Siddha, Suresh B wrote:
On Fri, Apr 14, 2006 at 11:50:10AM +1000, Peter Williams wrote:
Siddha, Suresh B wrote:
On Thu, Apr 13, 2006 at 04:57:15PM +1000, Peter Williams wrote:
Problem:
The move_tasks() function is designed to move UP TO the amount of load
it is asked to move and in doing this it skips over tasks looking for
ones whose load weights are less than or equal to the remaining load to
be moved. This is (in general) a good thing but it has the unfortunate
result of breaking one of the original load balancer's good points:
with previous load balancer code it was a good point.. because all tasks
were weighted the same from load balancer perspective.. but now the
imbalance represents what task to move (atleast in the working
cases :)
That's the option 4 case in my original mail. Are you suggesting that
it would have been the better option to adopt? If so, why?
No. I was not suggesting option-4. With this change in move_tasks, we will
be overriding the decision what ever we made while calculating imbalance.
Lets see for example, we have a simple DP system. With proc-0 running
one high priority and one low priority task, Proc-1 running one
low priority task. Ideally we would like to move low priority task from
P0 to P1. But with this patch, we may end up moving high priority task
from P0 to P1. But slowly after sometime(depending on high priority task
is on active/expired list), we will converge to the expected
solution..
Yes, there are problems with the active/expired arrays but they're no
worse with smpnice than they are without it.
Going to a single priority array would eliminate this problem not to
mention other problems to do with when to excuse tasks from being put on
the expired array at the end of their time slice (see Mike Galbraith's
patches). But I haven't had much luck pushing that barrow. :-)
you mean the highest priority task on the current active list of the new
run queue, right?
Good point. this_min_prio should probably be initialized to the minimum
of this_rq->curr->prio and this_rq->best_expired_prio rather just using
this_rq->curr->prio.
yes.
There will be some unnecessary movements of high priority tasks because of
this...
How so. At most one task per move_tasks() will be moved as a result of
this code that wouldn't have been moved otherwise. That task will be a
high priority task stuck behind a higher priority task on its current
queue that will be the highest priority on its new queue causing a
preempt and access to the CPU. I wouldn't call this unnecessary.
highest priority task can be in the expired list with normal priority
task running.. as in my above example.
Yes, but this will get it onto a CPU sooner so it's not all bad. Once
again it's no worse than without smpnice.
I said in the patch description that there are active/expired array
issues that make load balancing less than perfect with and without
smpnice. How far we go trying to eliminate them is the question.
If you have a better suggestion for how move_tasks() does its job, how
about providing a patch and with supporting arguments as to why its
better. If it is better then we can use it.
Peter, Are you sure that this is a converging solution? If we want to
Yes, I think we're getting there.
I think we need changes to try_to_wake_up() to help high priority tasks
find idle CPUs or CPUs where they would preempt when they wake up.
Leaving this to the load balancer is bad for these tasks latencies. I
think that this change needs to be done independently of smpnice as it
would be useful even without smpnice. I'm hoping Ingo or Nick will
comment on this proposal.
It would also help if you fixed the active load balance code so that
it's not necessary to distort normal load balancing to accommodate it.
I haven't had time to look at it myself (other than a quick glance) yet.
The only special check in find_busiest_group() helping MT/MC balancing
is pwr_now and pwr_move calculations..
What about the very messy code I had to put in so that
find_busiest_group() would return a group even if there were no queues
in the group with more than one task. Similar for find_busiest_queue().
These calculations will also help,
in future when we are dealing with sched groups which are not symmetric.
Asymmetries can be caused in scenarios like cpufreq, cpu logical hotplug..
I meant for you to move the (highly speculative) active load balancing
trigger out of load_balance() so that we can stop returning busiest
groups and queues that have no hope of participating in successful load
balancing.
I think we are unnecessarily behind active load balance...
I'm not sure what you mean here. I'm not behind it at all. I can see
the need for HT systems to be able to move the only running task of a
CPU to another CPU in another package but I think the implementation is
appalling. I think it needs a complete rewrite.
Peter
--
Peter Williams [email protected]
"Learning, n. The kind of ignorance distinguishing the studious."
-- Ambrose Bierce
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