[netperf-talk] CPU utilization & tickless kernels?

[Rick Jones]

On 12/07/2012 12:42 PM, Andrew Gallatin wrote:
> I added some debugging code to
> netcpu_procstat.c:calc_cpu_util_internal() to print out the
> total ticks, and it is vastly different for each CPU !!!
>
> I then got to wondering:  /proc/stat has a summary line, which
> appears to be what vmstat reads.  Why does netperf go through
> the trouble to read all the CPUs individually?  Why can't you
> just use the summary line?  Are there historical bugs with
> the summary line?

The initial reasons are lost in the mists of time.  It may have been 
simply because the initial HP-UX version did per-CPU utilization.  Today 
one reason for the per-CPU tracking is to enable reporting the ID and 
utilization of the most utilized CPU on either side.  That is included 
for helping with things like a four CPU system being able to have 25% 
overall CPU utilization with either 25% across the board, two CPUs at 
50% and two idle, or one at 100% and three idle, and all the other 
permuations.

> A very cheesy hack to use the summary line results in CPU
> utilization which agrees with vmstat's summary.
>
> Drew
>
> PS: FWIW, mpstat also gives none-sense results on this system.
> For example:
>
> 15:35:10     CPU    %usr   %nice    %sys %iowait    %irq   %soft  %steal
>   %guest   %idle
> 15:35:11     all    0.00    0.00   60.61    0.00    0.00    7.27    0.00
>     0.00   32.12
> 15:35:11       0    0.00    0.00    0.00    0.00    0.00   18.75    0.00
>     0.00   81.25
> 15:35:11       1    0.99    0.00   99.01    0.00    0.00    0.00    0.00
>     0.00    0.00
>
>
> By the per-cpu counters, we've got (81.25 + 0) / 2 or 40.6% idle,
> but the summary says 32% idle.

Well, misery loves company :)  As such, I'm glad that mpstat is 
similarly disagreeing.  Perhaps netperf needs to "interpret" the tick 
counts differently?  Scale them to the elapsed time somehow?  (Guessing) 
  Otherwise, perhaps the "bug" is in the source material.

Does top's per-CPU output look sane or mistaken?

rick

https://lkml.org/lkml/2012/2/6/20 has some issue with NOHZ and load 
averages - not sure if there is a relation there though.

I miss HP-UX's direct cycle counters, even if they were in an 
inconvenient lo/hi 32 bit format... "And you knew what you used then..."