I am using a raspberry pi to grab data through a SPI port. Attached to the SPI port is a PIC, programmed so that it samples on an A/D at 12.8kHz, and stores the samples in a 256 long buffer. (to cope with linux process pause and runs).
I have a linux program that reads the SPI and writes to a file. a couple of times per second I am getting missing data, I believe because linux is taking too long to return to my C program.
I understand that Linux (Raspbian) is not real-time and a process/program can be interrupted at any time, and that is why I am using a buffer in the PIC, to ride through these interruptions.
The problem is that sometimes the delay where the program waits is too large, and so I am missing some of the data. This happens maybe once every few seconds:
I would like suggestions for reducing the time, or at least setting a maximum time between process run time slices.
I have tried running with a nice level of -15 and -20.
Is there a way to reduce time slices?
Is there a memory disk on Raspbian that I can try writing my file to, incase it is the writing of the file that is causing it?
Here is some output from top:
top - 14:01:23 up 2:25, 3 users, load average: 1.91, 2.09, 2.18
Tasks: 76 total, 1 running, 75 sleeping, 0 stopped, 0 zombie
%Cpu(s): 7.6 us, 28.9 sy, 0.0 ni, 63.2 id, 0.0 wa, 0.0 hi, 0.3 si, 0.0 st
KiB Mem: 185732 total, 105708 used, 80024 free, 21112 buffers
KiB Swap: 102396 total, 0 used, 102396 free, 43916 cached
PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
353 root 20 0 0 0 0 D 15.3 0.0 17:01.87 spi0
13296 root 5 -15 2652 1468 1344 D 12.1 0.8 0:09.21 a.out
13386 pi 20 0 4708 2500 2064 R 1.6 1.3 0:00.40 top
4707 pi 20 0 4176 2572 1976 S 1.0 1.4 2:48.66 watch
11547 root 20 0 0 0 0 S 0.6 0.0 0:03.59 kworker/u2:1
16 root 20 0 0 0 0 S 0.3 0.0 0:05.60 kworker/0:1
2524 pi 20 0 9288 3296 2708 S 0.3 1.8 0:00.62 sshd
12448 root 20 0 0 0 0 S 0.3 0.0 0:02.98 kworker/u2:4
1 root 20 0 2168 1368 1260 S 0.0 0.7 0:02.22 init
2 root 20 0 0 0 0 S 0.0 0.0 0:00.00 kthreadd
3 root 20 0 0 0 0 S 0.0 0.0 0:06.88 ksoftirqd/0
5 root 0 -20 0 0 0 S 0.0 0.0 0:00.00
I can see from top that the a.out and spi0 process are using the most CPU time, but they are not maxing it out. Here you can see my process has a nice of -15.
Maybe one of the processes that is running is interfering? Here are my running processes, a.out is my SPI reading program.
pi@raspberrypi ~/frd/src/raspi/xcodefrd $ ps -ae
PID TTY TIME CMD
1 ? 00:00:02 init
2 ? 00:00:00 kthreadd
3 ? 00:00:07 ksoftirqd/0
5 ? 00:00:00 kworker/0:0H
7 ? 00:00:00 khelper
8 ? 00:00:00 kdevtmpfs
9 ? 00:00:00 netns
10 ? 00:00:00 perf
11 ? 00:00:00 khungtaskd
12 ? 00:00:00 writeback
13 ? 00:00:00 crypto
14 ? 00:00:00 bioset
15 ? 00:00:00 kblockd
16 ? 00:00:06 kworker/0:1
17 ? 00:00:00 rpciod
18 ? 00:00:00 kswapd0
19 ? 00:00:00 fsnotify_mark
20 ? 00:00:00 nfsiod
26 ? 00:00:00 kthrotld
27 ? 00:00:00 VCHIQ-0
28 ? 00:00:00 VCHIQr-0
29 ? 00:00:00 VCHIQs-0
30 ? 00:00:00 iscsi_eh
31 ? 00:00:00 dwc_otg
32 ? 00:00:00 DWC Notificatio
34 ? 00:00:00 kworker/0:2
35 ? 00:00:01 mmcqd/0
36 ? 00:00:00 VCHIQka-0
37 ? 00:00:00 SMIO
38 ? 00:00:00 deferwq
40 ? 00:00:00 jbd2/mmcblk0p6-
41 ? 00:00:00 ext4-rsv-conver
156 ? 00:00:00 udevd
286 ? 00:00:00 udevd
293 ? 00:00:00 udevd
321 ? 00:00:00 cfg80211
