Re: CPU utilization measure impacts on throughput?

Tom Pavel (
Mon, 17 Aug 1998 17:55:26 -0700

>>>>> On Mon, 17 Aug 1998, Danke Hirasawa <> writes:

> I have experienced that we got different throughput numbers in the
> TCP_STREAM test where we measured CPU utilization (with -c & -C options)
> as well as throughput measurements vs.. the another TCP_STREAM test
> where no CPU utilization measurement (without -c nor -C options).
> Usually throughputs of the test without CPU utilization measurements are
> higher.
> Does the measurement of CPU utilization (negatively) impact on
> throughput measurement?

I think the general answer to this depends on how the CPU measurement
is done. For example, there is special code to use a
pstat_getprocessor() call on HPs, which presumably gets CPU
utilization directly from the kernel without any overhead.

However, I have used netperf on Solaris, Digital Unix, AIX, and Linux.
On each of these systems, the USE_LOOPER CPU measurement technique is
used. This runs a "competing process" with the netperf process and
measures how much CPU time this (low-priority) looping process gets.
Running this CPU utilization test reduced some of my TCP bandwidths by
as much as 10%. I was quite surprised when I learned this.
Presumably, this happens because the schedulers in these OSes are not
perfect, and because things like paging and interrupts are a shared
resource despite the illusion of completely independent processes
presented to you by the OS.

Anyway, the short answer to your question is "Yes"...

Tom Pavel

Stanford Linear Accelerator Center