Peter Williams wrote:
Siddha, Suresh B wrote:
On Tue, Mar 28, 2006 at 05:00:50PM +1100, Peter Williams wrote:
[bits deleted]
Even with no HT and MC, this patch has still has issues in the presence
of different priority tasks... consider a simple DP system and run two
instances of high priority tasks(simple infinite loop) and two normal
priority
tasks. With "top" I observed that these normal priority tasks keep on
jumping
from one processor to another... Ideally with smpnice, we would assume
that each processor should have two tasks (one high priority and
another one with normal priority) ..
Yes, but you are failing to take into account the effect of the other
tasks on your system (e.g. top) that run from time to time. If their
burst of CPU use happens to coincide with some load balancing activity
they will cause an imbalance to be detected (that is different to that
which only considers your test tasks) and this will result in some tasks
being moved. Beware the Heisenberg Uncertainty Principle :-).
Notwithstanding the HUP, I've investigated this and have found that
there is more instability than expected and that it is due to a silly
bit of code (by me) at the end of find_busiest_queue() marked by the
comment:
/* or if there's a reasonable chance that *imbalance is big
* enough to cause a move
*/
that makes load balancing more aggressive. The functionality it
implemented should have been abandoned when the code was updated to use
average run queue loads instead of SCHED_LOAD_SCALE in the code that
handled small imbalances but wasn't. I'll send Andrew a patch that
removes the offending code shortly.
Peter
--
Peter Williams [email protected]
"Learning, n. The kind of ignorance distinguishing the studious."
-- Ambrose Bierce
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