Hi Arvid, I got curious and tried, also on 2.6.32-122-rtai - it turns out it has a significant improvement on rtai too, so that feature might go back even longer than 3.3.
I do see potential use for this, it would be in the plain kernel support of what is now 'sim mode' (aka posix threads, which are not RT-hardened wrt latency in any way) what we currently do is: Posix threads are used for for both 'vanilla' and 'RT PREEMPT' RTOS support; in case an RT PREEMPT kernel is detected its facilities are used (I'd need to read up on the specifics). If not, it falls back to plain scheduling, and thats where your suggestion could come it - it could improve RT latency when running on a vanilla kernel. here's the current code which is used for both vanilla and RT PREEMPT kernels: http://git.mah.priv.at/gitweb?p=emc2-dev.git;a=blob;f=src/rtapi/rt-preempt-user.c;h=d46994b172b030b98d601eba1a23707531794ed8;hb=refs/heads/rtos-integration-preview3 would you be willing to help plug that in? ideally it would be runtime flag; there will be a kernel autodetection routine which executes at the beginning of an RTAPI instance, and thats where a flag could be handed down from thanks for the suggestion! regards Michael Am 20.03.2013 um 00:20 schrieb Arvid Brodin: > Hello, > > Just out of curiosity, I wrote this program to test the latency using a > vanilla 3.4 kernel's "real-time scheduler". Maybe it is of interest to > someone on this list. > > $ uname -srvmp > Linux 3.3.8 #7 SMP PREEMPT Tue Jun 26 03:08:40 CEST 2012 x86_64 Intel(R) > Core(TM) i3-2120T CPU @ 2.60GHz > > Tests were made while running a 1080p x264 movie and "shaking" a large > glxgears window. > > Sample running as normal user without permission to change to realtime > scheduler, i.e. running under the normal scheduler (SCHED_OTHER): > > $ ./rt_test > Setting SCHED priority 1 > Could not set scheduler: Operation not permitted > CLOCK_REALTIME resolution: 1 ns > Establishing handler for signal 34 > timer ID is 0x1ae6010 > ... > Min, max diff -686, 694 µs; cumulative diff 1990 µs; overruns 0; > calls 70913 > Min, max diff -827, 1257 µs; cumulative diff 4003 µs; overruns 2; > calls 71970 > Min, max diff -645, 1193 µs; cumulative diff 4994 µs; overruns 1; > calls 73028 > Min, max diff -629, 635 µs; cumulative diff 4996 µs; overruns 0; > calls 74087 > Min, max diff -48, 65 µs; cumulative diff 4997 µs; overruns 0; > calls 75146 > Min, max diff -29, 34 µs; cumulative diff 4990 µs; overruns 0; > calls 76204 > Min, max diff -253, 160 µs; cumulative diff 4995 µs; overruns 0; > calls 77263 > Min, max diff -854, 868 µs; cumulative diff 4996 µs; overruns 0; > calls 78323 > Min, max diff -812, 1812 µs; cumulative diff 5990 µs; overruns 1; > calls 79381 > Min, max diff -635, 371 µs; cumulative diff 5990 µs; overruns 0; > calls 80441 > Min, max diff -536, 540 µs; cumulative diff 6000 µs; overruns 0; > calls 81500 > Min, max diff -855, 1229 µs; cumulative diff 6988 µs; overruns 1; > calls 82558 > Min, max diff -305, 435 µs; cumulative diff 7002 µs; overruns 0; > calls 83618 > Min, max diff -293, 304 µs; cumulative diff 6995 µs; overruns 0; > calls 84677 > Min, max diff -644, 647 µs; cumulative diff 6993 µs; overruns 0; > calls 85737 > > > Sample running under real-time scheduler (SCHED_FIFO): > # ulimit -r 1 > # ./rt_test > Setting SCHED priority 1 > CLOCK_REALTIME resolution: 1 ns > Establishing handler for signal 34 > timer ID is 0x1236010 > ... > Min, max diff -24, 36 µs; cumulative diff 2 µs; overruns 0; calls 35173 > Min, max diff -15, 20 µs; cumulative diff -3 µs; overruns 0; calls > 36178 > Min, max diff -30, 24 µs; cumulative diff -3 µs; overruns 0; calls > 37183 > Min, max diff -17, 14 µs; cumulative diff -1 µs; overruns 0; calls > 38187 > Min, max diff -27, 22 µs; cumulative diff -4 µs; overruns 0; calls > 39192 > Min, max diff -68, 80 µs; cumulative diff 2 µs; overruns 0; calls 40199 > Min, max diff -26, 32 µs; cumulative diff 0 µs; overruns 0; calls 41204 > Min, max diff -23, 29 µs; cumulative diff -3 µs; overruns 0; calls > 42209 > Min, max diff -29, 28 µs; cumulative diff 3 µs; overruns 0; calls 43215 > Min, max diff -24, 25 µs; cumulative diff -3 µs; overruns 0; calls > 44220 > Min, max diff -26, 28 µs; cumulative diff 2 µs; overruns 0; calls 45225 > Min, max diff -19, 25 µs; cumulative diff -3 µs; overruns 0; calls > 46231 > Min, max diff -47, 57 µs; cumulative diff -1 µs; overruns 0; calls > 48242 > > > It looks good enough for the servo thread, at least on my computer! > Here's the program, in case you want to try it yourself: > > Compile with > $ gcc -Wall -lrt rt_test.c -o rt_test > > --- > /* > * Latency/precision test program for POSIX timers under Linux > * real-time scheduling (SCHED_FIFO). > * > * Based on the sample code in 'man 2 timer_create', > * copyright (c) 2009 Linux Foundation, written by Michael Kerrisk > * <mtk.manpa...@gmail.com> > * and released under the "Verbatim" License: > * https://www.kernel.org/doc/man-pages/licenses.html#verbatim > * > * Modifications for this usage by Arvid Brodin <arv...@kth.se>. > */ > > #include <stdlib.h> > #include <unistd.h> > #include <stdio.h> > #include <sched.h> > #include <signal.h> > #include <time.h> > #include <errno.h> > > #define CLOCKID CLOCK_REALTIME > #define SIG SIGRTMIN > #define PERIOD 1000000 > > > #define errExit(msg) do { perror(msg); exit(EXIT_FAILURE); \ > } while (0) > > static inline int min(int a, int b) > { > return (a < b) ? a : b; > } > > static inline int max(int a, int b) > { > return (a > b) ? a : b; > } > > > /* Global variable to keep track of latency statistics etc. */ > static struct { > int mindiff; // Maximum negative deviation > int maxdiff; // Maximum positive deviation > int cumdiff; // Cumulative deviation from first call > int calls; // Total number of calls made > int overruns; // Number of completely missed calls > int last_ns; > } period_info; > > static void print_clockinfo() > { > struct timespec ts; > > if (clock_getres(CLOCK_REALTIME, &ts) != 0) { > printf("Could not get clock info.\n"); > return; > } > > printf("CLOCK_REALTIME resolution: %ld ns\n", ts.tv_nsec); > } > > /* > * Print statistics. Called from main loop. > */ > static void print_diffs() > { > printf("Min, max diff %5d, %5d µs; cumulative diff %d µs; overruns %d; > calls %d\n", > period_info.mindiff / 1000, > period_info.maxdiff / 1000, > period_info.cumdiff / 1000, > period_info.overruns, > period_info.calls); > period_info.mindiff = 0; > period_info.maxdiff = 0; > period_info.overruns = 0; > } > > > static struct sigaction sa; > > /* > * Signal handler. Called every PERIOD ns. > */ > static void handler(int sig, siginfo_t *si, void *uc) > { > timer_t *tidp; > struct timespec ts; > int overruns; > int diff; > > period_info.calls += 1; > > tidp = si->si_value.sival_ptr; > overruns = timer_getoverrun(*tidp); > if (overruns > 0) > period_info.overruns += overruns; > > clock_gettime(CLOCKID, &ts); > if (period_info.last_ns < 0) > // First call > period_info.last_ns = ts.tv_nsec - PERIOD; > else if (ts.tv_nsec < period_info.last_ns) > // Seconds wrapped > period_info.last_ns -= 1000000000; > > diff = ts.tv_nsec - period_info.last_ns - PERIOD; > period_info.mindiff = min(period_info.mindiff, diff); > period_info.maxdiff = max(period_info.maxdiff, diff); > period_info.cumdiff += diff; > > period_info.last_ns = ts.tv_nsec; > } > > /* > * Signal delivery causes system/library calls to return prematurely. > * See 'man 7 signal' for details. This function repeatedly call > * nanosleep() until (at least) 'sec' seconds has passed. > */ > static void sigsafe_sleep(int sec) > { > struct timespec rqtp, rmtp; > struct timespec *ts_req, *ts_rem, *ts_tmp; > int res; > > rqtp.tv_sec = sec; > rqtp.tv_nsec = 0; > > ts_req = &rqtp; > ts_rem = &rmtp; > res = nanosleep(ts_req, ts_rem); > while (res < 0 && errno == EINTR) { > ts_tmp = ts_req; > ts_req = ts_rem; > ts_rem = ts_tmp; > res = nanosleep(ts_req, ts_rem); > } > } > > int main(int argc, char *argv[]) > { > struct sched_param sched_param; > timer_t timerid; > struct sigevent sev; > struct itimerspec its; > int res; > const int sched_policy = SCHED_FIFO; > > // Scheduler > sched_param.sched_priority = sched_get_priority_min(sched_policy); > printf("Setting SCHED priority %d\n", sched_param.sched_priority); > res = sched_setscheduler(0, sched_policy, &sched_param); > if (res < 0) > perror("Could not set scheduler"); > > // Some init > print_clockinfo(); > period_info.mindiff = 0; > period_info.maxdiff = 0; > period_info.cumdiff = 0; > period_info.last_ns = -1; > > // Establish handler for timer signal > printf("Establishing handler for signal %d\n", SIG); > sa.sa_flags = SA_SIGINFO; > sa.sa_sigaction = handler; > sigemptyset(&sa.sa_mask); > if (sigaction(SIG, &sa, NULL) == -1) > errExit("sigaction"); > > // Create the timer > sev.sigev_notify = SIGEV_SIGNAL; > sev.sigev_signo = SIG; > sev.sigev_value.sival_ptr = &timerid; > if (timer_create(CLOCKID, &sev, &timerid) == -1) > errExit("timer_create"); > printf("timer ID is 0x%lx\n", (long) timerid); > > // Start the timer > its.it_value.tv_sec = 0; > its.it_value.tv_nsec = PERIOD; > its.it_interval.tv_sec = its.it_value.tv_sec; > its.it_interval.tv_nsec = its.it_value.tv_nsec; > > if (timer_settime(timerid, 0, &its, NULL) == -1) > errExit("timer_settime"); > > while (1) { > sigsafe_sleep(1); > print_diffs(); > } > > // Not reached > exit(EXIT_SUCCESS); > } > --- > > -- > Arvid > > ------------------------------------------------------------------------------ > Everyone hates slow websites. So do we. > Make your web apps faster with AppDynamics > Download AppDynamics Lite for free today: > http://p.sf.net/sfu/appdyn_d2d_mar > _______________________________________________ > Emc-developers mailing list > Emc-developers@lists.sourceforge.net > https://lists.sourceforge.net/lists/listinfo/emc-developers ------------------------------------------------------------------------------ Everyone hates slow websites. So do we. Make your web apps faster with AppDynamics Download AppDynamics Lite for free today: http://p.sf.net/sfu/appdyn_d2d_mar _______________________________________________ Emc-developers mailing list Emc-developers@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/emc-developers
