[PATCH -tip 0/1] posix-cpu-timers: fix CPUCLOCK_{PROF,VIRT} periodic tics precision
From: Stanislaw Gruszka
Date: Tue Aug 11 2009 - 10:32:38 EST
Hello.
POSIX 1.b cpu timers where clockid is based on CPUCLOCK_PROF and CPUCLOCK_VIRT,
have systematic timer error, the same as had itimers. Error can be showed by
program [1]. Results for unpatched and patched kernel are below:
Unpatched kernel:
CLK_PROF
Period 10000: counted time 14.51 , real time 15.99 , error -9.2 %
Period 9998: counted time 15.95 , real time 15.96 , error -0.1 %
Period 1000: counted time 7.99 , real time 15.98 , error -50.0 %
Period 11111: counted time 14.77 , real time 15.96 , error -7.5 %
Period 21315: counted time 15.45 , real time 15.96 , error -3.1 %
Period 1500: counted time 11.98 , real time 15.98 , error -25.0 %
Period 15000: counted time 14.96 , real time 15.96 , error -6.3 %
Period 50000: counted time 15.60 , real time 15.95 , error -2.2 %
Period 777777: counted time 15.56 , real time 15.95 , error -2.5 %
Period 212341: counted time 15.71 , real time 15.95 , error -1.5 %
Period 10001: counted time 14.50 , real time 15.96 , error -9.1 %
Period 1002: counted time 8.01 , real time 15.98 , error -49.9 %
Period 999000: counted time 14.98 , real time 15.95 , error -6.1 %
Period 2222: counted time 11.83 , real time 15.97 , error -26.0 %
Period 500: counted time 8.01 , real time 16.02 , error -50.0 %
CLK_VIRT
Period 10000: counted time 14.50 , real time 15.96 , error -9.2 %
Period 9998: counted time 15.95 , real time 15.96 , error -0.1 %
Period 1000: counted time 7.99 , real time 15.98 , error -50.0 %
Period 11111: counted time 14.77 , real time 15.97 , error -7.5 %
Period 21315: counted time 15.45 , real time 15.96 , error -3.2 %
Period 1500: counted time 11.99 , real time 15.99 , error -25.0 %
Period 15000: counted time 14.96 , real time 15.96 , error -6.3 %
Period 50000: counted time 15.60 , real time 15.96 , error -2.2 %
Period 777777: counted time 15.56 , real time 15.95 , error -2.5 %
Period 212341: counted time 15.71 , real time 15.96 , error -1.5 %
Period 10001: counted time 14.50 , real time 15.96 , error -9.1 %
Period 1002: counted time 8.01 , real time 15.99 , error -49.9 %
Period 999000: counted time 14.98 , real time 15.95 , error -6.1 %
Period 2222: counted time 11.83 , real time 15.98 , error -26.0 %
Period 500: counted time 8.01 , real time 16.01 , error -50.0 %
Patched kernel:
CLK_PROF
Period 10000: counted time 16.05 , real time 16.06 , error -0.1 %
Period 9998: counted time 16.30 , real time 16.30 , error -0.0 %
Period 1000: counted time 16.45 , real time 16.45 , error -0.0 %
Period 11111: counted time 15.99 , real time 16.00 , error -0.1 %
Period 21315: counted time 15.99 , real time 15.99 , error -0.1 %
Period 1500: counted time 16.03 , real time 16.03 , error -0.0 %
Period 15000: counted time 15.99 , real time 16.00 , error -0.0 %
Period 50000: counted time 15.95 , real time 15.99 , error -0.3 %
Period 777777: counted time 15.56 , real time 15.99 , error -2.7 %
Period 212341: counted time 16.35 , real time 16.37 , error -0.1 %
Period 10001: counted time 15.99 , real time 16.00 , error -0.0 %
Period 1002: counted time 16.05 , real time 16.05 , error -0.0 %
Period 999000: counted time 15.98 , real time 15.99 , error -0.1 %
Period 2222: counted time 16.02 , real time 16.02 , error -0.0 %
Period 500: counted time 8.02 , real time 16.05 , error -50.0 %
CLK_VIRT
Period 10000: counted time 15.99 , real time 16.00 , error -0.1 %
Period 9998: counted time 16.02 , real time 16.03 , error -0.1 %
Period 1000: counted time 16.04 , real time 16.05 , error -0.1 %
Period 11111: counted time 15.99 , real time 16.00 , error -0.1 %
Period 21315: counted time 16.39 , real time 16.40 , error -0.1 %
Period 1500: counted time 16.02 , real time 16.03 , error -0.0 %
Period 15000: counted time 15.97 , real time 16.00 , error -0.1 %
Period 50000: counted time 15.95 , real time 16.00 , error -0.3 %
Period 777777: counted time 15.56 , real time 16.00 , error -2.8 %
Period 212341: counted time 16.14 , real time 16.34 , error -1.2 %
Period 10001: counted time 15.98 , real time 16.00 , error -0.1 %
Period 1002: counted time 16.04 , real time 16.05 , error -0.0 %
Period 999000: counted time 15.98 , real time 16.00 , error -0.1 %
Period 2222: counted time 16.01 , real time 16.02 , error -0.1 %
Period 500: counted time 8.02 , real time 16.05 , error -50.0 %
Even if probably nobody use these timers, I think we should fix them
because we have this API in kernel and the timers perform bad. Patch
is against Ingo's -tip (-sched-devel) tree where are my previous itmiers
patches.
Cheers
Stanislaw
[1] Program for measurement POSIX.1b CPUCLOCK_{PROF,VIRT} cpu timers precision
#include <sys/time.h>
#include <signal.h>
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#include <string.h>
#include <unistd.h>
#define ARRAY_SIZE(a) (sizeof(a)/sizeof(a[0]))
const int test_periods_us[] = {
10000,
9998,
1000,
11111,
21315,
1500,
15000,
50000,
777777,
212341,
10001,
1002,
999000,
2222,
500,
};
volatile int prof_counter;
void handler(int signr)
{
prof_counter++;
}
#define CPUCLOCK_PROF 0
#define CPUCLOCK_VIRT 1
#define CPUCLOCK_SCHED 2
#define MAKE_PROCESS_CPUCLOCK(pid, clock) \
((~(clockid_t) (pid) << 3) | (clockid_t) (clock))
#define CLK_PROF MAKE_PROCESS_CPUCLOCK(0, CPUCLOCK_PROF)
#define CLK_VIRT MAKE_PROCESS_CPUCLOCK(0, CPUCLOCK_VIRT)
#define CLK_SCHED MAKE_PROCESS_CPUCLOCK(0, CPUCLOCK_SCHED)
void test_func(void)
{
long long i = 0;
long long count = 0;
for(i=0; i<3000000000LL; i++)
count++;
}
double timeval_diff(const struct timeval *start, const struct timeval *end)
{
return (end->tv_sec - start->tv_sec) + (end->tv_usec - start->tv_usec)/1000000.0;
}
void measure_itimer_error(int period_us, clockid_t clk_id)
{
struct sigaction act;
struct timeval start, end;
struct sigevent evt;
double real_time, counted_time;
timer_t tid;
prof_counter = 0;
struct timespec period_ts = {
.tv_sec = 0,
.tv_nsec = period_us * 1000,
};
struct itimerspec timer_spec = {
.it_interval = period_ts,
.it_value = period_ts
};
act.sa_handler = handler;
sigemptyset(&act.sa_mask);
act.sa_flags = 0;
if (sigaction(SIGPROF, &act, NULL) < 0) {
printf("sigaction failed\n");
exit(1);
}
memset(&evt, 0, sizeof(evt));
evt.sigev_notify = SIGEV_SIGNAL;
evt.sigev_signo = SIGPROF;
if (timer_create(clk_id, &evt, &tid) < 0) {
perror("timer_create");
exit(1);
}
if (timer_settime(tid, 0, &timer_spec, NULL) < 0) {
perror("timer_settime");
exit(1);
}
/* run a busy loop and measure it */
gettimeofday(&start, NULL);
test_func();
gettimeofday(&end, NULL);
/* disable the timer */
memset(&timer_spec, 0, sizeof(timer_spec));
if (timer_settime(tid, 0, &timer_spec, NULL) < 0) {
perror("timer_settime");
exit(1);
}
if (timer_delete(tid) < 0) {
perror("timer_delete");
exit(1);
}
counted_time = prof_counter * period_us / 1000000.0;
real_time = timeval_diff(&start, &end);
printf("Period %d:\tcounted time %.2f , real time %.2f , error %.1f %%\n",
period_us, counted_time, real_time, (counted_time/real_time - 1.0)*100.0);
}
int main()
{
int i;
printf("CLK_PROF\n");
for (i=0; i<ARRAY_SIZE(test_periods_us); i++)
measure_itimer_error(test_periods_us[i], CLK_PROF);
printf("CLK_VIRT\n");
for (i=0; i<ARRAY_SIZE(test_periods_us); i++)
measure_itimer_error(test_periods_us[i], CLK_VIRT);
return 0;
}
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