Re: AW: AW: Re[4]: [HACKERS] Allowing WAL fsync to be done via O_SYNC

[Tom Lane]

Zeugswetter Andreas SB  <[EMAIL PROTECTED]> writes:
>> HPUX has usleep, but the man page says
>> 
>> The usleep() function is included for its historical usage. The
>> setitimer() function is preferred over this function.

> I doubt that setitimer has microsecond precision on HPUX.

Well, if you insist on beating this into the ground:

$ cat timetest.c
#include 
#include 
#include 

int main(int argc, char** argv)
{
int i;
int delay;

delay = atoi(argv[1]);

for (i = 0; i < 1000; i++)
usleep(delay);

return 0;
}
$ gcc -O -Wall timetest.c
$ time ./a.out 1

real0m20.02s
user0m0.04s
sys 0m0.09s
$ time ./a.out 1000

real0m20.04s
user0m0.04s
sys 0m0.09s
$ time ./a.out 1

real0m20.01s
user0m0.03s
sys 0m0.08s
$ time ./a.out 2

real0m30.03s
user0m0.04s
sys 0m0.09s
$
$ cat timetest2.c
#include 
#include 
#include 
#include 
#include 

typedef void (*pqsigfunc) (int);

pqsigfunc
pqsignal(int signo, pqsigfunc func)
{
struct sigaction act,
oact;

act.sa_handler = func;
sigemptyset(&act.sa_mask);
act.sa_flags = 0;
if (signo != SIGALRM)
act.sa_flags |= SA_RESTART;
if (sigaction(signo, &act, &oact) < 0)
return SIG_ERR;
return oact.sa_handler;
}

void
catch_alarm(int sig)
{
}

int main(int argc, char** argv)
{
int i;
struct itimerval iv;
int delay;

delay = atoi(argv[1]);

pqsignal(SIGALRM, catch_alarm);

for (i = 0; i < 1000; i++)
{
iv.it_value.tv_sec = 0;
iv.it_value.tv_usec = delay;
iv.it_interval.tv_sec = 0;
iv.it_interval.tv_usec = 0;
setitimer(ITIMER_REAL, &iv, NULL);
pause();
}

return 0;
}
$ gcc -O -Wall timetest2.c
$ time ./a.out 1

real0m20.04s
user0m0.01s
sys 0m0.05s
$ time ./a.out 1000

real0m20.02s
user0m0.01s
sys 0m0.06s
$ time ./a.out 1

real0m20.01s
user0m0.01s
sys 0m0.05s
$ time ./a.out 2

real0m30.01s
user0m0.01s
sys 0m0.06s
$

The usleep man page implies that usleep is actually implemented as a
setitimer call, which would explain the interchangeable results.  In
any case, neither one is useful for timing sub-clock-tick intervals;
in fact they're worse than select().

Anyone else want to try these examples on other platforms?

regards, tom lane

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AW: AW: Re[4]: [HACKERS] Allowing WAL fsync to be done via O_SYNC

[Zeugswetter Andreas SB]

> >> It's great as long as you never block, but it sucks for making things
> >> wait, because the wait interval will be some multiple of 10 msec rather
> >> than just the time till the lock comes free.
> 
> > On the AIX platform usleep (3) is able to really sleep microseconds without 
> > busying the cpu when called for more than approx. 100 us (the longer the interval,
> > the less busy the cpu gets) .
> > Would this not be ideal for spin_lock, or is usleep not very common ?
> > Linux sais it is in the BSD 4.3 standard.
> 
> HPUX has usleep, but the man page says
> 
>  The usleep() function is included for its historical usage. The
>  setitimer() function is preferred over this function.

I doubt that setitimer has microsecond precision on HPUX.

> In any case, I would expect that all these functions offer accuracy
> no better than the scheduler's regular clock cycle (~ 100Hz) on most
> kernels.

Not on AIX, and I don't beleive that for the majority of other UNIX platforms eighter. 
I do however suspect, that some implementations need a busy loop, which would, 
if at all, only be acceptable on an SMP system.

Andreas

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