Re: [Discuss-gnuradio] FIFO latency

> On Sat, May 28, 2011 at 22:06, Marcus D. Leech<mleech@ripnet.com> wrote:
>> I evaluated latency of a FIFO (actually an ordinary pipe, but the kernel
>> mechanisms are identical), and measured 30usecs average on my
>> 1.2GHz AMD Phenom system with plenty 'o memory.
>>
>> I sent timestamps across the FIFO (struct timeval), and the reader grabbed
>> the local time of day, and computed the difference. There's
>> a fair amount of uncertainty on the reader due to gettimeofday() call
>> overhead. But 30usec on a wimpy CPU is certainly comfortably
>> below 1msec.
> gettimeofday() is a fast function. But if you want real high-fidelity
> - read CPU clock counter. Just make sure your app runs on a one
> selected core.
>
> Could you post your app and raw results? I'm interested in
> min/mean/max values and distribution graphs. Because max values do
> play role when playing with real-time.
>
====== latency_writer.c ========
#include <stdio.h>
#include <time.h>
#include <sys/time.h>
main ()
{
struct timeval tv;
while (1)
{
gettimeofday (&tv, NULL);
fwrite (&tv, sizeof(tv), 1, stdout);
fflush (stdout);
usleep (250000);
}
}

============ latency_reader.c ==============
#include <stdio.h>
#include <time.h>
#include <sys/time.h>
main ()
{
struct timeval now;
struct timeval sender;
long long int t1, t2;

while (fread (&sender, sizeof(sender), 1, stdin) != 0)
{
gettimeofday (&now, NULL);
t1 = sender.tv_sec * 1000000;
t1 += sender.tv_usec;

t2 = now.tv_sec * 1000000;
t2 += now.tv_usec;
fprintf (stderr, "%lld\n", t2 - t1);
}
}

I just run it like:

./latency_writer | ./latency_reader

--
Marcus Leech
Principal Investigator
Shirleys Bay Radio Astronomy Consortium
http://www.sbrac.org

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