A collection of coding snippets, tips, hints and random thoughts.
From time to time I have to edit files on a SunOS 5.10 server. I use Emacs with tramp for this. However, after some time I get this error message from tramp:
File error: Couldn’t find exit status of `test -e …
It seems that the ssh connection goes bad for some reason. After this you won’t be able to save the file anymore. You can work around this by running M-x tramp-cleanup-all-connections and then saving again.
Since I’ve been using my 256MB Model B Pi as a server, I had been getting regular page allocation failures of the following kind:
sshd: page allocation failure: order:0, mode:0x20
This is rather annoying and seems to affect stability as well, like disrupted ssh or smb connections. It seems that the kernel has a setting to affect the minimum free memory that it keeps for allocations, which may be too low. So in the /etc/sysctl.conf file you can edit the last lines to bump this value from 8 Mbegabytes to 16 Megabytes:
# rpi tweaks
vm.swappiness=1
vm.min_free_kbytes = 16184
Settings are effective after a reboot. For me, this seems to fix the problem. But to be extra sure, I also changed the memory split from 192/64 to 224/32. Since my Pi runs headless, 64 MBytes seemed too much simply for a framebuffer console that is not even used. So after these tweaks my Pi shows 216 MByte of available memory and is running for some days now without page allocation errors.
My kernel.log and debug log files had grown to over 2 GBytes each. This was due to log messages of this kind:
delay: estimated 0, actual 133
As another blog stated, this can be fixed by adding a parameter to the sound-usb-audio module of Alsa. So I did the same thing, creating a file /etc/modprobe.d/snd_usb_audio.conf with this content:
options snd-usb-audio nrpacks=1
After restarting Alsa or a reboot, the problems were gone, the logfiles kept small.
I strongly discourage the usage of rt2800 based WiFi dongles with the Raspberry Pi. For some weeks I have been debugging why my Raspberry Pi freezes and crashes all the time. And it turns out that it is the WiFi dongle. I was using an Edimax EW-7711UTn, but the chip is also being used in other devices. There seems to be a bug report for the Raspbian kernel, and I have spent quite some time on the Raspberry forum at Stackexchange. Currently, I hooked up the Raspberry Pi via Ethernet directly to my router. This works fine, but the router is upstairs, while the Raspberry is supposed to be downstairs, connected to my stereo. So this is only a stopgap measure until I have found a WiFi dongle that does not crash the Pi. Hints are welcome.
Update: I now switched to a TP-Link TL-WN725N with a RTL8188CUS chip. This adapter is much smaller (no external antenna) and surprisingly also quite a bit faster. I achieve ~1.9 MByte/sec sustained point to point datarates via SMB. So far no crashes or other problems. I will continue testing, but it looks much better than with the Ralink chip.
Update 2: Almost four months into testing the RTL8188CUS based WiFi dongle and it runs absolutely fine. I have uptimes of almost a month without any problems. After that the Pi degrades due to some of the daemons leaking or memory fragmentation. But I think this is bearable!
The other day I installed the mpdas, which is the audio scrobbler for the music player daemon. Since there’s no debian package for the Raspberry Pi, I compiled mpdas from scratch and installed it. Now I don’t want to run it manually each time the Raspberry Pi boots up. So I found a nice template for writing your own debian-style init-script. I changed it a little and also installed the daemon tool, to turn the interactive mpdas program into a daemon. Just run apt-get install daemon to install it. Then put the following file under /etc/init.d/mpdas and run update-rc.d mpdas defaults. Then mpdas will be run automatically upon boot. Oh, one more thing: put your mpdas configuration under /usr/local/etc/mpdasrc or adjust the DAEMONOPTS in the init script accordingly.
