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SubjectRe: [Announce] [patch] Modular Scheduler Core and Completely Fair Scheduler [CFS]

* Matt Mackall <mpm@selenic.com> wrote:

> Look at what happened with I/O scheduling. Opening things up to some
> new ideas by making it possible to select your I/O scheduler took us
> from 10 years of stagnation to healthy, competitive development, which
> gave us a substantially better I/O scheduler.

actually, 2-3 years ago we already had IO schedulers, and my opinion
against plugsched back then (also shared by Nick and Linus) was very
much considering them. There are at least 4 reasons why I/O schedulers
are different from CPU schedulers:

1) CPUs are a non-persistent resource shared by _all_ tasks and
workloads in the system. Disks are _persistent_ resources very much
attached to specific workloads. (If tasks had to be 'persistent' to
the CPU they were started on we'd have much different scheduling
technology, and there would be much less complexity.) More analogous
to CPU schedulers would perhaps be VM/MM schedulers, and those tend
to be hard to modularize in a technologically sane way too. (and
unlike disks there's no good generic way to attach VM/MM schedulers
to particular workloads.) So it's apples to oranges.

in practice it comes down to having one good scheduler that runs all
workloads on a system reasonably well. And given that a very large
portion of system runs mixed workloads, the demand for one good
scheduler is pretty high. While i can run with mixed IO schedulers
just fine.

2) plugsched did not allow on the fly selection of schedulers, nor did
it allow a per CPU selection of schedulers. IO schedulers you can
change per disk, on the fly, making them much more useful in
practice. Also, IO schedulers (while definitely not being slow!) are
alot less performance sensitive than CPU schedulers.

3) I/O schedulers are pretty damn clean code, and plugsched, at least
the last version i saw of it, didnt come even close.

4) the good thing that happened to I/O, after years of stagnation isnt
I/O schedulers. The good thing that happened to I/O is called Jens
Axboe. If you care about the I/O subystem then print that name out
and hang it on the wall. That and only that is what mattered.

all in one, while there are definitely uses (embedded would like to have
a smaller/different scheduler, etc.), the technical case for
modularization for the sake of selectability is alot lower for CPU
schedulers than it is for I/O schedulers.

nor was the non-modularity of some piece of code ever an impediment to
competition. May i remind you of the pretty competitive SLAB allocator
landscape, resulting in things like the SLOB allocator, written by
yourself? ;-)

Ingo
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