Peter Williams wrote:
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.
Thanks this works and I'm now convinced that there's hopping occurring
but I disagree on the cause (see below).
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?
The upshot of this is that the code near "however we may" etc. never
gets executed. So what must be happening is that *imbalance is greater
than SCHED_LOAD_SCALE so it goes out unchanged (as the fact that the
problem occurs with hard spinners lets try_to_wake_up() off the hook).
But, as I also said in another e-mail, there must be another task with
equal or higher priority than the task (that's doing the hopping) on its
CPU for it to be booted off. I would surmise that the prime candidate
here would be the migration thread for the CPU in question as it's a
real time task and seems to run fairly frequently.
The obvious way to handle this is too NOT move tasks from a run queue
where the migration thread is running unless the number of running tasks
on that CPU is greater than 2. I'll try to come up with a patch that
does this without causing too much complexity.
Peter
--
Peter Williams [email protected]
"Learning, n. The kind of ignorance distinguishing the studious."
-- Ambrose Bierce
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to [email protected]
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
[Index of Archives]
[Kernel Newbies]
[Netfilter]
[Bugtraq]
[Photo]
[Stuff]
[Gimp]
[Yosemite News]
[MIPS Linux]
[ARM Linux]
[Linux Security]
[Linux RAID]
[Video 4 Linux]
[Linux for the blind]
[Linux Resources]
