FFTW 3.3.3 on ARM

Performance increases roughly an order of magnitude from ARM soft float to ARM hard float, and another from ARM hard float to Intel.

Toradex's T20: NVIDIA Tegra 2 without NEON:

ib256 = In-place (input array overwritten with output) backwards 256
ob256 = Out-of-place (input array is preserved)  backwards 256
// gcc version 4.5.2 (Sourcery G++ Lite 2011.03-41)
// Soft float, -fPIC
./bench -s ib256
Problem: ib256, setup: 7.16 s, time: 143.97 us, ``mflops'': 71.127

//gcc version 4.7.3 20121001 (prerelease) (crosstool-NG linaro-1.13.1-4.7-2012.10-20121022 - Linaro GCC 2012.10) 

// Hard float, -fPIC

Problem: ib256, setup: 7.20 s, time: 13.83 us, ``mflops'': 740.52

Problem: ob256, setup: 3.38 s, time: 12.45 us, ``mflops'': 822.8

Toradex's T30: NVIDIA Tegra 3 with NEON:

//gcc version 4.7.3 20121001 (prerelease) (crosstool-NG linaro-1.13.1-4.7-2012.10-20121022 - Linaro GCC 2012.10) 

// Hard float, -fPIC, -mfpu=neon

Problem: ib256, setup: 6.98 s, time: 10.85 us, ``mflops'': 943.9
Problem: ob256, setup: 3.25 s, time: 9.87 us, ``mflops'': 1037.6
// Hard float, -fPIC, -mfpu=neon, --enable--neon
Problem: ib256, setup: 10.09 s, time: 8.14 us, ``mflops'': 1258.2
Problem: ob256, setup: 5.15 s, time: 7.18 us, ``mflops'': 1425.1

An i7-950 running an Ubuntu VM:

// --enable-sse2, -fPIC, -m32

Problem: ib256, setup: 42.86 ms, time: 845.89 ns, ``mflops'': 12106

Problem: ob256, setup: 21.93 ms, time: 665.65 ns, ``mflops'': 15383

High resolution timers on ARM

Our embedded device currently runs Linux without high resolution timers and CONFIG_HZ=100.

I compiled the 2.6.33 kernel with CONFIG_HIGH_RES_TIMERS=y and no other changes.

I wanted to know a little more about how clock interrupts are handled with this kernel option.  Clock interrupts must happen more frequently than 10 ms in order to support higher resolution timers, or else nanosleep must busy wait in the kernel if and only if no other processes are runnable.

checking the interrupt count in /proc/interrupts between sleeps:
cat /proc/interrupt; sleep 1; cat /proc/interrupt
CPU0                                             
 26:     628010          SC  ost0                
 26:     628114          SC  ost0                

Reports 100 interrupts in a second, or 10 ms, the jiffy time.  The clock event source seems have the same interrupt frequency as before.  That can't be right, so...

I forced interrupts to happen with greater frequency:
http://www.advenage.com/topics/linux-timer-interrupt-frequency.php (see end of post for code)

CPU0                                                         
 26:     642285          SC  ost0                            
kernel timer interrupt frequency is approx. 4016 Hz or higher
 26:     643307          SC  ost0                       

Reports ~1000 interrupts during the program execution, far greater than the jiffy.  Clearly the interrupt generating clock must be reprogrammed when a wait time less than a jiffy is requested.

01	#include <signal.h>

02	#include <stdio.h>

03	#include <stdlib.h>

04	#include <string.h>

05	#include <sys/time.h>

06

07

08	#define USECREQ 250

09	#define LOOPS   1000

10

11	void event_handler (int signum)

12

{

13	static unsigned long cnt = 0;

14	static struct timeval tsFirst;

15	if (cnt == 0) {

16	gettimeofday (&tsFirst, 0);

17

}

18

cnt ++;

19	if (cnt >= LOOPS) {

20	struct timeval tsNow;

21	struct timeval diff;

22	setitimer (ITIMER_REAL, NULL, NULL);

23	gettimeofday (&tsNow, 0);

24	timersub(&tsNow, &tsFirst, &diff);

25	unsigned long long udiff = (diff.tv_sec * 1000000) + diff.tv_usec;

26	double delta = (double)(udiff/cnt)/1000000;

27	int hz = (unsigned)(1.0/delta);

28	printf ("kernel timer interrupt frequency is approx. %d Hz", hz);

29	if (hz >= (int) (1.0/((double)(USECREQ)/1000000))) {

30	printf (" or higher");

31

}

32	printf ("\n");

33

exit (0);

34

}

35

}

36

37	int main (int argc, char **argv)

38

{

39	struct sigaction sa;

40	struct itimerval timer;

41

42	memset (&sa, 0, sizeof (sa));

43	sa.sa_handler = &event_handler;

44	sigaction (SIGALRM, &sa, NULL);

45	timer.it_value.tv_sec = 0;

46	timer.it_value.tv_usec = USECREQ;

47	timer.it_interval.tv_sec = 0;

48	timer.it_interval.tv_usec = USECREQ;

49	setitimer (ITIMER_REAL, &timer, NULL);

50	while (1);

51

}