Messages in this thread | ![/](/images/icornerl.gif) | | Date | Mon, 13 Oct 2003 14:46:49 -0700 | From | Tom Marshall <> | Subject | missed itimer signals in 2.6 |
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It seems that with the 2.6 kernel, ITIMER_REAL becomes inaccurate when the interval is set under about 50ms. Using a 10ms interval, I get 455 signals in five seconds (about 9% loss). The test machine has virtually no load and this is reproducible across machines of varying speeds. The test application is attached.
Typical output under 2.4.22:
i=10, ticks=501 (expected 500), elapsed=5.008537, drift=+0.001468 i=20, ticks=251 (expected 250), elapsed=5.019925, drift=+0.000076 i=30, ticks=167 (expected 166), elapsed=5.009974, drift=+0.000027 i=40, ticks=126 (expected 125), elapsed=5.039981, drift=+0.000020 i=50, ticks=101 (expected 100), elapsed=5.049980, drift=+0.000021
Typical output under 2.6.0-test7:
i=10, ticks=455 (expected 500), elapsed=5.003323, drift=-0.453316 i=20, ticks=239 (expected 250), elapsed=5.018035, drift=-0.238033 i=30, ticks=162 (expected 166), elapsed=5.021115, drift=-0.161113 i=40, ticks=122 (expected 125), elapsed=5.001113, drift=-0.121111 i=50, ticks=99 (expected 100), elapsed=5.048102, drift=-0.098101
-- Nothing is illegal if a hundred businessmen decide to do it, and that's true anywhere in the world. -- Andrew Young /* * test itimer */
#include <stdlib.h> #include <stdio.h>
#include <unistd.h> #include <signal.h> #include <sys/time.h> #include <time.h>
#define TEST_INTERVAL_SEC 5
static inline void tv_sub(struct timeval* ptv1, struct timeval* ptv2) { ptv1->tv_sec -= ptv2->tv_sec; ptv1->tv_usec -= ptv2->tv_usec; while (ptv1->tv_usec < 0) { ptv1->tv_sec--; ptv1->tv_usec += 1000*1000; } }
static uint g_ticks; static uint g_interval; static struct timeval g_now;
void itimer_handler(int sig) { g_ticks++; g_now.tv_usec += g_interval*1000; while (g_now.tv_usec > 1000*1000) { g_now.tv_usec -= 1000*1000; g_now.tv_sec++; } }
void start_itimer(uint ms) { struct timeval tv; struct itimerval val;
tv.tv_sec = ms/1000; tv.tv_usec = (ms%1000)*1000;
val.it_interval = tv; val.it_value = tv;
signal(SIGALRM, itimer_handler); g_ticks = 0; g_interval = ms; gettimeofday(&g_now, NULL); setitimer(ITIMER_REAL, &val, NULL); }
void stop_itimer(void) { struct timeval tv; struct itimerval val;
tv.tv_sec = tv.tv_usec = 0; val.it_interval = tv; val.it_value = tv;
setitimer(ITIMER_REAL, &val, NULL); signal(SIGALRM, SIG_DFL); }
int main(void) { int i; struct timeval tv_start; struct timeval tv_now;
char drift_sign; struct timeval tv_drift; struct timeval tv_elapsed;
for (i = 10; i <= 50; i += 10) { gettimeofday(&tv_start, NULL); start_itimer(i); do { usleep(1000*1000); gettimeofday(&tv_now, NULL); tv_elapsed = tv_now; tv_sub(&tv_elapsed, &tv_start); } while (tv_elapsed.tv_sec < TEST_INTERVAL_SEC); stop_itimer();
if (g_now.tv_sec > tv_now.tv_sec || (g_now.tv_sec == tv_now.tv_sec && g_now.tv_usec > tv_now.tv_usec)) { drift_sign = '+'; tv_drift = g_now; tv_sub(&tv_drift, &tv_now); } else { drift_sign='-'; tv_drift = tv_now; tv_sub(&tv_drift, &g_now); } printf("i=%d, ticks=%u (expected %u), elapsed=%ld.%06ld, drift=%c%ld.%06ld\n", i, g_ticks, (TEST_INTERVAL_SEC*1000/i), tv_elapsed.tv_sec, tv_elapsed.tv_usec, drift_sign, tv_drift.tv_sec, tv_drift.tv_usec); } return 0; } [unhandled content-type:application/pgp-signature] | ![\](/images/icornerr.gif) |