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Torvalds Explains Scheduler Decision
Firedog writes "There's been a lot of recent debate over why Linus Torvalds chose the new CFS process scheduler written by Ingo Molnar over the SD process scheduler written by Con Kolivas, ranging from discussing the quality of the code to favoritism and outright conspiracy theories. KernelTrap is now reporting Linus Torvalds' official stance as to why he chose the code that he did. 'People who think SD was "perfect" were simply ignoring reality,' Linus is quoted as saying. He goes on to explain that he selected the Completely Fair Scheduler because it had a maintainer who has proven himself willing and able to address problems as they are discovered. In the end, the relevance to normal Linux users is twofold: one is the question as to whether or not the Linux development model is working, and the other is the question as to whether the recently released 2.6.23 kernel will deliver an improved desktop experience."
What do CFS and SD stand for in this case? The summary and linked articles do not describe this.
Having lurked on http://www.lkml.org/ for several years, I find Linus to be rather rude. May be it's because English is not his first language...so words are not well chosen. I must say though, that I excuse him because he produces, [or helps produce] a very useful product on the world today. That is the Linux kernel.
Why can't there be a flag that determines what scheduler is used at runtime, with both schedulers built into the kernel? I thought the whole point of Linux is that it is customizable and modular - I know this doesn't necessarily apply to the kernel, but why not?
;)
I know very little about operating systems, schedulers, and maintaining large projects, so please excuse any ignorance in my post
Have you improved it to the point that when the system is borderline out of main memory or has a moderately high load average it actually *works* as a desktop system?
E.g. when Firefox is consuming 65-70% of main memory and slower than #%#$ and you know it is waiting on swapped out pages and your swap rate is measured in the dozens to hundreds rather than hundreds to thousands (on vmstat)? (I mean really, how can one take an operating system seriously when only memory is at 100% and not CPU + memory + Disk I/O?)
The real issue, for those who have read comments that Con has made in interviews, seems to be the lack of concern on the part of most of the "in-crowd" Linux developers for performance on the desktop. In part this seems driven by the fact that the people who actually get paid to maintain Linux, benchmark it and "improve" it only care about its performance in server farms and *not* on the individual desktop. I will weigh in on the side of desktop user out there (that wants the Linux sitting beneath their desk to devote its every waking minute to making *them* happy) by saying that if my mplayer "hangs" in the middle of a song (only to continue with a loud burst of noise 10 seconds later) when the CPU is busy with "nice -19" processes, my Firefox browser takes half a minute to scroll a page or open a screen) when memory is tight, and it takes minutes to bring up a tab or minimized program I haven't touched in 3 days and return them to a functional state then the operating system *Has a PROBLEM*.
Con was very clear in his interviews that the problem is the lack of caring about *desktop* performance. Given my comments in the previous paragraph -- some of these areas may be very difficult to benchmark and as a result one is left with nothing but handwaving and loud voices when it comes to discussions about whether the problem exists and how it should be fixed.
I will say this, in the mid-'90s I used X-windows under Unixware on *Pentium 1s* as a desktop machine. I now use X-windows under Linux on a Pentium 4 (with 5-10x more main memory) as a desktop machine. I would argue that my desktop user experience is as problematic now as it was then *despite* the hardware improvements. That IMO is what Con felt was the problem he was trying to address. That is what it would appear the core Linux developers may be failing to address. Con's points raised my awareness level to the extent that I actually went investigating to see whether there were open source distributions of the BeOS and/or Darwin (which is based on Mach) available since they are based on different OS architecture models and might be more end-user friendly [1]. I was hoping to find something I could run in a VM under Linux on my current hardware without major file system surgery. But I have little confidence that such an approach would fix core problems with the Linux scheduling and paging systems. I would *love* to see a real side-by-side comparison of Linux vs. FreeBSD for desktop users with an emphasis on how BSD scheduling, paging and swapping may be different (better?).
(And as a side note, I could care *zero* about the performance of Linux in file server applications!)
1. I did use both Nextstep (on Pentiums) and IRIX (on a R4000) for a while and found both to provide better end-user experience than Unixware (X) or Windows (95-98). I am disappointed that Linux barely manages to match those experiences given the hardware available nearly a decade later.
OMG people even have to steal comments these days?
I wrote this comment on kerneltrap.
Christ, that's incredible. Nice one "bconway".
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