[Devel] Re: [RFC] [PATCH 0/3] Add group fairness to CFS

[Peter Williams]

Srivatsa Vaddagiri wrote:
> Here's an attempt to extend CFS (v13) to be fair at a group level, rather than
> just at task level. The patch is in a very premature state (passes
> simple tests, smp load balance not supported yet) at this point. I am sending 
> it out early to know if this is a good direction to proceed.
> 
> Salient points which needs discussion:
> 
> 1. This patch reuses CFS core to achieve fairness at group level also.
> 
>    To make this possible, CFS core has been abstracted to deal with generic 
>    schedulable "entities" (tasks, users etc).
> 
> 2. The per-cpu rb-tree has been split to be per-group per-cpu.
> 
>    schedule() now becomes two step on every cpu : pick a group first (from
>    group rb-tree) and a task within that group next (from that group's task
>    rb-tree)
> 
> 3. Grouping mechanism - I have used 'uid' as the basis of grouping for
>    timebeing (since that grouping concept is already in mainline today).
>    The patch can be adapted to a more generic process grouping mechanism
>    (like http://lkml.org/lkml/2007/4/27/146) later.
> 
> Some results below, obtained on a 4way (with HT) Intel Xeon box. All 
> number are reflective of single CPU performance (tests were forced to 
> run on single cpu since load balance is not yet supported).
> 
> 
> 			         uid "vatsa"	           uid "guest"
> 		             (make -s -j4 bzImage)    (make -s -j20 bzImage)
> 
> 2.6.22-rc1		          772.02 sec		497.42 sec (real)
> 2.6.22-rc1+cfs-v13 	          780.62 sec		478.35 sec (real)
> 2.6.22-rc1+cfs-v13+this patch     776.36 sec		776.68 sec (real)

This would seem to indicate that being fair between groups isn't always 
a good thing.  With 2.6.22-rc1 and 2.6.22-rc1+cfs-v13 "guest" gets his 
build done in about 2/3 the time of "vatsa" without seriously 
inconveniencing "vatsa".  All making scheduling fair between the groups 
has done is penalize "guest" without significantly improving matters for 
"vatsa" (he gains a mere 4 seconds out of 780).

BUT I imagine that this is an artefact caused by the use of HT 
technology and that if the test were run on a computer without HT the 
results would be more impressive.

Peter
-- 
Peter Williams                                   pwil3058 at bigpond.net.au

"Learning, n. The kind of ignorance distinguishing the studious."
  -- Ambrose Bierce
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