyway, the issue is moot as there's clear opposition to the unification idea.

Too bad - there was heavy initial opposition to the arch/x86 unification as
well [and heavy opposition to tools/perf/ as well], still both worked out
extremely well :-)

Ingo