Package: manpages-dev
Version: 2.49-1
Severity: normal
File: /usr/share/man/man2/times.2.gz
I have a problem with the following sentence in times(2) manual page,
RETURN VALUE section:
Since Linux 2.6, this point is (2^32/HZ) - 300 (i.e., about 429 million)
seconds before system boot time.
The problem is that in order to be able to use the return value (map it to my
wall-clock) I have to know the value of HZ. It appears that the value of HZ
depends on the kernel configuration option (recent Linux versions can have
100Hz, 250Hz, 1000Hz etc), so it's not possible to compile once and use
the same binary on other machines. On Google I also found some discussion
that HZ can be 1024 on some architectures
So this HZ variable is a very bad idea.
Google says that sysconf(_SC_CLK_TCK) should be used instead of HZ.
This seems to be reasonable and works for time (jiffies) numbers in /proc,
but it does NOT work for the return value, consider the following:
1) I compiled my kernel (2.6.20.11 with completely fair scheduler 14 on
for Pentium 4) with the following options:
CONFIG_HZ_1000=y
CONFIG_HZ=1000
2) sysconf(_SC_CLK_TCK) returns 100, and it works correctly with timing
numbers in the structure and /proc/#/stat (i.e. starttime field).
3) expression 2^32/sysconf(_SC_CLK_TCK)-300 evaluates to 42949372
this is about 42 million (not about 429 million as in manual)
4) I found the following formula to be useful to convert the returned
ticks to milli seconds since boot:
struct tms mytimes;
uint64_t ms=(times(&mytimes)*1000/sysconf(_SC_CLK_TCK) - (1LLU<<32) + 951416);
The idea is that times(&mytimes) returns the number of ticks since arbitrary
point in time, I multiply it with 1000/sysconf(_SC_CLK_TCK) which is the
number of milliseconds per one tick. Then the offset of the arbitrary point
in time is adjusted.
Interestingly, the constant 951416 is not firm (it seems to be shrinking by
100ms per hour or so), but (1LLU<<32) is not to be divided and the constant
difference is far from 300 seconds as manual says.
I just wonder what is really happening here.
I used the following program to find out the formula and the difference
between the values of times(2) and /proc/#/stat startime:
#include <sys/times.h>
#include <sys/types.h>
#include <unistd.h>
#include <iostream>
#include <fstream>
using namespace std;
long long myStartTime(){
char statname[64] = "";
char info[64];
snprintf(statname, 64, "/proc/%d/stat", getpid());
ifstream stat(statname);
stat.width(64); stat>>info; // PID
if (atol(info)!=getpid()) {
cout << "bad proc stat file" << endl;
exit(EXIT_FAILURE);
}
stat.width(64); stat>>info; // filename of executable
stat.width(64); stat>>info; // state
stat.width(64); stat>>info; // parent PID
stat.width(64); stat>>info; // process group
stat.width(64); stat>>info; // session
stat.width(64); stat>>info; // tty nr
stat.width(64); stat>>info; // process group ID of tty owner
stat.width(64); stat>>info; // flags
stat.width(64); stat>>info; // No. minor faults
stat.width(64); stat>>info; // No. minor faults in children
stat.width(64); stat>>info; // No. major faults
stat.width(64); stat>>info; // No. major faults in children
stat.width(64); stat>>info; // No. jiffies in user mode
stat.width(64); stat>>info; // No. jiffies in kernel mode
stat.width(64); stat>>info; // children jiffies in user mode
stat.width(64); stat>>info; // children jiffies in kernel mode
stat.width(64); stat>>info; // priority
stat.width(64); stat>>info; // nice value
stat.width(64); stat>>info; // 0 (zero)
stat.width(64); stat>>info; // jiffies before next SIGALARM
stat.width(64); stat>>info; // jiffies since start after boot
return (atoll(info)*1000/sysconf(_SC_CLK_TCK));
}
long ticks_per_sec = sysconf(_SC_CLK_TCK);
inline long long ticks2ms(long long &ticks) {
return (ticks*1000/ticks_per_sec - (1LLU<<32) + 951416);
}
struct tms mytimes;
long long start, before, after;
int main(){
before = times(&mytimes);
sleep(1);
after = times(&mytimes);
before = ticks2ms(before);
after = ticks2ms(after);
start = myStartTime();
long long diff;
if (before>start) {
diff = (before-start);
cout << "Too late by " << diff << "ms" << endl;
} else if (start>before) {
diff = (start - before);
cout << "TOO EARLY BY " << diff << "ms" << endl;
cout << "startt="<<start<<"ms\nbefore=" << before
<< "ms after="<<after << "ms duration="<<(after-before)<<"ms" << endl;
} else cout << "GOOD" << endl;
}
-- System Information:
Debian Release: lenny/sid
APT prefers testing
APT policy: (990, 'testing'), (500, 'stable'), (50, 'unstable'), (1,
'experimental')
Architecture: i386 (i686)
Kernel: Linux 2.6.20.11-cfs-v13 (PREEMPT)
Locale: LANG=lt_LT.UTF-8, LC_CTYPE=lt_LT.UTF-8 (charmap=UTF-8)
Shell: /bin/sh linked to /bin/bash
Versions of packages manpages-dev depends on:
ii manpages 2.43-0 Manual pages about using a GNU/Lin
manpages-dev recommends no packages.
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