Re: [Announce] [patch] Modular Scheduler Core and Completely Fair Scheduler [CFS]

[Ingo Molnar <mingo@elte.hu>]

* Daniel Walker <dwalker@mvista.com> wrote:

> I'm not in love with the current or other schedulers, so I'm 
> indifferent to this change. However, I was reviewing your release 
> notes and the patch and found myself wonder what the logarithmic 
> complexity of this new scheduler is .. I assumed it would also be 
> constant time , but the __enqueue_task_fair doesn't appear to be 
> constant time (rbtree insert complexity).. [...]

i've been worried about that myself and i've done extensive measurements 
before choosing this implementation. The rbtree turned out to be a quite 
compact data structure: we get it quite cheaply as part of the task 
structure cachemisses - which have to be touched anyway. For 1000 tasks 
it's a loop of ~10 - that's still very fast and bound in practice.

here's a test i did under CFS. Lets take some ridiculous load: 1000 
infinite loop tasks running at SCHED_BATCH on a single CPU (all inserted 
into the same rbtree), and lets run lat_ctx:

  neptune:~/l> uptime
  22:51:23 up 8 min,  2 users,  load average: 713.06, 254.64, 91.51

  neptune:~/l> ./lat_ctx -s 0 2
  "size=0k ovr=1.61
  2 1.41

lets stop the 1000 tasks and only have ~2 tasks in the runqueue:

  neptune:~/l> ./lat_ctx -s 0 2

  "size=0k ovr=1.70
  2 1.16

so the overhead is 0.25 usecs. Considering the load (1000 tasks trash 
the cache like crazy already), this is more than acceptable.

	Ingo
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