On Mon, Nov 22, 2010 at 09:05:23AM -0500, Thor Lancelot Simon wrote: > We've been trying to do some profiling on a system which is a > dual-processor x86 running NetBSD/i386 5.1. > > It has not gone well. If we enable profiling for more than 0.2 seconds, > the system locks up. Even more distressingly, sometimes attempts to > turn off profiling simply don't: kgmon -h reports that it turned off > profiling, but actually it didn't. To quote one of my coworkers: > > > .2 seconds was fine. .3 was not. Even though it claimed to have turned > > profiling off, profiling was actually still on. > > > > I broke into the debugger and _gmonparam was 0: GMON_PROF_ON. Manually > > patching it to 3 in my debugger curiously did not work. It kept being set > > to 0. > > > > With DDB write command, I was able to set it to 3, and after a while, the > > system came back and I was able to obtain the trace. > > Another disturbing effect we've noticed is that the sysmon sme_worker goes > crazy when profiling is turned on: it appears to loop sending cv_broadcast() > a million times, and to be scheduled every few ticks instead of once every > 30 seconds. I cannot account for why this would be by (quick) code > examination. Unfortunately disabling it does not solve the system hangs > when we turn on profiling on the MP kernel, however. > > The only even remotely unusual thing about our kernel is that it's built > with HZ=1024. We have confirmed that hz = profhz = stathz = 1024 at runtime, > however. > > Does anyone have profiling working on a multiprocessor system right now? > On i386? With netbsd-5?
It worked during the 5.0 development cycle, but it has been a very long time since I tried it. With the current setup a spinlock is taken on EVERY function entry, so it does have a severe impact on the system. Although unless the box is being hammered at the time, what you suggest seems to indicate a more severe problem. It wants for per-CPU buffers that get merged when the data is read out.. As a workaround have you considered using tprof? Not as useful as gprof though.
