Chris Friesen wrote:
Rich Paredes wrote:
So since cpumax5 has a lower nice value and thus a higher priority (25 in
this case), shouldn't it be given it's own cpu. If I give cpumax5 a
nice
value of -20, it does start using it's own cpu.
My explanation would be that since the scheduler tries to limit cpu
affinity, the nice value of 0 isn't enough to get the scheduler to move
this process to another processors run queue. I could be totally wrong
here though.
I think you are correct. The load balancer doesn't think that this is
enough of an imbalance to go through the effort of swapping two
processes around.
The kernel in use (2.6.5) doesn't take nice into account during load
balancing and just allocates the 5 tasks among the 4 CPUs in a way that
tries to give each CPU the same number of tasks. It also tries not to
move tasks around too much so when it has found a solution that
satisfies that criterion it leaves the tasks there.
5 tasks among 4 CPUs means 1 task each for 3 of the CPUs and 2 tasks for
the other CPU. As nice isn't taken into account it is purely down to
chance whether or not the high priority task ends up being one of those
that gets a CPU to itself or has to share with another task. Some
elementary probability theory should enable the probability of a "good"
outcome (i.e. the high priority task not having to share) to be calculated.
This is an example of the type of situation that the smpnice patches
were designed to handle. They take nice into account and should ensure
that the high priority does get a CPU to itself in this scenario. They
are scheduled for release in the 2.6.18 kernel.
Peter
--
Peter Williams [email protected]
"Learning, n. The kind of ignorance distinguishing the studious."
-- Ambrose Bierce
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