On Wed, 2007-01-24 at 10:41 +0100, John wrote: > I'm using the POSIX timers API. My platform is x86 running Linux > 2.6.18.6 patched with the high-resolution timer subsystem. > > http://www.tglx.de/hrtimers.html > > I've written a small "de-jittering engine" that receives packets in > small bursts due to network jitter (typical average rate of 1000 packets > per second), and re-sends them at a "smooth" rate. > > Just before I re-send a packet, I arm a one-shot timer in order to > receive a signal when it is time to send the next packet. > > I've noticed a strange phenomenon that I cannot explain. > > Sometimes (rarely) the one-shot timer will expire more than 50 µs later > than expected. This would seem normal, except that it happens periodically. > > For example, my app had been running normally for 2 minutes when it > started printing diagnostics (see below). > > The first T_NEXT_POP is the date the timer was supposed to expire, > > NOW is the date the timer was handled after returning from sigwaitinfo > (I am aware that blocking signals, and handling them at a specific point > in the code will add some latency) > > The second T_NEXT_POP is the date the next timer is supposed to expire. > > DIFF is the difference between real and expected dates. > > (All dates are CLOCK_MONOTONIC by the way.) > > As you can see, the first diagnostic came at 472.410501014... Then > another diagnostic almost exactly two seconds apart 9 times in a row! > > My process is the only SCHED_FIFO process on the system. There are no > user-space processes with a higher priority. AFAICT, only a kernel > thread could keep the CPU away from my app. > > Is there a periodic kernel thread that runs every 2 seconds, cannot be > preempted, and runs for over 50 µs??
This sounds like a BIOS SMI issue. Can you reproduce this behavior on different hardware? thanks -john - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
