Re: usleep

Re: usleep

[Alan Bort]

for the #include statemen I assume you are using some sort of C
languagbe (even C, C++ or C#) Well... my man of usleep says that it
makes the system wait for any number of microseconds... make the maths.

Anyway... I really don't remebmer the value of a microsecond... I THINK
it was something like "1 x 10 ^ -6" or something alike... (excuse the
quotation aberration).


El jue, 12-06-2003 a las 01:55, Lee Chin escribió:
> I would think the following code would wait for 1 second each itteration before printing hello, but it waits way too long.  Replacing the for loop body with a 
> usleep(1000000) works great... what am I missing here?
> 
> Thanks
> Lee
> 
> #include <unistd.h>
> 
> int main()
> {
>     while (1)
>     {
>         int i;
> 
>         printf("hello\n");
>         for(i = 0; i < 50; i++)
>         {
>             usleep(1000);
>         }
> 
>     }
> 
> }
-- 
Alan Bort
Linux Registered User 298277 -Country Manager- [http://counter.li.org]
[ http://www.linuxquestions.org ] Username: Ciccio
[ http://es.tldp.org ]
Ciccio.-

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Re: usleep

[Alan Bort]

QNX. do some research on that OS. Long time ago I heard that it was a
real time OS and that programming and running programs on it was VERY
much more accurate than the standard INTEL. I don't really know what you
want... if you wan precission then this might be your solution... if you
dont... then try the bes aproximation possible and use it...

El jue, 12-06-2003 a las 03:38, Ray Olszewski escribió:
> At 08:17 PM 6/11/2003 -0400, Mark Hahn wrote:
> > > An i386 has a hardware timer that beats about 72 times per second, leaving
> >
> >well, it has several hardware counter/timers, but that setting
> >is not special (and isn't it only used by DOS?)
> 
> Actually, I wrote that from memory (of timer limitations I encountered when 
> programming in VB on Windows 2000), and I think I got the actual numbers wrong.
> 
> > > me uncertain as to how accurate timings below 14 msec can be on this
> > > hardware using the kernel's internal timer.
> >
> >timers can be set MUCH faster than that.
> 
> In principle, yes. usleep() timers can be *set* to 1 usec. That's not the 
> question. The question is how *accurate* timers based on usleep() can be at 
> low settings.
> 
> This experiment says they are not very accurate. I changed Lee's loop to be:
> 
>    for(i = 0; i < 500; i++)
>                  {
>             usleep(10);
>          }
> 
> Now 500 x 10 = 5000 usec, or 1/200 seconds. But the loop actually takes 10 
> seconds to iterate. This implies that usleep()  has a minimum delay of 
> 10/500 seconds, or 20 msec, built into it (at least on my test system).
> 
> My best guess is that this limitation derives either from the timer code in 
> the Linux kernel or from the system hardware that code relies on.
> 
> 
> 
> -
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-- 
Alan Bort
Linux Registered User 298277 -Country Manager- [http://counter.li.org]
[ http://www.linuxquestions.org ] Username: Ciccio
[ http://es.tldp.org ]
Ciccio.-

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usleep

[Lee Chin]

I would think the following code would wait for 1 second each itteration before printing hello, but it waits way too long.  Replacing the for loop body with a 
usleep(1000000) works great... what am I missing here?

Thanks
Lee

#include <unistd.h>

int main()
{
    while (1)
    {
        int i;

        printf("hello\n");
        for(i = 0; i < 50; i++)
        {
            usleep(1000);
        }

    }

}

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Re: usleep

[CaT]

On Wed, Jun 11, 2003 at 05:55:02PM -0500, Lee Chin wrote:
> I would think the following code would wait for 1 second each itteration
> before printing hello, but it waits way too long.  Replacing the for
> loop body with a usleep(1000000) works great... what am I missing here?

usleep = micro sleep = sleep in 1/1000000 second incrememnts.

