On 2021-09-30 17:08:54 [+0206], John Ogness wrote: > > Where does this "--latency=" option apply to? > > It is the value written to /dev/cpu_dma_latency, which AFAIK writes the > maximum acceptable latency (in microseconds). This translates to the > allowed C states. cyclictest currently writes 0, which should keep the > processor in C0. For example, setting it to 1-5, should allow C0 and C1. > > Using --laptop will cause cyclictest to avoid touching > /dev/cpu_dma_latency. But nobody would know that unless they looked at > the code. Exactly. We want to measure not to change the defaults before we start doing so. > IMHO, systems should be configured for production use and cyclictest > should just _measure_ latencies at a specified priority level. But by > default cyclictest is adjusting global system behavior during > measurements, thus providing results that the system (as it is actually > configured) would not be able to provide. > > I realize that nobody wants to touch defaults. But I'm not sure users > are aware how important the --laptop option is for realistic > measurements. In fact, the description of --laptop even encourages users > _not_ to use it. :-/ We don't want to touch the defaults? Not so long ago I was curious why the -b argument didn't stop the trace. Apparently I didn't use the --tracemark option which is required nowadays. > John Ogness Sebastian
