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Linux Gets Completely Fair Scheduler
SchedFred writes "KernelTrap is reporting that CFS, Ingo Molnar's Completely Fair Scheduler, was just merged into the Linux kernel. The new CPU scheduler includes a pluggable framework that completely replaces Molnar's earlier O(1) scheduler, and is described to 'model an "ideal, precise multi-tasking CPU" on real hardware. CFS tries to run the task with the "gravest need" for more CPU time. So CFS always tries to split up CPU time between runnable tasks as close to "ideal multitasking hardware" as possible.' The new CPU scheduler should improve the desktop Linux experience, and will be part of the upcoming 2.6.23 kernel."
I know enough about process scheduling to fill a ketchup cup at the nearest burger joint, but it struck me that this sounds like the debate about "network neutrality" vs "tiered service." The O(1) was supposed to be a very generic decision-making system that made a decision in a very agnostic way (to simplify the work down to a predictable consistent order of work). This CFS strikes me as a system which will have a much higher level of complexity and context awareness, which sounds like some processes will get more than others. The intention is to make it fair in the real world but not necessarily balanced, since not all processes are alike in their needs or expectations of task switching.
This is just rambling on, and admittedly it may be straining a metaphor too far, so don't go crazy biting my head off for not knowing all things about the kernel. See 'ketchup cup' above.
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Yep. I've seen this in CFS testing actually. Pretty much all of the work that the X server does can be assumed to be "on behalf of" somebody, but in the end those cycles still belong to the X server. So an app can be thoroughly abusive, spam the X server with requests, fill up queues, and prevent anyone else from using the server to do anything useful -- and yet it still gets priority because as far as the scheduler can tell it's a perfectly nice I/O-bound task that spends most of its time waiting for the X server to get back to it. As I recall, Linus provided a "fix" for that particular problem sometime early in 2.6 or late in 2.5 -- but later retracted it because it did more harm than good -- any simple solution you might think of has already been tried and thrown away.
:)
Oh, and the abusive app that likes to make X servers choke? Firefox. Ugh. Hate that thing.