#!/bin/bash
### BEGIN INIT INFO
# Provides: mpdas
# Required-Start: $remote_fs $syslog $mpd
# Required-Stop: $remote_fs $syslog
# Default-Start: 2 3 4 5
# Default-Stop: 0 1 6
# X-Interactive: true
# Short-Description: Audio Scrobbler for mpd
# Description: Starts the Audio Scrobbler for the mpd music player daemon.
### END INIT INFO
DAEMON_PATH="/usr/bin/"
DAEMON=daemon
DAEMONOPTS="-u pi -r -X /usr/local/bin/mpdas"
NAME=mpdas
DESC="The mpdas audio scrobbler for mpd"
PIDFILE=/var/run/$NAME.pid
SCRIPTNAME=/etc/init.d/$NAME
case "$1" in
start)
printf "%-50s" "Starting $NAME..."
cd $DAEMON_PATH
PID=`$DAEMON $DAEMONOPTS > /dev/null 2>&1 & echo $!`
#echo "Saving PID" $PID " to " $PIDFILE
if [ -z $PID ]; then
printf "%sn" "Fail"
else
echo $PID > $PIDFILE
printf "%sn" "Ok"
fi
;;
status)
printf "%-50s" "Checking $NAME..."
if [ -f $PIDFILE ]; then
PID=`cat $PIDFILE`
if [ -z "`ps axf | grep ${PID} | grep -v grep`" ]; then
printf "%sn" "Process dead but pidfile exists"
else
echo "Running"
fi
else
printf "%sn" "Service not running"
fi
;;
stop)
printf "%-50s" "Stopping $NAME"
PID=`cat $PIDFILE`
cd $DAEMON_PATH
if [ -f $PIDFILE ]; then
kill -HUP $PID
printf "%sn" "Ok"
rm -f $PIDFILE
else
printf "%sn" "pidfile not found"
fi
;;
restart)
$0 stop
$0 start
;;
*)
echo "Usage: $0 {status|start|stop|restart}"
exit 1
esac
My WiFi router sometimes goes haywire and the Pi won’t notice when the WiFi connection is up again. So I wrote this little script:
#!/bin/bash
TESTIP=192.168.1.1
ping -c4 ${TESTIP} > /dev/null
if [ $? != 0 ]
then
logger -t $0 "WiFi seems down, restarting"
ifdown --force wlan0
ifup wlan0
else
logger -t $0 "WiFi seems up."
fi
You can put this script under /usr/local/bin and add the following line to the system wide /etc/crontab:
*/5 * * * * root /usr/local/bin/testwifi.sh
This will check every five minutes if the connection is still up, and restart it, if the router cannot be pinged. If you dislike all the syslog messages, you can comment them out in the script.
My corresponding /etc/network/interfaces looks like this (I uninstalled all the network managers):
auto lo
iface lo inet loopback
iface eth0 inet static
address 192.168.3.42
netmask 255.255.255.0
auto wlan0
iface wlan0 inet dhcp
pre-up wpa_supplicant -Dwext -i wlan0 -c /etc/wpa_supplicant/wpa_supplicant.conf -B
post-down killall wpa_supplicant ; rmmod 8192cu ; modprobe 8192cu
iface default inet dhcp
The wpa_supplicant.conf should be easy to generate, there are lots of guides on the web for this.
Since my Raspberry Pi runs headless, and the analog audio output is not that great, I decided to add a USB sound card to my little machine. I took a Roland UA-1G, which I was using before on an OpenWRT machine. The device was immediately recognized:
Bus 001 Device 007: ID 0582:00ea Roland Corp.
However, ALSA will prohibit the card from becoming sound device #0, thus being the default. For that you have to comment out the following line in /etc/modprobe.d/alsa-base.conf:
# Keep snd-usb-audio from beeing loaded as first soundcard
#options snd-usb-audio index=-2
After rebooting or restarting ALSA, the Roland will become the default sound device:
$ aplay -l
**** List of PLAYBACK Hardware Devices ****
card 0: UA1G [UA-1G], device 0: USB Audio [USB Audio]
Subdevices: 0/1
Subdevice #0: subdevice #0
card 1: ALSA [bcm2835 ALSA], device 0: bcm2835 ALSA [bcm2835 ALSA]
Subdevices: 7/8
Subdevice #0: subdevice #0
Subdevice #1: subdevice #1
Subdevice #2: subdevice #2
Subdevice #3: subdevice #3
Subdevice #4: subdevice #4
Subdevice #5: subdevice #5
Subdevice #6: subdevice #6
Subdevice #7: subdevice #7
I have set up my Raspberry Pi as a music and file server. My requirements were:
