Messages in this thread | | | Date | Mon, 05 Mar 2012 20:57:50 +0100 | From | Lesław Kopeć <> | Subject | Re: Inconsistent load average on tickless kernels |
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On 29.02.2012 13:06, Peter Zijlstra wrote:
> Missing here is a kernel build with CONFIG_NO_HZ but booted with > nohz=off; this would be an interesting data point because it includes > all the funny code but still ticks are the right frequency.
You've asked for it and you got it. I have rebooted some servers with nohz=off parameter set on kernels complied with CONFIG_NO_HZ=y. They're the ones listed below with 'off' suffix.
On 29.02.2012 17:24, Peter Zijlstra wrote:
> Hrmm, this suggests we age too hard with nohz code.. in your test case > is there significant idle time? That is, suppose you run each cpu at 30% > what is the period of you load? Running 3s out of 10s is significantly > different from running .3ms out of 1ms.
It's definitely more similar to the second case - very frequent, but short bursts of activity. A single process does a tiny bit of computation mixed with a fair amount of network activity on each request. There are 80 such processes which are responsible for majority of system load.
On 29.02.2012 18:03, Peter Zijlstra wrote:
>> The only thing I could find is that on nohz we can confuse the per-rq >> sample period, does the below make a difference? > > Uhm, something like so that is.. > > --- > kernel/sched/core.c | 3 ++- > 1 files changed, 2 insertions(+), 1 deletions(-) > > diff --git a/kernel/sched/core.c b/kernel/sched/core.c > index d7c4322..44f61df 100644 > --- a/kernel/sched/core.c > +++ b/kernel/sched/core.c > @@ -2380,7 +2380,8 @@ static void calc_load_account_active(struct rq *this_rq) > if (delta) > atomic_long_add(delta, &calc_load_tasks); > > - this_rq->calc_load_update += LOAD_FREQ; > + while (!time_before(jiffies, this_rq->calc_load_update)) > + this_rq->calc_load_update += LOAD_FREQ; > } > > /* >
I have compiled another batch of kernels with this patch applied (they're the ones with 'patch0' suffix). The only difference was the patch had to go to kernel/sched.c, but that's what you get when not using the latest sources. Anyway, here are the results accompanied by a pretty picture [1]:
std off patch0 2.6.32.55-no-hz 0.76 0.91 - 2.6.32.55-no-hz-74f5187ac8 6.41 9.40 4.93 2.6.32.55-no-hz-0f004f5a69 0.78 0.92 0.90 2.6.37-rc5-no-hz-0f004f5a69 0.95 0.92 0.86 2.6.37-rc5-no-hz-pre-0f004f5a69 9.16 10.47 8.02
It seems that the patch didn't help much on kernels with 0f004f5a69 applied. The ones with just 74f5187ac8 are reporting a more plausible values, but slightly lower than the ones without patch0. Am I right to assume that the correct load values are the ones produced by kernels complied with CONFIG_NO_HZ=n? Should they be the baseline?
I can run additional tests if you have other leads to follow. Is there a particular kernel version I should focus on? If not I will continue to use the current bundle. I'm also planning to give the latest stable release a spin.
[1] http://img835.imageshack.us/img835/2204/kernelload.png
-- Lesław Kopeć
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