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SubjectRe: [ANNOUNCE/RFC] Really Fair Scheduler
Hi,

On Fri, 31 Aug 2007, Ingo Molnar wrote:

Maybe I should explain for everyone else (especially after seeing some of
the comments on kerneltrap), why I reacted somewhat irritated on what
looks like such an innocent mail.
The problem is without the necessary background one can't know how wrong
such statements as this are, the level of confidence is amazing though:

> Peter's work fully solves the rounding corner-cases you described as:

I'd expect Ingo to know better, but the more he refuses to answer my
questions, the more I doubt it, at least than it comes to the math part.

While Peter's patches are interesting, they are only a small step to what
I'm trying to achieve. With these patches the basic CFS math pretty much
looks like this:

sum_{t}^{T}(round(time_{t} * round(WMULT / weight_{t}) * WEIGTH0 / WMULT))
= sum_{r}^{R}(round(time_{r} * round(WMULT / weight_sum) * WEIGTH0 / WMULT))
It's based on this equation:

sum_{t}^{T}(time_{t} / weight_{t}) = sum_{r}^{R}(time_{r} / weight_sum)
This is the time a single task gets relative to the the runtime of all
tasks. These sums are incrementally added/substracted to wait_runtime and
it should stay around zero.

All Peter's wait_runtime-scale patch does is to move the weight_{t} from
one side to the other and that's it, it changes _nothing_ about the
rounding above - "the rounding corner-cases" are still there.
In my announcement I describe now in quite some detail, how I get rid of
this rounding effect. The only rounding from above equation which is still
left is "round(WMULT / weight_{t})", but this is luckily a constant and so
is the time one task gets relative to another (unless reniced).
Anyone who has actually read and understood what I wrote will hopefully
realize what complete nonsense a statement like this is:

> So the most intrusive (math) aspects of your patch have been implemented
> already for CFS (almost a month ago), in a finegrained way.

I'm not repeating again the whole math here, if anyone has questions about
it, I'll try my best to answer them. So instead here are some of the
intrusive aspects, which supposedly have been implemented already.

One key difference is that I don't maintain the global sum (fair_clock)
directly anymore, I can calculate it if needed, but it's not used to
update wait_runtime anymore. This has the advantage that the whole
rounding involved in it has no influence anymore on how much time a task
gets. Without this value the whole math around how to schedule a task is
quite different as well.

Another key difference is that I got rid of (WEIGTH0 / WMULT), this has
the advantage that it completely gets rid of the problematic rounding and
the scheduler can be now finally precise as the hardware allows it.
OTOH this has consequences for the range of values, as they can and are
expected to overflow now after some time, which the math has to take into
account.

One positive side effect of these overflows is that I can reduce the
resolution the scheduler is working with and thus I can get rid of pretty
much all of the 64bit math, if the reduced resolution is sufficient, e.g.
for all archs which have a jiffies based scheduler clock, but even
embedded archs might like it.

> So if you'd like to work with us on this and get items that make sense
> merged (which we'd very much like to see happen), could you please
> re-shape the rest of your changes and ideas (such as whether to use
> ready-queueing or a runqueue concept, which does look interesting) ontop
> of Peter's queue, and please do it as a finegrained, reviewable,
> mergable series of patches, like Peter did. Thanks!

The funny thing is it should be no that hard to split the patch, but that
wasn't point. The point is to discuss the differences - how the different
approach effects the scheduling decisions, as the new scheduler maintains
somewhat different values. If I'm now told "it's just the same thing
mathematically", which is provably nonsense, I'm a little stunned and the
point that aggravates me is that most people simply are going to believe
Ingo, because they don't understand the issue (and they don't really have
to). I'm still amazed how easily Ingo can just ignore the main part of my
work and still gets away with it. The last thing I want is yet another
flame war, all I want is that this work is taken seriously, but it took
Ingo less than 9 hours to get to a thorough judgement...

bye, Roman
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