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Linux Gets O(1) SMP Patch As Late Christmas Gift
bodin writes: "Now that new-year's parties are over things are getting boring again. For those who want to see and perhaps even try something more complex, Ingo Molnar is
announcing this patch that is a pretty radical rewrite of the Linux
scheduler. This is big stuff!"
Before attempting to install this on my test box I'd like to know exactly what the performance inplications are to specific types of applications and services. For example I am extremely interested in improving JVM performance on a Linux box.
Any information or direction about this would be very helpful.
Wow. I read this guy's description of what he has done. I hope he's a teacher, because his explanation of those complex issues was a joy to read.
I have a BP6 dual-celeron Debian machine which already gives me the benefits of countless hours of volunteer time, including the SMP kernel and ReiserFS, along with dozens of free development tools. Now I see this guy working like a dog to tune the heck out of the scheduler for SMP machines, and I know that when I eventually run the 2.6 kernel, I'm again going to reap the benefits of his work.
It's almost enough to make me learn to hack the Linux kernel out of a sense of obligation.
Patrick Doyle
I mod down every jackass who puts his moderation policy in his sig. Oh, wait a sec....
This is a terrible idea. The fact is, for 99% of applications the scheduler isn't a bottleneck. Even on Alan Cox's worst-case 8-way SMP machines the scheduler doesn't eat tremendous amounts of CPU; _way_ more than it should, but an approach like Ingo Molnar's or Davide Libenzi's will get rid of most of that overhead.
But the real reason it's a terrible idea is because of the cache-line impact. At this level of the system, keeping the CPU cache intact and full of process data is critical to good performance. Otherwise when you switch tasks you wind up having to go out to main memory, which is sloooow. So compact code is more important than fast code in a lot of these situations, and a neural net like you describe is going to be enormous relative to the approaches of Davide and Mingo.
And Linus rightly pointed out that as simple as the current scheduler is, it's basically the only part of the kernel that's pretty much the same code as it was 8 years ago, mostly because it does work very well for the vast majority of cases. Not to say that Mingo, Davide, Rob Love, et al aren't doing a huge boon, and on 4-way and bigger systems it's more of a concern, but e.g. the VM and I/O subsystems are areas that could reap far greater performance rewards for the most common cases (and the VM is getting a ton of eyes, and the I/O subsystem has been the focus of most of the early 2.5 work thanks to Jens Axboe and others)
Sumner Hayes
rage, rage against the dying of the light
Cool!