Re: [proaudio] Real-time for audio

[Grant <emailgrant@xxxxxxxxx>]

>>> With a rt kernel, you can setup both the hardware priorities and the software
>>> priorities. That mean that tasks accessing some hardware, the sound card, will
>>> have the priority over the other tasks, and that tasks executed by a given user
>>> or group [the audio group] will have priority over the other. rtirq will dot
>>> that for you. Just emerge it and add it into the default run level.
>>
>> Is jack necessary for me then?  What I want to do is use my USB DAC
>> and mpd (music player app) in real time.  Maybe jack is necessary if I
>> want to know *how* real time it is?
>>
>>> Be aware that if you don't really need a rt kernel, it is best to not use it,
>>> because such a kernel can cause compilation failures with gcc.
>>> http://bugs.gentoo.org/show_bug.cgi?id=190462
>>> http://bugs.gentoo.org/show_bug.cgi?id=20600
>>> For that raeson, if you want to use a rt kernel, it is good practice to have
>>> another kernel of the same version, vanilla or gentoo sources, and reboot on
>>> this non rt kernel when using emerge.
>>
>> I noticed this when trying to emerge nvidia-drivers.
>>
>> Thanks,
>> Grant
>
> Hi Grant,
>   I thought we had this discussion maybe 6 months ago? Maybe I'm
> wrong about that.
>
>   For pure audio playback the real-time kernel and Jack provide
> almost NO value. The real-time kernel is able to give more responsive
> attention to certain drivers or apps. Jack is one of them and if your
> mpd app - whatever that is - is a Jack app then it inherits the
> attention. However this only matters when you need to do something in
> 'real time'. There is nothing, in my opinion, 'real time' about
> reading a stream of pre-recorded data off a disk, buffering it up and
> then sending it to a sound card. Why does the sound card need to
> receive data in 5mS vs 25mS? The only difference that the buffer time
> causes is to delay the start of playback. Once playback starts as long
> as there is no time the buffer runs dry there is no difference in the
> sound you hear.
>
>   I think that for your application using the real-time kernel and
> Jack is adding nothing but complication and that's likely to cause
> more problems than it's going to solve.

How low of a latency are you able to keep stable?  The output of my
jackd calculates 5.8ms.  Here's the command I'm using now:

jackd -R -P85 -c h -d alsa -d hw:1,0 -r44100 -p256 -n3 -S -P

I've made all of the optimizations I could from these guides:

http://irc.esben-stien.name/mediawiki/index.php/Setting_Up_Real_Time_Operation_on_GNU/Linux_Systems
http://proaudio.tuxfamily.org/wiki/index.php?title=Howto_RT_Kernel

I can't get rtirq to work though.  I've emerged it and set it to start
during the default runlevel, but I get the following and it doesn't
look like my USB DAC has been prioritized.  Any idea why it isn't
working?  Do you see any room for improvement in my output?

