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

Ingo Molnar
2007-04-13T22:30:37Z

* 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