Messages in this thread | | | Subject | Re: [PATCH RFC] cpuidle/drivers/menu: Remove get_loadavg in the performance multiplier | From | Daniel Lezcano <> | Date | Wed, 3 Oct 2018 12:25:20 +0200 |
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On 03/10/2018 10:58, Rafael J. Wysocki wrote: > On Tuesday, September 25, 2018 11:38:44 AM CEST Daniel Lezcano wrote: >> The function get_loadavg() returns almost always zero. To be more >> precise, statistically speaking for a total of 1023379 times passing >> to the function, the load is equal to zero 1020728 times, greater than >> 100, 610 times, the remaining is between 0 and 5. >> >> I'm putting in question this metric. Is it worth to keep it? > > The honest answer is that I'm not sure. > > Using the CPU load for driving idle state selection is questionable in general > (there need not be any connection between the two in principle) and this > particular function is questionable either. If it really makes no difference > in addition to that, then yes it would be good to get rid of it. > > That said it looks like that may be quite workload-dependent, so maybe there > are workloads where it does make a difference? Like networking or similar?
Perhaps initially the load was not the same:
commit 69d25870f20c4b2563304f2b79c5300dd60a067e Author: Arjan van de Ven <arjan@infradead.org> Date: Mon Sep 21 17:04:08 2009 -0700
cpuidle: fix the menu governor to boost IO performance
The function get_loadavg() was:
static int get_loadavg(void) { unsigned long this = this_cpu_load();
return LOAD_INT(this) * 10 + LOAD_FRAC(this) / 10; }
With:
unsigned long this_cpu_load(void) { struct rq *this = this_rq(); return this->cpu_load[0]; }
Now we pass the load we got from get_iowait_load() which is:
void get_iowait_load(unsigned long *nr_waiters, unsigned long *load) { struct rq *rq = this_rq(); *nr_waiters = atomic_read(&rq->nr_iowait); *load = rq->load.weight; }
Colin Cross did some measurements a few years ago and concluded it is zero most of the time, otherwise the number is so high it has no meaning. The get_loadavg() call is removed from Android since 2011 [1].
I can hack my server (bi Xeon - 6 cores), add statistics regarding this metric and let it run during several days, sure a lot of different workloads will happen on it (compilation, network, throughput computing, ...) in a real use case manner. That is what I did with an ARM64 evaluation board.
However I won't be able to do that right now and that will take a bit of time. If you think it is really useful, I can plan to do that in the next weeks.
-- Daniel
[1] https://android.googlesource.com/kernel/common/+/4dedd9f124703207895777ac6e91dacde0f7cc17
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