Con Kolivas Returns, With a Desktop-Oriented Linux Scheduler

myvirtualid writes "Con Kolivas has done what he swore never to do: returned to the Linux kernel and written a new — and, according to him — waaay better scheduler for the desktop environment. In fact, BFS appears to outperform existing schedulers right up until one hits a 16-CPU machine, at which point he guesses performance would degrade somewhat. According to Kolivas, BFS 'was designed to be forward looking only, make the most of lower spec machines, and not scale to massive hardware. i.e. [sic] it is a desktop orientated scheduler, with extremely low latencies for excellent interactivity by design rather than 'calculated,' with rigid fairness, nice priority distribution and extreme scalability within normal load levels.'"

Comments

Re:BFS is the Brain Fuck Scheduler.

[Fizzl]

from the dare-not-speak-its-name dept.

Re:Glory!

[Trepidity]

My question is: is it in the kernel tree yet? Is this that 2.6.31 scheduler change I heard about earlier yesterday, or is it something Completely Different?

No, and probably won't ever be, though perhaps some ideas will be borrowed.

From his FAQ:

Are you looking at getting this into mainline?

LOL.

No really, are you?

LOL.

Really really, are you?

No. They would be crazy to use this scheduler anyway since it won't scale to
their 4096 cpu machines. The only way is to rewrite it to work that way, or
to have more than one scheduler in the kernel. I don't want to do the former,
and mainline doesn't want to do the latter. Besides, apparently I'm a bad
maintainer, which makes sense since for some reason I seem to want to have
a career, a life, raise a family with kids and have hobbies, all of which
have nothing to do with linux.

Can it be made to scale to 4096 CPUs?

Sure I guess you could run one runqueue per CPU package instead of a global
one and so on, but I have no intention whatsoever at doing that because it
will compromise the performance where *I* care.

The "bad maintainer" part is referring to bad blood over the adoption of Ingo Molnar's CFS over Kolivas's own RSDL, in particular at least one LKML poster suggesting that, all else being equal, it'd be better to merge Molnar's code, as he was more likely to be a reliable maintainer (Molnar's more tied into the workings of the mainline kernel development/merging/etc.).

Re:Glory!

[kav2k]

Oh yeah, and which other scheduler's, if any, did this guy write?

SD scheduler

forward looking

[Trepidity]

Took me a while to figure out what "forward looking" means in this context, since "forward-looking scheduler" doesn't seem to be common terminology, and I assumed he wasn't talking about his grand forward-looking vision for schedulerdom.

Based on some previous arguments he's had, it sounds like he opposes the common heuristic of upping interactive process priority by keeping track of how long processes sleep--- processes that sleep a lot are probably I/O bound, and should get a priority boost so they can run on the (less frequent than for CPU-bound processes) occasions when they're ready. Kolivas wants schedulers to be forward-looking in the sense that they decide how to schedule without looking at process run history, by looking purely at who's ready to run, available timeslices, priorities, etc.

Re:Cool, but what does that spec mean?

[Trepidity]

He means something different by it--- that the scheduler should only look forward, not look back to per-process history in making its scheduling decisions. A common hack/heuristic to improve interactive performance is to boost the priority of processes that sleep a lot, since CPU-bound jobs sleep rarely, while interactive processes sleep a lot. Kolivas think that's a hack that obscures the real problems with interactive performance, and leads to unpredictable performance since it doesn't fix the underlying issues. So wants to design schedulers with good interactive performance that make decisions based purely on the current set of running processes and priorities, and the upcoming timeslices.

Re:Glory!

[Trepidity]

Yeah, that makes sense, but he seems to have taken it personally. It sounds like part of it stems from his feeling that Molnar unnecessarily wrote a replacement using his ideas and got credit for it, instead of helping out to turn one of Kolivas's fair-scheduling proposals into something that could be merged. Though from what I can tell Molnar's replies are all pretty friendly, and he seemed keen to provide appropriate credit.

Re:Glory!

[Hurricane78]

What is that? You don't have the choice of scheduler in your kernel? I'm using the Zen sources, and I get to choose between least half a dozen schedulers, including other settings. I am certain that this scheduler will make it into that patchset, and that I will enable it, as soon as zen-sources-2.6.31 get installed on my system.

After all this is Linux! Not some one-company-one-kernel monoculture!

Re:Who cares?

[TheRaven64]

If you're interested, the clang team have done a lot of profiling of exactly what takes time when compiling. It's particularly interesting how much of a bottleneck preprocessing is with gcc and, more importantly, distcc (which sends the preprocessed sources over the network for compilation). Most of the results are on the web site, with a few in the mailing list archives.

Re:Glory!

[shutdown -p now]

The same is pretty much true of .Net's Windows.Forms. It's a bit faster than Swing, although not by much (some parts are actually slower - System.Drawing vs Java2D, for example), so it's a little more forgiving of doing work in the UI thread. It will still bite you in a non-trivial application. Of course, the framework provides absolutely no help in writing a multithreaded application, and all of the tools, examples and documentation make writing a multi-threaded application far more difficult than it should be.

Yes, and things like Control.Invoke to marshal invocations from background threads to UI, and especially BackgroundWorker, which are there specifically to provide a high-level (i.e. without locks) API for worker threads, with progress reporting and cancellation, must be just figments of my imagination?

Have you actually written any WinForms code in .NET 2.0+?

Re:great news

[pthisis]

The impression I've gotten from reading various past "Con threads", is that while he tries in the beginning, Con doesn't deal well with this process; he can't keep his ego submerged, gets frustrated, and everything (perhaps including Con himself last time I read one of these threads) ends up unravelling.

Agreed; Con seems not to be able to work well in the process.

e.g. Ingo ran a bunch of benchmarks on BFS and made a long post to LKML explaining his results, that, while critical of its performance on a series of benchmarks, bent over backwards to be very polite in tone, with things like:

First and foremost, let me say that i'm happy that you are hacking the Linux scheduler again. It's perhaps proof that hacking the scheduler is one of the most addictive things on the planet ;-) ...

General interactivity of BFS seemed good to me - except for the pipe test when there was significant lag over a minute. I think it's some starvation bug, not an inherent design property of BFS, so i'm looking forward to re-test it with the fix. ...
I hope to be able to work with you on this, please dont hesitate sending patches if you wish - and we'll also be following BFS for good ideas and code to adopt to mainline.

And Con responded with a very defensive and confrontational tone:
I'm not interested in a long protracted discussion about this since I'm too busy to live linux the way full time developers do, so I'll keep it short, and perhaps you'll understand my intent better if the FAQ wasn't clear enough.

Do you know what a normal desktop PC looks like? No, a more realistic question based on what you chose to benchmark to prove your point would be: Do you know what normal people actually do on them?

Feel free to treat the question as rhetorical.

Full exchange here:
http://thread.gmane.org/gmane.linux.kernel/886319