On Fri, Jul 3, 2009 at 7:50 PM, Jon Masters<jonathan@xxxxxxxxxxxxxx> wrote: > On Fri, 2009-07-03 at 11:54 +0000, jbl wrote: > >> [11:52:41] <jbl:#linux-rt> +jonmasters: ping >> [11:53:27] <jbl> +how long should be max latencies seen with hwlat detector? > > It's a US holiday, so I'll reply by email as I'm not really around for > IRC, but I don't want to forget to add something to this - especially as > people seem to be trying to also track down new latencies in RT itself. > I don't think these are SMI related - though now you can check. First I wondered if I should reply via twitter or blog ... just to be more 2.0 ;) > The default for the latency detector is to regard any unexplained time > interval over (greater than, not greater than or equal to) 10us as a > latency worthy of some regard. But you can also configure it down to 1us > since anything over 0us represents some kind of interruption - it's all > down to what threshold you actually care about (few people notice 1us). > > Anyway. How high should it go? Theoretically, that is an open ended > question. But we have seen in practice latencies up to many milliseconds > or even higher - I don't expect you'll go over 100ms and in reality, I > don't expect it to be anything like that, maybe hundreds of us on a > modern enough system. But the range is really down to whatever crap the > BIOS is doing when it's taking your CPU cycles away, and that could > literally be anything (reformulating the coke recipe isn't unlikely). > > If you're seeing something irregular, for widely long periods of time it > is less likely to be the kind of SMIs we see, which do tend to be > semi-predictable as they rely upon counters for most of the > non-asynchronous (e.g. hardware emulation) activities. But if you see > something short many times per second, or something longer on a > predictable schedule then it is quite likely to be an SMI hitting you. I was wondering about the "samples recorded" value. First I thought there is something wrong since that value was always zero on my test machine. I expected that this this value would reflect somehow duration / window length. I guess the documentation could be extended to explain a bit how to work with various settings of sample window and width to find out the exact interval of the disturbance. Thanks, Jan -- To unsubscribe from this list: send the line "unsubscribe linux-rt-users" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
