Siddha, Suresh B wrote:
On Wed, Feb 15, 2006 at 09:40:32AM +1100, Peter Williams wrote:
Siddha, Suresh B wrote:
On a 4P(8-way with HT), if you run a -20 task(a simple infinite loop)
it hops from one processor to another processor... you can observe it
using top.
How do you get top to display which CPU tasks are running on?
In the interactive mode, you can select the "last used cpu" field to display.
or you can use /proc/pid/stat
find_busiest_group() thinks there is an imbalance and ultimately the
idle cpu kicks active load balance on busy cpu, resulting in the hopping.
I'm still having trouble getting my head around this. A task shouldn't be moved unless there's at least one other task on its current CPU, it
Because of the highest priority task, weighted load of that cpu
will be > SCHED_LOAD_SCALE. Because of this, an idle cpu in find_busiest_group() thinks that there is an imbalance.. This is due to
the code near the comment "however we may be ablet to increase total CPU power used by ...". That piece of code assumes that a unit load
is represented by SCHED_LOAD_SCALE (which is no longer true with smpnice)
and finally results in "pwr_move > pwr_now".. This will make the idle cpu
try to pull that process from busiest cpu and the process will ultimately move
with the help of active load balance...
But as I pointed out, with the code as it is in the recent -mm kernels, the amount of biased load (i.e. NICE_TO_BIAS_PRIO(0)) that find_busiest_group() sets *imbalance to in these circumstances is too small for a task with nice less than zero to be moved i.e. move_tasks() will skip it. Or are you just referring to the vanilla kernels?