-- 
Martin's distress was in contrast to the bitter satisfaction of some
of his fellow marines as they surveyed the scene. "The Iraqis are sick
people and we are the chemotherapy," said Corporal Ryan Dupre. "I am
starting to hate this country. Wait till I get hold of a friggin' Iraqi.
No, I won't get hold of one. I'll just kill him."
	- http://www.informationclearinghouse.info/article2479.htm
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Re: usleep

[Mark Hahn]

> I would think the following code would wait for 1 second each itteration
> before printing hello,

well, usleep takes microseconds, so as you've written it,
you should expect it to take 1ms per usleep or 50ms between hello's.
further, the man page doesn't claim that it'll actually sleep
for exactly the specified microseconds.

here:
#include <unistd.h>
#include <stdio.h>
int main() {
    int c = 20;
    while (c--) {
        int i;
        printf("hello\n");
        for(i = 0; i < 50; i++) {
            usleep(1000);
        }
    }
    return 0;
}

(not that a correct program needs both the stdio and a return from main.)

| [hahn@hahn hahn]$ /usr/bin/time ./chin
| hello
...
| hello
| 0.00user 0.00system 0:19.99elapsed 0%CPU (0avgtext+0avgdata 0maxresident)k
| 0inputs+0outputs (71major+10minor)pagefaults 0swaps
| [hahn@hahn hahn]$ setrealtime time ./chin
| hello
...
| hello
| 0.13user 0.87system 0:01.00elapsed 100%CPU (0avgtext+0avgdata 0maxresident)k
| 0inputs+0outputs (71major+10minor)pagefaults 0swaps

hmm, so on its own, 20 iterations of your loop take 20 seconds,
one second per iteration, and the usleep is actually lasting 20ms.
this is the traditional behavior (and correct, since the man page 
permits this kind of fuzziness).

if run in realtime mode, 20*50 usleep(1000)'s take 1.00 seconds, cool.

also, if I add:
#include <sys/time.h>

void myusleep(unsigned us) {
    struct timeval tv;
    tv.tv_sec = 0;
    tv.tv_usec = us;
    select(0,0,0,0,&tv);
}

then it behaves pretty much exactly as expected.  (good code would handle
the case where us>1000000 and where select is interrupted by a signal.)


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Re: usleep

[Ray Olszewski]

At 05:55 PM 6/11/2003 -0500, Lee Chin wrote:
>I would think the following code would wait for 1 second each itteration 
>before printing hello, but it waits way too long.  Replacing the for loop 
>body with a
>usleep(1000000) works great... what am I missing here?

Why would you expect this loop to wait 1 second? The usleep() call is for 
1/1000 of a second, and you call it 50 times. 50/1000 seconds is hardly 
equal to 1 second.

In fact, compiled on my system, the loop as written does introduce just 
about a 1 second delay (30 iterations take 29 seconds,  timed crudely with 
a wristwatch ... just right, since the initial printf() happens before the 
first loop run).

The ability of a system to resolve time is limited by the kernel and the 
hardware. If you read the man page for nanosleep() -- the POSIX replacement 
for usleep -- it specifies this limitation on that call's behavior. SO does 
the man page for usleep, though the writing here is fuzzier.

An i386 has a hardware timer that beats about 72 times per second, leaving 
me uncertain as to how accurate timings below 14 msec can be on this 
hardware using the kernel's internal timer.

If you need more accurate timing, you might want to look at using /dev/rtc 
(described in the kernel Documentation directory), which relies on IRQ8 and 
the system's real-time-clock hardware.

The test system I used, BTW, is a P-III 1 Ghz, and I ran the program with 
an ssh connection as STDOUT.


>Thanks
>Lee
>
>#include <unistd.h>
>
>int main()
>{
>     while (1)
>     {
>         int i;
>
>         printf("hello\n");
>         for(i = 0; i < 50; i++)
>         {
>             usleep(1000);
>         }
>
>     }
>
>}


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Re: usleep

[Mark Hahn]

> An i386 has a hardware timer that beats about 72 times per second, leaving 

well, it has several hardware counter/timers, but that setting 
is not special (and isn't it only used by DOS?)

> me uncertain as to how accurate timings below 14 msec can be on this 
> hardware using the kernel's internal timer.

timers can be set MUCH faster than that.

