Re: Comments on the KSE option

On Sat, 28 Oct 2006, David Xu wrote:

> 3) Third, it adds overhead to scheduler (I have already post a number) and 
> might make locking more diffcult for per-cpu queue like scheduler, since now 
> you always have to contend the ksegrp runqueue lock between many CPUs, also 
> because you have build the fairness in the scheduler and every scheduler 
> must obey the ksegrp algorithm, it may make more diffcult to implement 
> another alogrithm and replace it, see 4).

This is my single biggest concern: our scheduling, thread/process, and context 
management paths in the kernel are currently extremely complex.  This has a 
number of impacts: it makes it extremely hard to read and understand, it adds 
significant overhead, and it makes it quite hard to modify and optimize for 
increasing numbers of processors.  We need to be planning on a world of 128 
hardware threads/machine on commodity server hardware in the immediate future, 
which means that the current "giant sched_lock" cannot continue much longer. 
Kip's prototypes of breaking out sched_lock as part of the sun4v work have 
been able to benefit significantly from the reduced complexity of a KSE-free 
kernel, and it's fairly clear that the task of improving schedule scalability 
is dramatically simpler when the kernel model for threading is more simple. 
Regardless of where the specific NO_KSE option in the kernel goes, reducing 
kernel scheduler/etc complexity should be a first order of business, because 
effective SMP work really depends on that happening.

Robert N M Watson
Computer Laboratory
University of Cambridge