On Thu, 15 Sep 2011, Andriy Gapon wrote:
This is more of a "just for the record" email.
I think I've already stated the following observations, but I suspect that they
drowned in the noise of a thread in which I mentioned them.
1. Incorrect topology is built for single-package SMP systems.
That topology has two levels ("shared nothing" and "shared package") with
exactly
the same CPU sets. That doesn't work well with the rebalancing algorithm which
assumes that each level is a proper/strict subset of its parent.
2. CPU load comparison algorithms are biased towards lower logical CPU IDs.
With all other things being equal the algorithms will always pick a CPU with a
lower ID. This creates certain load asymmetry and predictable patterns in load
distribution.
If all other things truly are equal why does selecting a lower cpu number
matter?
Another observation.
It seems that ULE makes a decision about thread-to-CPU affinity at the time
when a
thread gets switched out. This looks logical from the implementation point of
view. But it doesn't seem logical from a general point of view - when the
thread
will be becoming running again its affinity profile may become completely
different. I think that it would depend on how much a thread actually spends
not
running.
The decision is made at sched_add() time. sched_pickcpu() does the work
and selects the run-queue we will be added to. We consider the CPU that
the thread was last running on but the decision is made at the time that a
run queue must be selected.
Jeff
--
Andriy Gapon
_______________________________________________
freebsd-current@freebsd.org mailing list
http://lists.freebsd.org/mailman/listinfo/freebsd-current
To unsubscribe, send any mail to "freebsd-current-unsubscr...@freebsd.org"
_______________________________________________
freebsd-current@freebsd.org mailing list
http://lists.freebsd.org/mailman/listinfo/freebsd-current
To unsubscribe, send any mail to "freebsd-current-unsubscr...@freebsd.org"
