[RFT][patch] Scheduling for HTT and not only
Alexander Motin
mav at FreeBSD.org
Sat Mar 3 07:40:31 UTC 2012
Hi.
On 03/03/12 05:24, Adrian Chadd wrote:
> mav@, can you please take a look at George's traces and see if there's
> anything obviously silly going on?
> He's reporting that your ULE work hasn't improved his (very) degenerate case.
As I can see, my patch has nothing to do with the problem. My patch
improves SMP load balancing, while in this case problem is different. In
some cases, when not all CPUs are busy, my patch could mask the problem
by using more CPUs, but not in this case when dnets consumes all
available CPUs.
I still not feel very comfortable with ULE math, but as I understand, in
both illustrated cases there is a conflict between clearly CPU-bound
dnets threads, that consume all available CPU and never do voluntary
context switches, and more or less interactive other threads. If other
threads detected to be "interactive" in ULE terms, they should preempt
dnets threads and everything will be fine. But "batch" (in ULE terms)
threads never preempt each other, switching context only about 10 times
per second, as hardcoded in sched_slice variable. Kernel build by
definition consumes too much CPU time to be marked "interactive".
exo-helper-1 thread in interact.out could potentially be marked
"interactive", but possibly once it consumed some CPU to become "batch",
it is difficult for it to get back, as waiting in a runq is not counted
as sleep and each time it is getting running, it has some new work to
do, so it remains "batch". May be if CPU time accounting was more
precise it would work better (by accounting those short periods when
threads really sleeps voluntary), but not with present sampled logic
with 1ms granularity. As result, while dnets threads each time consume
full 100ms time slices, other threads are starving, getting running only
10 times per second to voluntary switch out in just a few milliseconds.
> On 2 March 2012 16:14, George Mitchell<george+freebsd at m5p.com> wrote:
>> On 03/02/12 18:06, Adrian Chadd wrote:
>>>
>>> Hi George,
>>>
>>> Have you thought about providing schedgraph traces with your
>>> particular workload?
>>>
>>> I'm sure that'll help out the scheduler hackers quite a bit.
>>>
>>> THanks,
>>>
>>>
>>> Adrian
>>>
>>
>> I posted a couple back in December but I haven't created any more
>> recently:
>>
>> http://www.m5p.com/~george/ktr-ule-problem.out
>> http://www.m5p.com/~george/ktr-ule-interact.out
>>
>> To the best of my knowledge, no one ever examined them. -- George
--
Alexander Motin
More information about the freebsd-hackers
mailing list