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Re: usleep

[Ray Olszewski]

At 08:17 PM 6/11/2003 -0400, Mark Hahn wrote:
> > An i386 has a hardware timer that beats about 72 times per second, leaving
>
>well, it has several hardware counter/timers, but that setting
>is not special (and isn't it only used by DOS?)

Actually, I wrote that from memory (of timer limitations I encountered when 
programming in VB on Windows 2000), and I think I got the actual numbers wrong.

> > me uncertain as to how accurate timings below 14 msec can be on this
> > hardware using the kernel's internal timer.
>
>timers can be set MUCH faster than that.

In principle, yes. usleep() timers can be *set* to 1 usec. That's not the 
question. The question is how *accurate* timers based on usleep() can be at 
low settings.

This experiment says they are not very accurate. I changed Lee's loop to be:

   for(i = 0; i < 500; i++)
                 {
            usleep(10);
         }

Now 500 x 10 = 5000 usec, or 1/200 seconds. But the loop actually takes 10 
seconds to iterate. This implies that usleep()  has a minimum delay of 
10/500 seconds, or 20 msec, built into it (at least on my test system).

My best guess is that this limitation derives either from the timer code in 
the Linux kernel or from the system hardware that code relies on.



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Re: usleep

[Lee Chin]

I'm not sure what you are saying, but yes there are 1 million microseconds in a second.... 1000000 micro seconds that is.  So my loop seems correct.  Also, I am running this on a 2 GhZ processor, so I doubt the 1000 calls to usleep can add that much overhead.  I think something is wrong with the library or the kernel.
----- Original Message -----
From: Alan Bort <333101@personal.net.py>
Date: 11 Jun 2003 19:08:34 +0300
To: Lee Chin <leechin@mail.com>
Subject: Re: usleep

> for the #include statemen I assume you are using some sort of C
> languagbe (even C, C++ or C#) Well... my man of usleep says that it
> makes the system wait for any number of microseconds... make the maths.
> 
> Anyway... I really don't remebmer the value of a microsecond... I THINK
> it was something like "1 x 10 ^ -6" or something alike... (excuse the
> quotation aberration).
> 
> 
> El jue, 12-06-2003 a las 01:55, Lee Chin escribió:
> > I would think the following code would wait for 1 second each itteration before printing hello, but it waits way too long.  Replacing the for loop body with a 
> > usleep(1000000) works great... what am I missing here?
> > 
> > Thanks
> > Lee
> > 
> > #include <unistd.h>
> > 
> > int main()
> > {
> >     while (1)
> >     {
> >         int i;
> > 
> >         printf("hello\n");
> >         for(i = 0; i < 50; i++)
> >         {
> >             usleep(1000);
> >         }
> > 
> >     }
> > 
> > }
> -- 
> Alan Bort
> Linux Registered User 298277 -Country Manager- [http://counter.li.org]
> [ http://www.linuxquestions.org ] Username: Ciccio
> [ http://es.tldp.org ]
> Ciccio.-
> 

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Re: usleep

[Lee Chin]

Thanks... that helpes (I initially posed my code wrong... my loop I intended to post was a for(i = 0; i < 1000; i++)

Any way... what is setrealtime?  I dont have it on my linux machine

Thanks
Lee
----- Original Message -----
From: Mark Hahn <hahn@physics.mcmaster.ca>
Date: Wed, 11 Jun 2003 19:45:17 -0400 (EDT)
To: Lee Chin <leechin@mail.com>
Subject: Re: usleep

