Re: [PATCH RFC] rdtscbench: a nohz_full validation and benchmarking tool

[Clark Williams <williams@redhat.com>]

On Fri, 21 Aug 2015 15:45:58 -0500
Isaac Griswold-Steiner <isaac.griswold.steiner@gmail.com> wrote:

> From: Isaac Griswold-Steiner <isaac.griswoldsteiner@ni.com>
> 
> rdtscbench is a cyclictest-like tool that spawns a thread per cpu. Each thread
> measures the difference in cycle count (using the tsc) during the execution of a
> tight loop.
> 
> This is a simple tool intended to be used for the validation of nohz_full CPU
> configurations. As the validation of nohz_full CPUs is the objective, the tool
> avoids the usage of system calls, timers, or anything that might break nohz_full.
> 


Isaac,

A question and a request.

Was there any particular reason you used sleep() rather than clock_nanosleep() in your cycles_per_second function? I see that you did ten samples but wondered if the slop from a HZ-based wakeup might still introduce some error, as opposed to a more precise programmed wakeup. 

Also, I'd appreciate it if you would expand a bit on the usage section in your README file, specifically how you tune a system prior to running rdtscbench, what output indicates that your tuning is *not* working, versus when to know you're doing the right things. It's probably as simple as saying "if the max latency numbers are spiking you have a problem" but it's good to be explicit about that sort of thing. 

Thanks,
Clark