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SubjectRe: sched: Consequences of integrating the Per Entity Load Tracking Metric into the Load Balancer
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On Wed, 2013-01-02 at 09:52 +0530, Preeti U Murthy wrote: 
> Hi everyone,
> I have been looking at how different workloads react when the per entity
> load tracking metric is integrated into the load balancer and what are
> the possible reasons for it.
>
> I had posted the integration patch earlier:
> https://lkml.org/lkml/2012/11/15/391
>
> Essentially what I am doing is:
> 1.I have disabled CONFIG_FAIR_GROUP_SCHED to make the analysis simple
> 2.I have replaced cfs_rq->load.weight in weighted_cpuload() with
> cfs.runnable_load_avg,the active load tracking metric.
> 3.I have replaced se.load.weight in task_h_load() with
> se.load.avg.contrib,the per entity load tracking metric.
> 4.The load balancer will end up using these metrics.
>
> After conducting experiments on several workloads I found out that the
> performance of the workloads with the above integration would neither
> improve nor deteriorate.And this observation was consistent.
>
> Ideally the performance should have improved considering,that the metric
> does better tracking of load.
>
> Let me explain with a simple example as to why we should see a
> performance improvement ideally:Consider 2 80% tasks and 1 40% task.
>
> With integration:
> ----------------
>
> 40%
> 80% 40%
> cpu1 cpu2
>
> The above will be the scenario when the tasks fork initially.And this is
> a perfectly balanced system,hence no more load balancing.And proper
> distribution of loads on the cpu.
>
> Without integration
> -------------------
>
> 40% 40%
> 80% 40% 80% 40%
> cpu1 cpu2 OR cpu1 cpu2
>
> Because the view is that all the tasks as having the same load.The load
> balancer could ping pong tasks between these two situations.
>
> When I performed this experiment,I did not see an improvement in the
> performance though in the former case.On further observation I found
> that the following was actually happening.
>
> With integration
> ----------------
>
> Initially 40% task sleeps 40% task wakes up
> and select_idle_sibling()
> decides to wake it up on cpu1
>
> 40% -> -> 40%
> 80% 40% 80% 40% 80% 40%
> cpu1 cpu2 cpu1 cpu2 cpu1 cpu2
>
>
> This makes load balance trigger movement of 40% from cpu1 back to
> cpu2.Hence the stability that the load balancer was trying to achieve is
> gone.Hence the culprit boils down to select_idle_sibling.How is it the
> culprit and how is it hindering performance of the workloads?
>
> *What is the way ahead with the per entity load tracking metric in the
> load balancer then?*

select_idle_sibling() is all about dynamic, lowering latency and
cranking up cores during ramp-up to boost throughput. If you want the
system to achieve a stable state with periodic balancing, you need to
turn select_idle_sibling() the heck off. Once you've gotten the box
mostly committed, it's just an overhead/bounce source anyway.

> In replies to a post by Paul in https://lkml.org/lkml/2012/12/6/105,
> he mentions the following:
>
> "It is my intuition that the greatest carnage here is actually caused
> by wake-up load-balancing getting in the way of periodic in
> establishing a steady state. I suspect more mileage would result from
> reducing the interference wake-up load-balancing has with steady
> state."
>
> "The whole point of using blocked load is so that you can converge on a
> steady state where you don't NEED to move tasks. What disrupts this is
> we naturally prefer idle cpus on wake-up balance to reduce wake-up
> latency. I think the better answer is making these two processes load
> balancing() and select_idle_sibling() more co-operative."

The down-side of steady state seeking via load tracking being that you
want to take N% average load tasks, and stack them on top of each other,
which does nothing good for those tasks when they overlap in execution.
Long term balance looks all pretty, but if one or more of them could
have slipped into an idle shared cache, it's still a latency hit and
utilization loss. You are at odds with select_idle_sibling()'s mission.
It cares about the here and now, while load tracking cares about fuzzy
long-term averages.

> I had not realised how this would happen until I saw it happening in the
> above experiment.

select_idle_sibling()'s job is to be annoying as hell.. and it does that
very well :)

> Based on what Paul explained above let us use the runnable load + the
> blocked load for calculating the load on a cfs runqueue rather than just
> the runnable load(which is what i am doing now) and see its consequence.
>
> Initially: 40% task sleeps
>
> 40%
> 80% 40% -> 80% 40%
> cpu1 cpu2 cpu1 cpu2
>
> So initially the load on cpu1 is say 80 and on cpu2 also it is
> 80.Balanced.Now when 40% task sleeps,the total load on cpu2=runnable
> load+blocked load.which is still 80.
>
> As a consequence,firstly,during periodic load balancing the load is not
> moved from cpu1 to cpu2 when the 40% task sleeps.(It sees the load on
> cpu2 as 80 and not as 40).
> Hence the above scenario remains the same.On wake up,what happens?
>
> Here comes the point of making both load balancing and wake up
> balance(select_idle_sibling) co operative. How about we always schedule
> the woken up task on the prev_cpu? This seems more sensible considering
> load balancing considers blocked load as being a part of the load of cpu2.

Once committed, that works fine.. until load fluctuates. Don't plug the
holes opportunistically, you lose throughput.

> If we do that,we end up scheduling the 40% task back on cpu2.Back to the
> scenario which load balancing intended.Hence a steady state is
> maintained no matter what unless other tasks show up.
>
> Note that considering prev_cpu as the default cpu to run the woken up
> task on is possible only because we use blocked load for load balancing
> purposes.
>
> The above steps of using blocked load and selecting the prev_cpu as the
> target for the woken up task seems to me to be the next step.This could
> allow the load balance with the per entity load tracking metric to
> behave as it is supposed to without anything else disrupting it.And here
> i expect a performance improvement.
>
> Please do let me know your suggestions.This will greatly help take the
> right steps here on, in achieving the correct integration.

Again, I think you want a knob to turn select_idle_sibling() off. Load
tracking goal is to smooth, select_idle_sibling() goal is to peak.

-Mike



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