Hi Peter,
Thank you very much for your feedback.
On 10/25/2012 09:26 PM, Peter Zijlstra wrote:
> OK, so I tried reading a few patches and I'm completely failing.. maybe
> its late and my brain stopped working, but it simply doesn't make any
> sense.
>
> Most changelogs and comments aren't really helping either. At best they
> mention what you're doing, not why and how. This means I get to
> basically duplicate your entire thought pattern and I might as well do
> the patches myself.
>
> I also don't see the 'big' picture of what you're doing, you start out
> by some weird avoid short running task movement.. why is that a good
> start?
>
> I would have expected a series like this to replace rq->cpu_load /
> rq->load with a saner metric and go from there.. instead it looks like
> its going about at things completely backwards. Fixing small details
> instead of the big things.
Let me see if I get what you are saying here right.You want to replace for
example cfs_rq->load.weight with a saner metric because it does not consider
the run time of the sched entities queued on it,merely their priorities.If this
is right,in this patchset I believe cfs_rq->runnable_load_avg would be that
right metric because it considers the run time of the sched entities queued on
it.
So why didnt I replace? I added cfs_rq->runnable_load_avg as an additional
metric instead of replacing the older metric.I let the old metric be as a dead
metric and used the newer metric as an alternative.so if this alternate metric
does not do us good we have the old metric to fall back on.
> At best they mention what you're doing, not why and how.
So the above explains *what* I am doing.
*How* am i doing it: Everywhere the scheduler needs to make a decision on:
a.find_busiest_group/find_idlest_group/update_sg_lb_stats:use sum of
cfs_rq->runnable_load_avg to decide this instead of sum of cfs_rq->load.weight.
b.find_busiest_queue/find_idlest_queue: use cfs_rq->runnable_load_avg to
decide this instead of cfs_rq->load.weight
c.move_tasks: use se->avg.load_avg_contrib to decide the weight of of each
task instead of se->load.weight as the former reflects the runtime of the sched
entity and hence its actual load.
This is what my patches3-13 do.Merely *Replace*.
*Why am I doing it*: Feed the load balancer with a more realistic metric to do
load balancing and observe the consequences.
> you start out by some weird avoid short running task movement.
> why is that a good start?
The short running tasks are not really burdening you,they have very little run
time,so why move them?
Throughout the concept of load balancing the focus is on *balancing the
*existing* load* between the sched groups.But not really evaluating the
*absolute load* of any given sched group.
Why is this *the start*? This is like a round of elimination before the actual
interview round ;) Try to have only those sched groups as candidates for load
balancing that are sufficiently loaded.[Patch1]
This *sufficiently loaded* is decided by the new metric explained in the *How*
above.
> I also don't see the 'big' picture of what you're doing
Patch1: is explained above.*End result:Good candidates for lb.*
Patch2:
10%
10%
10% 100%
------ ------
SCHED_GP1 SCHED_GP2
Before Patch After Patch
-----------------------------------
SCHED_GP1 load:3072 SCHED_GP1:512
SCHED_GP2 load:1024 SCHED_GP2:1024
SCHED_GP1 is busiest SCHED_GP2 is busiest:
But Patch2 re-decides between GP1 and GP2 to check if the latency of tasks is
getting affected although there is less load on GP1.If yes it is a better *busy
* gp.
*End Result: Better candidates for lb*
Rest of the patches: now that we have our busy sched group,let us load balance
with the aid of the new metric.
*End Result: Hopefully a more sensible movement of loads*
This is how I build the picture.
Regards
Preeti
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