On Wed, 19 Sep 2007, Siddha, Suresh B wrote:
On Tue, Sep 18, 2007 at 11:03:59PM -0700, Tong Li wrote:
This patch attempts to improve CFS's SMP global fairness based on the new
virtual time design.
Removed vruntime adjustment in set_task_cpu() as it skews global fairness.
Modified small_imbalance logic in find_busiest_group(). If there's small
imbalance, move tasks from busiest to local sched_group only if the local
group contains a CPU whose min_vruntime is the maximum among all CPUs in
the same sched_domain. This prevents any CPU from advancing too far ahead
in virtual time and avoids tasks thrashing between two CPUs without
utilizing other CPUs in the system. For example, for 10 tasks on 8 CPUs,
since the load is not evenly divisible by the number of CPUs, we want the
extra load to have a fair use of every CPU in the system.
Tested with a microbenchmark running 10 nice-0 tasks on 8 CPUs. Each task
Just as an experiment, can you run 82 tasks on 8 CPUs. Current
imbalance_pct logic will not detect and fix the global fairness issue
even with this patch.
Right. For 82 tasks on 8 CPUs, the max error is 7.07% and min is -14.12%.
It could be fixed by removing the imbalance_pct check (i.e., making
imbalance_pct effectively 1). The cost would be more migrations, so I
wasn't sure if that was the approach people would agree on.
if (*imbalance < busiest_load_per_task) {
- unsigned long tmp, pwr_now, pwr_move;
- unsigned int imbn;
-
small_imbalance:
- pwr_move = pwr_now = 0;
- imbn = 2;
- if (this_nr_running) {
- this_load_per_task /= this_nr_running;
- if (busiest_load_per_task > this_load_per_task)
- imbn = 1;
- } else
- this_load_per_task = SCHED_LOAD_SCALE;
-
- if (max_load - this_load + SCHED_LOAD_SCALE_FUZZ >=
- busiest_load_per_task * imbn) {
- *imbalance = busiest_load_per_task;
- return busiest;
- }
This patch removes quite a few lines and all this is logic is not for
fairness :( Especially the above portion handles some of the HT/MC
optimizations.
What are the optimizations? I was trying to simplify the code to use only
vruntime to control balancing when there's small imbalance, but if it
breaks non-fairness related optimizations, we can certainly add the code
back.
tong
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