> > I would think the following code would wait for 1 second each itteration
> > before printing hello,
> 
> well, usleep takes microseconds, so as you've written it,
> you should expect it to take 1ms per usleep or 50ms between hello's.
> further, the man page doesn't claim that it'll actually sleep
> for exactly the specified microseconds.
> 
> here:
> #include <unistd.h>
> #include <stdio.h>
> int main() {
>     int c = 20;
>     while (c--) {
>         int i;
>         printf("hello\n");
>         for(i = 0; i < 50; i++) {
>             usleep(1000);
>         }
>     }
>     return 0;
> }
> 
> (not that a correct program needs both the stdio and a return from main.)
> 
> | [hahn@hahn hahn]$ /usr/bin/time ./chin
> | hello
> ...
> | hello
> | 0.00user 0.00system 0:19.99elapsed 0%CPU (0avgtext+0avgdata 0maxresident)k
> | 0inputs+0outputs (71major+10minor)pagefaults 0swaps
> | [hahn@hahn hahn]$ setrealtime time ./chin
> | hello
> ...
> | hello
> | 0.13user 0.87system 0:01.00elapsed 100%CPU (0avgtext+0avgdata 0maxresident)k
> | 0inputs+0outputs (71major+10minor)pagefaults 0swaps
> 
> hmm, so on its own, 20 iterations of your loop take 20 seconds,
> one second per iteration, and the usleep is actually lasting 20ms.
> this is the traditional behavior (and correct, since the man page 
> permits this kind of fuzziness).
> 
> if run in realtime mode, 20*50 usleep(1000)'s take 1.00 seconds, cool.
> 
> also, if I add:
> #include <sys/time.h>
> 
> void myusleep(unsigned us) {
>     struct timeval tv;
>     tv.tv_sec = 0;
>     tv.tv_usec = us;
>     select(0,0,0,0,&tv);
> }
> 
> then it behaves pretty much exactly as expected.  (good code would handle
> the case where us>1000000 and where select is interrupted by a signal.)
> 
> 

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Re: usleep

[Mark Hahn]

> Any way... what is setrealtime?  I dont have it on my linux machine

it's a simple tool I wrote a long time ago, code below.
***** N O T E *******
it can effectively freeze your machine, since it gives you a process
which will not be prempted!  it *is* useful, though, if you know
what you're doing...
***** N O T E *******

#include <unistd.h>
#include <stdlib.h>
#include <stdio.h>
#include <sched.h>
/* setrealtime: run procs in realtime.
   author: Mark Hahn <hahn@mcmaster.ca> */

int main(int argc, char *argv[]) {
    static struct sched_param sched_parms;
    int pid, wrapper=0;

    if (argc <= 1)
        return 1;

    pid = atoi(argv[1]);

    if (!pid || argc != 2) {
        wrapper = 1;
        pid = getpid();
    }
    if (!pid)
        return 1;
 
    sched_parms.sched_priority = sched_get_priority_min(SCHED_FIFO);
    if (sched_setscheduler(pid, SCHED_FIFO, &sched_parms) == -1) {
        perror("cannot set realtime scheduling policy");
        return 1;
    }
    if (wrapper) {
        setuid(getuid());
        execvp(argv[1],&argv[1]);
        perror("exec failed");
        return 1;
    }
    return 0;
}

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Re: usleep

[Lee Chin]

Sorry everyone, I had initially posed my for loop wrong... I meant to say for(i = 0; i < 1000; i++).

Mark Hahn clarified the issue and usleep is not actually accurate (provably) and the man page actually has a disclaimer.  To me, it seems like usleep(2000) * 50 itterations actually gives a reasonably accurate 1 second delay on machines from 800 mgz to 2 ghz.

Mark Hahn actually gave me some more accurate code to do usleep too.  Thanks Mark!
Thanks
Lee
----- Original Message -----
From: CaT <cat@zip.com.au>
Date: Thu, 12 Jun 2003 09:21:46 +1000
To: Lee Chin <leechin@mail.com>
Subject: Re: usleep

> On Wed, Jun 11, 2003 at 05:55:02PM -0500, Lee Chin wrote:
> > I would think the following code would wait for 1 second each itteration
> > before printing hello, but it waits way too long.  Replacing the for
> > loop body with a usleep(1000000) works great... what am I missing here?
> 
> usleep = micro sleep = sleep in 1/1000000 second incrememnts.
> 
> -- 
> Martin's distress was in contrast to the bitter satisfaction of some
> of his fellow marines as they surveyed the scene. "The Iraqis are sick
> people and we are the chemotherapy," said Corporal Ryan Dupre. "I am
> starting to hate this country. Wait till I get hold of a friggin' Iraqi.
> No, I won't get hold of one. I'll just kill him."
> 	- http://www.informationclearinghouse.info/article2479.htm

-- 
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RE: usleep

[Riley Williams]

Hi Ray.

