[netperf-talk] FreeBSD sysctl cpu utilization diffs

[Rick Jones]

Andrew Gallatin wrote:
> I use the attached patch to netperf 2.4.1 to measure cpu utilization
> on FreeBSD.  The existing method in 2.2.4 and 2.4.1 confused me, and I
> did not want to wait for a calibration period.

If the mechanism is what I think it was, basically, the idea was to 
compare how fast something increments when the system is "idle" vs when 
the system is running netperf.  Hence the calibration period.

> The FreeBSD kern.cp_time is quite simple.  It returns a copy of an
> array of counters maintained by the kernel.  One of these counters is
> incremented stathz times per second on each processor.  The choice of
> counter (user, nice, sys, intr, idle) depends on the state of the CPU
> when the stathz clock tick happens.  This array is global (not
> per-CPU) and is protected by a spinlock in SMP versions of the FreeBSD
> kernel.

That just shows they haven't gotten to very high CPU counts yet :)

>  See the statclock() routine in sys/kern/kern_clock.c
> (http://www.freebsd.org/cgi/cvsweb.cgi/src/sys/kern/kern_clock.c?rev=1.187&content-type=text/x-cvsweb-markup)
> 
> The patch simply takes a snapshot of the counters when the test
> starts, another snapshot when the test ends, and caclulates the 
> CPU utilization by figuring out how much idle time there was as
> compared to the other, non-idle CPU states.
> 
> I have tested my patch on FreeBSD 5.4 i386 UP, 6.0 i386 SMP, and
> 7.0 amd64 SMP and the CPU time reported by netperf agrees with
> the CPU time reported by the FreeBSD CPU monitoring tools (vmstat,
> systat -vm, iostat).

Since they are likely using the same mechanism I would hope so :)

What does it look like compared to the looper method?  One of the 
concerns I have about all these "statistical" sampling methods is 
whether or not they remain accurate under a very high interrupt load 
such as that produced by netperf.

Not that some of the non-statistical mechanisms upon which netperf 
relies are golden either.

> 
> I was not sure what calibrate_idle_rate() should return.  As a
> stopgap, I had it return sysconf(_SC_CLK_TCK); like the linux
> procstat code, but I have no idea if that is correct, or if
> it even matters.  I suspect that I just need to return a non-zero
> value.

For a CPU method that requires no calibration, it doesn't really matter 
what is returned.

rick jones

> 
> Thanks for listening,
> 
> Drew
> 
> 
> 
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