On Thu, 30 Sep 2021, John Ogness wrote: > On 2021-09-30, John Kacur <jkacur@xxxxxxxxxx> wrote: > >>>> With hackbench, the system is sufficiently busy to avoid the going > >>>> into idle. > >>> > >>> Not just that. cyclictest's usage of /dev/cpu_dma_latency has the side > >>> effect that it may disable some of the PM stuff in the system. > >>> So your system appears good but then when cyclictest is gone, the > >>> numbers go up. > >>> > >>> Maybe we should drop that so we observe a system without altering its > >>> behaviour? > >> > >> +1 > >> > >> Developers wanting to explicitly cause this behavior can use --latency= > >> to enable it. Having it on as a default is misleading. > > > > Where does this "--latency=" option apply to? I see this option was dropped from the help, will add it back. > > 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. > > 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. The thing is, we are not just trying to measure an environment, we are also simulating a realtime application. If your realtime environment doesn't disable c-states, then your realtime application probably should. It's always a question of are we trying to measure a worse case scenario or a best case scenario. If I remove this default we're going to get a slew of reports from people using cyclictest wondering why they aren't getting good realtime performance. > > 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. :-/ I agree, but I just wanted to quickly inform people on this email thread, how to disable the default of writing to cpu_dma_latency, before we create any patches to improve this.