 > Now 500 x 10 = 5000 usec, or 1/200 seconds. But the loop
 > actually takes 10 seconds to iterate. This implies that
 > usleep() has a minimum delay of 10/500 seconds, or 20
 > msec, built into it (at least on my test system).

The standard PC BIOS sets up the hardware timer with a tick
interval of 18.2 ms which is probably what you are seeing.
Memory says that the reason for this is that there are 2^16
(65,536) such ticks per hour, and that apparently made the
original IBM PC BIOS somewhat simpler as a result.

Best wishes from Riley.
---
 * Nothing as pretty as a smile, nothing as ugly as a frown.

---
Outgoing mail is certified Virus Free.
Checked by AVG anti-virus system (http://www.grisoft.com).
Version: 6.0.488 / Virus Database: 287 - Release Date: 5-Jun-2003

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RE: usleep

[Ray Olszewski]

At 05:14 AM 6/12/2003 +0100, Riley Williams wrote:
>Hi Ray.
>
>  > Now 500 x 10 = 5000 usec, or 1/200 seconds. But the loop
>  > actually takes 10 seconds to iterate. This implies that
>  > usleep() has a minimum delay of 10/500 seconds, or 20
>  > msec, built into it (at least on my test system).
>
>The standard PC BIOS sets up the hardware timer with a tick
>interval of 18.2 ms which is probably what you are seeing.
>Memory says that the reason for this is that there are 2^16
>(65,536) such ticks per hour, and that apparently made the
>original IBM PC BIOS somewhat simpler as a result.
>
>Best wishes from Riley.

Thanks, Riley. This is indeed what I was remembering, albeit imperfectly.

Having done a bit more reading, I find that the standard Linux kernel 
causes modern computers (Pentium or newer) to generate "heartbeat" 
interrupts on IRQ0 at the rate of 100 per second, that is, every 10 msec. 
(This is separate from the RTC, which generates interrupts on IRQ8 at a 
rate that can be controlled more readily than by editing kernel source.) 
You can verify this by hand, by comparing uptime (in seconds) to the IRQ0 
entry in /proc/interrupts. (I actually get 99.998 per second this way, 
perhaps because ntp reseting of real time affects uptime reports.)

For that reason, on a standard kernel (and a customized one is apparently a 
big deal, because a lot of stuff assumes the kernel gets ticks at this 
rate) nothing can be timed at a finer timing than 10 msec using kernel time 
facilities ... for finer timing, you have to use the more flexible facility 
of the IRQ8 RTC, which can be programmed to provide up to 8192 beats per 
second.

386s and 486s might still,  I think, have the coarser timing you describe 
from DOS days. I no longer have any running here to check, though.



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Re: usleep

[Steven Smith]

[-- Attachment #1: Type: text/plain, Size: 1518 bytes --]

> I would think the following code would wait for 1 second each
> itteration before printing hello, but it waits way too long.
> Replacing the for loop body with a usleep(1000000) works
> great... what am I missing here?
Essentially, that usleep() isn't actually terribly accurate for short
waits.  First, you always give up the rest of your timeslice, which
delays you a few milliseconds (more if there's contention for the
processor).  Second, kernel timers will always fire a short period
after the actual specified time, which gives you a further small,
roughly constant delay.  Third, you aren't actually woken up
immediately after the timer goes off; you simply become eligible for
scheduling again.

This means that when you call usleep(x), you actually end up sleeping
for x + n us, where n is a few thousand.  If you call usleep(1000000),
this will probably be less than one percent of the total time slept,
so nobody really cares.  If you call usleep(1000), it's likely to be
several times the actual sleep time required, which you almost
certainly do care about.

If you really can't call usleep() or nanosleep() with a big number,
the only option is probably to spin: the kernel can't cope with really
fine-grained scheduling.  If you're root, you can use a real time
process, in which case the kernel will do the spinning for you for
delays of less than 2ms.  Depending on the exact delay being used and
how you calibrate the busy wait, this may be slightly more accurate.

Steven Smith,
sos22@cam.ac.uk.