353 ? 00:18:14 spi0
411 ? 00:00:00 kworker/0:1H
1792 ? 00:00:06 ifplugd
1795 ? 00:00:01 ifplugd
1797 ? 00:00:01 ifplugd
1800 ? 00:00:00 wpa_supplicant
1877 ? 00:00:00 dhclient
2246 ? 00:00:00 rsyslogd
2248 ? 00:00:00 thd
2296 ? 00:00:00 cron
2326 ? 00:00:00 dbus-daemon
2393 ? 00:00:01 ntpd
2422 ? 00:00:00 sshd
2513 tty1 00:00:00 getty
2514 tty2 00:00:00 getty
2515 tty3 00:00:00 getty
2516 tty4 00:00:00 getty
2517 tty5 00:00:00 getty
2518 tty6 00:00:00 getty
2519 ? 00:00:00 getty
2520 ? 00:00:00 sshd
2524 ? 00:00:04 sshd
2525 pts/0 00:00:02 bash
2545 ? 00:00:00 sshd
2549 ? 00:00:39 sshd
2550 ? 00:00:13 sftp-server
2551 ? 00:00:00 sshd
2555 ? 00:00:00 sshd
2556 pts/1 00:00:01 bash
2583 ? 00:00:00 sshd
2587 ? 00:00:03 sshd
2588 pts/2 00:00:01 bash
4707 pts/2 00:02:56 watch
8843 ? 00:00:06 kworker/u2:0
11547 ? 00:00:04 kworker/u2:1
12448 ? 00:00:03 kworker/u2:4
13186 ? 00:00:03 kworker/u2:2
13295 pts/0 00:00:00 sudo
13296 pts/0 00:01:09 a.out
13690 ? 00:00:01 kworker/u2:3
14083 pts/0 00:00:00 ps
This is what my data stream looks like, you can spot the bit where the SPI reads were delayed:
You may of course want a look at my code. I have tried different buffer sizes.
#include <stdio.h>
#include <stdlib.h>
#include <wiringPi.h>
#include <sys/timeb.h>
#include <time.h>
#include <sys/timeb.h>
#include "frdSPI.h"
#include "frdBuffer.h"
#define TRIGGERVAL 50
#define BUFF_SIZE 32
int main(void){
unsigned char buffer[BUFF_SIZE];
int i, sample, channel = 0 , triggerdifference, oldsample, qtytowriteout = 0;
unsigned long int samplenumber=0;
unsigned long int differencenumber=0;
FILE *fp;
char filename[100];
struct timeb recordtime;
// Following is test code to see how to make filename and use it to write a file.
ftime( &recordtime );
sprintf( filename, "/frd/data/%ld%03ld.startup", recordtime.time, recordtime.millitm);
fp = fopen( filename ,"w" );
fprintf( fp, "test and more %lu", samplenumber);
fclose(fp);
if ( wiringPiSPISetup (channel, 500000) < 0){
fprintf (stderr, "SPI Setup failed! Check module is loaded and run app as root.\n");
exit(-1);
}
while(1){
wiringPiSPIDataRW ( channel, &buffer, BUFF_SIZE );
for(i=0; i<BUFF_SIZE; i++){
sample=parseSPIDataStream(buffer[i]);
if( sample > -1 ){
samplenumber++;
oldsample = circularbufferreadwrite( sample );
if( calculateDifferenceValue(sample, &triggerdifference))
{
differencenumber++;
if( triggerdifference > TRIGGERVAL || triggerdifference < (-1 * TRIGGERVAL) ){
//printf("%d \n", triggerdifference);
if( qtytowriteout == 0 )
{
ftime( &recordtime );
sprintf( filename, "/tmp/%ld%03ld.csv", recordtime.time, recordtime.millitm);
fp = fopen(filename,"w");
samplenumber=0;
}
if( qtytowriteout < 4000 )
qtytowriteout = qtytowriteout + 12800;
}
}
if( qtytowriteout && (fp!=(FILE *)NULL))
{
//printf("%lu,%d\n", samplenumber, oldsample);
fprintf(fp, "%lu,%d\n", samplenumber, oldsample);
qtytowriteout--;
if( qtytowriteout == 0 ){
fclose(fp);
printf("\b");
}
}
}
}
}
}
Thank you for any help and suggestions in advance.
EDIT I just ran dd if=/dev/zero of=/dev/null as well, to take up some CPU time, and this has made the problems significantly worse, so I think I am on the right track, but I could do with some pointers as to how to fix it. Here is a graph of the data when using more CPU time:
