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

Sat May 26 18:29:51 PDT 2007

William Lee Irwin III wrote:
> Srivatsa Vaddagiri wrote:
>>> Ingo/Peter, any thoughts here?  CFS and smpnice probably is "broken" 
>>> with respect to such example as above albeit for nice-based tasks.
> 
> On Sat, May 26, 2007 at 10:17:42AM +1000, Peter Williams wrote:
>> See above.  I think that faced with cpu affinity use by the system 
>> administrator that smpnice will tend towards a task to cpu allocation 
>> that is (close to) the best that can be achieved without violating the 
>> cpu affinity assignments.  (It may take a little longer than normal but 
>> it should get there eventually.)
>> You have to assume that the system administrator knows what (s)he's 
>> doing and is willing to accept the impact of their policy decision on 
>> the overall system performance.
>> Having said that, if it was deemed necessary you could probably increase 
>> the speed at which the load balancer converged on a good result in the 
>> face of cpu affinity by keeping a "pinned weighted load" value for each 
>> run queue and using that to modify find_busiest_group() and 
>> find_busiest_queue() to be a bit smarter.   But I'm not sure that it 
>> would be worth the added complexity.
> 
> Just in case anyone was looking for algorithms...
> 
> Lag should be considered in lieu of load because lag

What's the definition of lag here?

> is what the
> scheduler is trying to minimize;

This isn't always the case.  Some may prefer fairness to minimal lag. 
Others may prefer particular tasks to receive preferential treatment.

> load is not directly relevant, but
> appears to have some sort of relationship. Also, instead of pinned,
> unpinned should be considered.

If you have total and pinned you can get unpinned.  It's probably 
cheaper to maintain data for pinned than unpinned as there's less of it 
on normal systems.

> It's unpinned that load balancing can
> actually migrate.

True but see previous comment.

> Using the signed minimax pseudonorm (i.e. the highest
> signed lag, where positive is higher than all negative regardless of
> magnitude) on unpinned lags yields a rather natural load balancing
> algorithm consisting of migrating from highest to lowest signed lag,
> with progressively longer periods for periodic balancing across
> progressively higher levels of hierarchy in sched_domains etc. as usual.
> Basically skip over pinned tasks as far as lag goes.
> 
> The trick with all that comes when tasks are pinned within a set of
> cpus (especially crossing sched_domains) instead of to a single cpu.

Yes, this makes the cost of maintaining the required data higher which 
makes keeping pinned data more attractive than unpinned.

BTW keeping data for sets of CPU affinities could cause problems as the 
number of possible sets is quite large (being 2 to the power of the 
number of CPUs).  So you need an algorithm based on pinned data for 
single CPUs that knows the pinning isn't necessarily exclusive rather 
than one based on sets of CPUs.  As I understand it (which may be 
wrong), the mechanism you describe below takes that approach.

> There one can just consider a cpu to enter a periodic load balance
> cycle, and then consider pushing and pulling, perhaps what could be
> called the "exchange lags" for the pair of cpus. That would be the
> minimax lag pseudonorms for the tasks migratable to both cpus of the
> pair. That makes the notion of moving things from highest to lowest
> lag (where load is now considered) unambiguous apart from whether all
> this converges, but not when to actually try to load balance vs. when
> not to, or when it's urgent vs. when it should be done periodically.
> 
> To clarify that, an O(cpus**2) notion appears to be necessary, namely
> the largest exchange lag differential between any pair of cpus. There
> is also the open question of whether moving tasks between cpus with the
> highest exchange lag differential will actually reduce it or whether it
> runs the risk of increasing it by creating a larger exchange lag
> differential between different pairs of cpus. A similar open question
> is raised by localizing balancing decisions to sched_domains. What
> remains clear is that any such movement reduces the worst-case lag in
> the whole system. Because of that, the worst-case lag in the whole
> system monotonically decreases as balancing decisions are made, and
> that much is subject to an infinite descent argument. Unfortunately,
> determining the largest exchange lag differential appears to be more
> complex than merely finding the highest and lowest lags. Bipartite
> forms of the problem also arise from sched_domains.
> 
> I doubt anyone's really paying any sort of attention, so I'll not
> really bother working out much more in the way of details with respect
> to load balancing. It may be that there are better ways to communicate
> algorithmic notions than prose descriptions. However, it's doubtful I'll
> produce anything in a timely enough fashion to attract or hold interest.
> 
> The smpnice affair is better phrased in terms of task weighting. It's
> simple to honor nice in such an arrangement. First unravel the
> grouping hierarchy, then weight by nice. This looks like
> 
> task    nice    hier1   hier2   ...     hierN
> t_1     w_n1    w_h11   w_h21   ...     w_hN1
> t_2     w_n2    w_h12   w_h22   ...     w_hN2
> ...
> 
> For the example of nice 0 vs. nice 10 as distinct users with 10%
> steppings between nice levels, one would have
> 
> task    nice   hier1
> t_1     1      1
> t_2     0.3855 1
> 
> w_1, the weight of t_1, would be
>         (w_h11*w_n1/(w_h11*w_n1 + w_h12*w_n2))
>                 = (1*1/(1 + 1*0.3855..))
>                 = 0.7217..
> w_2, the weight of t_2, would be
>         (w_h12*w_n2/(w_h11*w_n1 + w_h12*w_n2))
>                 = (1*0.3855../(1 + 1*0.3855..))
>                 = 0.27826..
> This just so happens to work out to being the same as if t_1 and t_2
> had their respective nice numbers without the scheduler grouping, which
> is basically what everyone wants to happen.
> 
> It's more obvious how to extend it to more tasks than levels of
> hierarchy. An example of that follows:
> 
> task    nice    hier1   hier2   ...     hierN
> t_1     0.3     0.6     *       ...     *
> t_2     0.7     0.4     *       ...     *
> 
> hier2 through hierN are ignorable since t_1 and t_2 are both the only
> members at those levels of hierarchy. We then get something just like
> the above example, w_1 = 0.3*0.6/(0.3*0.6+0.7*0.4) = 0.3913.. and
> w2 = 0.7*0.4/(0.3*0.6+0.7*0.4) = 0.6087..
> 
> It's more interesting with enough tasks to have more meaningful levels
> of hierarchy.
> 
> task    nice    hier1   hier2
> t_1     0.7     0.6     0.6
> t_2     0.3     0.6     0.4
> t_3     0.7     0.4     0.6
> t_4     0.3     0.4     0.4
> 
> where t_1 and t_2 share a hier1 grouping and t_3 and t_4 also share
> a hier1 grouping, but the hier1 grouping for t_1 and t_2 is distinct
> from the hier1 grouping for t_3 and t_4. All hier2 groupings are
> distinct. So t_1 would have pre-nice weight 0.6*0.6, t_2 0.6*0.4,
> t_3 0.6*0.4, and t_4 0.4*0.4 (the numbers were chosen so denominators
> conveniently collapse to 1). Now that the hierarchy is flattened,
> nice numbers can be factored in for t_1's final weight being
> 0.7*0.36/(0.7*0.36+0.3*0.24+0.7*0.24+0.3*0.16) = 0.252/0.54 = 0.467..
> and the others being 0.133.. (t_2), 0.311.. (t_3), and 0.0889.. (t_4).
> 
> In such a manner nice numbers obey the principle of least surprise.

Is it just me or did you stray from the topic of handling cpu affinity 
during load balancing to hierarchical load balancing?  I couldn't see 
anything in the above explanation that would improve the handling of cpu 
affinity.

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

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