[-- Attachment #2: Type: application/pgp-signature, Size: 189 bytes --]

Re: usleep

[Lee Chin]

Mark,
Two questions

1) Do threads inherrit the schedule priority?  Or do I need to set this for every thread

2) If there is nothing else runnable in my system, will this help? or is it effectively the same?

Thanks
Lee
----- Original Message -----
From: Mark Hahn <hahn@physics.mcmaster.ca>
Date: Wed, 11 Jun 2003 21:02:17 -0400 (EDT)
To: Lee Chin <leechin@mail.com>
Subject: Re: usleep

> > Any way... what is setrealtime?  I dont have it on my linux machine
> 
> it's a simple tool I wrote a long time ago, code below.
> ***** N O T E *******
> it can effectively freeze your machine, since it gives you a process
> which will not be prempted!  it *is* useful, though, if you know
> what you're doing...
> ***** N O T E *******
> 
> #include <unistd.h>
> #include <stdlib.h>
> #include <stdio.h>
> #include <sched.h>
> /* setrealtime: run procs in realtime.
>    author: Mark Hahn <hahn@mcmaster.ca> */
> 
> int main(int argc, char *argv[]) {
>     static struct sched_param sched_parms;
>     int pid, wrapper=0;
> 
>     if (argc <= 1)
>         return 1;
> 
>     pid = atoi(argv[1]);
> 
>     if (!pid || argc != 2) {
>         wrapper = 1;
>         pid = getpid();
>     }
>     if (!pid)
>         return 1;
>  
>     sched_parms.sched_priority = sched_get_priority_min(SCHED_FIFO);
>     if (sched_setscheduler(pid, SCHED_FIFO, &sched_parms) == -1) {
>         perror("cannot set realtime scheduling policy");
>         return 1;
>     }
>     if (wrapper) {
>         setuid(getuid());
>         execvp(argv[1],&argv[1]);
>         perror("exec failed");
>         return 1;
>     }
>     return 0;
> }
> 

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Re: usleep

[Stephen Samuel]

On my box, (a P3/450) usleep seems to have an overhead of about 10000usec
and a minimum response time of about 20000

/* my test prog:
  * (based on yours)
  */

#include <unistd.h>
#include <sys/time.h>

struct timeval tv1,tv2;
#define PAUSE 100000

int main()
{
     while (1)
     {
         int i;
         double sum=0.0,t;
         gettimeofday(&tv2, NULL);
         printf("hello\n");
         for(i = 0; i < 20 ; i++)
         {
                 gettimeofday(&tv1, NULL);
             usleep(PAUSE);
                gettimeofday(&tv2, NULL);
                printf("slept %9.6fs\n", t=tv2.tv_sec+tv2.tv_usec*.000001 -tv1.tv_sec-tv1.tv_usec*.000001 );
                sum+=t;
         }
         printf ("average %9.6fs\n",sum/i);
         sleep (5);
     }
}



Lee Chin wrote:
 > I would think the following code would wait for 1 second each itteration before printing hello, but it waits way too long.  Replacing the for loop body with a
 > usleep(1000000) works great... what am I missing here?
 >
 > Thanks
 > Lee



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		   http://www.bcgreen.com/~samuel/
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doubt to touch the jewel within each person and bring it to life.

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Re: usleep

[Mark Hahn]

> On my box, (a P3/450) usleep seems to have an overhead of about 10000usec
> and a minimum response time of about 20000

which is perfectly correct/expected.  
although "overhead" isn't quite right: instead, say
it has a resolution of 1/HZ and a minimum of 2/HZ.

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Re: usleep

[Stephen Samuel]

Ray Olszewski wrote:
 > At 05:55 PM 6/11/2003 -0500, Lee Chin wrote:

 > In fact, compiled on my system, the loop as written does introduce just
 > about a 1 second delay (30 iterations take 29 seconds,  timed crudely
 > with a wristwatch ... just right, since the initial printf() happens
 > before the first loop run).

Who needs a wristwatch when you're running on unix.

time the_command

will give you much more accurate timings than you could ever
get with a wristwatch.


-- 
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		   http://www.bcgreen.com/~samuel/
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doubt to touch the jewel within each person and bring it to life.

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