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

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Date:	Sat, 14 Apr 2007 00:30:17 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Daniel Walker <dwalker@...sta.com>
Cc:	linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Con Kolivas <kernel@...ivas.org>,
	Nick Piggin <npiggin@...e.de>, Mike Galbraith <efault@....de>,
	Arjan van de Ven <arjan@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [Announce] [patch] Modular Scheduler Core and Completely Fair Scheduler [CFS]

* Daniel Walker <dwalker@...sta.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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