# ps -Leo rtprio,cmd
RTPRIO CMD
     - init [3]
     - [kthreadd]
    99 [migration/0]
    49 [sirq-high/0]
    49 [sirq-timer/0]
    49 [sirq-net-tx/0]
    49 [sirq-net-rx/0]
    49 [sirq-block/0]
    49 [sirq-block-iopo]
    49 [sirq-tasklet/0]
    49 [sirq-sched/0]
    49 [sirq-hrtimer/0]
    49 [sirq-rcu/0]
    99 [posixcputmr/0]
     - [desched/0]
    99 [migration/1]
    99 [posixcputmr/1]
    49 [sirq-high/1]
    49 [sirq-timer/1]
    49 [sirq-net-tx/1]
    49 [sirq-net-rx/1]
    49 [sirq-block/1]
    49 [sirq-block-iopo]
    49 [sirq-tasklet/1]
    49 [sirq-sched/1]
    49 [sirq-hrtimer/1]
    49 [sirq-rcu/1]
     - [desched/1]
     1 [events/0]
     1 [events/1]
     - [khelper]
     - [async/mgr]
     - [sync_supers]
     - [bdi-default]
     - [kblockd/0]
     - [kblockd/1]
     - [kacpid]
     - [kacpi_notify]
     - [kacpi_hotplug]
    50 [irq/9-acpi]
     - [ata/0]
     - [ata/1]
     - [ata_aux]
     - [ksuspend_usbd]
     - [khubd]
     - [kseriod]
     - [cfg80211]
     - [rpciod/0]
     - [rpciod/1]
     - [kswapd0]
     - [aio/0]
     - [aio/1]
     - [crypto/0]
     - [crypto/1]
    50 [irq/7-parport0]
    50 [irq/14-ide0]
    50 [irq/28-ahci]
     - [scsi_eh_0]
     - [scsi_eh_1]
     - [scsi_eh_2]
     - [scsi_eh_3]
     - [scsi_eh_4]
     - [scsi_eh_5]
    50 [irq/19-ath]
     - [phy0]
    50 [irq/21-ehci_hcd]
    50 [irq/20-ehci_hcd]
    50 [irq/23-ohci_hcd]
    50 [irq/22-ohci_hcd]
    50 [irq/12-i8042]
    50 [irq/1-i8042]
     - [kpsmoused]
    50 [irq/8-rtc0]
     - [edac-poller]
     - [usbhid_resumer]
    50 [irq/21-hda_inte]
     - [hd-audio0]
     - [kjournald]
     - [scsi_eh_6]
     - [usb-storage]
     - /sbin/udevd --daemon
     - [flush-8:0]
     - /sbin/portmap
     - /sbin/rpc.statd --no-notify
     - /usr/sbin/rpc.mountd
     - [lockd]
     - [nfsd]
     - [nfsd]
     - [nfsd]
     - [nfsd]
     - [nfsd]
     - [nfsd]
     - [nfsd]
     - [nfsd]
     - metalog [MASTER]
     - metalog [KERNEL]
     - /usr/sbin/acpid
     - /usr/bin/dbus-daemon --system
     - /sbin/wpa_supplicant -c/etc/wpa_supplicant/wpa_supplicant.conf -C/var/run
     - /bin/wpa_cli -a/etc/wpa_supplicant/wpa_cli.sh -p/var/run/wpa_supplicant -
     - /usr/sbin/cupsd -C /etc/cups/cupsd.conf
     - /usr/sbin/hald --use-syslog --verbose=no
     - /usr/sbin/console-kit-daemon
     - /usr/sbin/console-kit-daemon
     - /usr/sbin/console-kit-daemon
     - /usr/sbin/console-kit-daemon
     - /usr/sbin/console-kit-daemon
     - /usr/sbin/console-kit-daemon
     - /usr/sbin/console-kit-daemon
     - /usr/sbin/console-kit-daemon
     - /usr/sbin/console-kit-daemon
     - /usr/sbin/console-kit-daemon
     - /usr/sbin/console-kit-daemon
     - /usr/sbin/console-kit-daemon
     - /usr/sbin/console-kit-daemon
     - /usr/sbin/console-kit-daemon
     - /usr/sbin/console-kit-daemon
     - /usr/sbin/console-kit-daemon
     - /usr/sbin/console-kit-daemon
     - /usr/sbin/console-kit-daemon
     - /usr/sbin/console-kit-daemon
     - /usr/sbin/console-kit-daemon
     - /usr/sbin/console-kit-daemon
     - /usr/sbin/console-kit-daemon
     - /usr/sbin/console-kit-daemon
     - /usr/sbin/console-kit-daemon
     - /usr/sbin/console-kit-daemon
     - /usr/sbin/console-kit-daemon
     - /usr/sbin/console-kit-daemon
     - /usr/sbin/console-kit-daemon
     - /usr/sbin/console-kit-daemon
     - /usr/sbin/console-kit-daemon
     - /usr/sbin/console-kit-daemon
     - /usr/sbin/console-kit-daemon
     - /usr/sbin/console-kit-daemon
     - /usr/sbin/console-kit-daemon
     - /usr/sbin/console-kit-daemon
     - /usr/sbin/console-kit-daemon
     - /usr/sbin/console-kit-daemon
     - /usr/sbin/console-kit-daemon
     - /usr/sbin/console-kit-daemon
     - /usr/sbin/console-kit-daemon
     - /usr/sbin/console-kit-daemon
     - /usr/sbin/console-kit-daemon
     - /usr/sbin/console-kit-daemon
     - /usr/sbin/console-kit-daemon
     - /usr/sbin/console-kit-daemon
     - /usr/sbin/console-kit-daemon
     - /usr/sbin/console-kit-daemon
     - /usr/sbin/console-kit-daemon
     - /usr/sbin/console-kit-daemon
     - /usr/sbin/console-kit-daemon
     - /usr/sbin/console-kit-daemon
     - /usr/sbin/console-kit-daemon
     - /usr/sbin/console-kit-daemon
     - /usr/sbin/console-kit-daemon
     - /usr/sbin/console-kit-daemon
     - /usr/sbin/console-kit-daemon
     - /usr/sbin/console-kit-daemon
     - /usr/sbin/console-kit-daemon
     - /usr/sbin/console-kit-daemon
     - /usr/sbin/console-kit-daemon
     - /usr/sbin/console-kit-daemon
     - /usr/sbin/console-kit-daemon
     - /usr/sbin/console-kit-daemon
     - /usr/sbin/console-kit-daemon
     - hald-runner
     - hald-addon-input: Listening on /dev/input/event4 /dev/input/event2 /dev/i
     - hald-addon-storage: polling /dev/sdb (every 2 sec)
     - hald-addon-storage: polling /dev/sr0 (every 2 sec)
     - hald-addon-acpi: listening on acpid socket /var/run/acpid.socket
     - /usr/sbin/ntpd -p /var/run/ntpd.pid -u ntp:ntp
     - /usr/sbin/hddtemp -d -l 127.0.0.1 /dev/sda
     - /usr/sbin/sshd
     - /usr/sbin/cron
     - /usr/bin/gdm
     - /usr/bin/gdm
     - /usr/bin/X :0 -audit 0 -auth /var/gdm/:0.Xauth -nolisten tcp vt7
     - /usr/sbin/xinetd -pidfile /var/run/xinetd.pid -stayalive -reuse
     - /sbin/agetty 38400 tty1 linux
     - /sbin/agetty 38400 tty2 linux
     - /sbin/agetty 38400 tty3 linux
     - /sbin/agetty 38400 tty4 linux
     - /sbin/agetty 38400 tty5 linux
     - /sbin/agetty 38400 tty6 linux
     - /bin/sh /etc/xdg/xfce4/xinitrc -- /etc/X11/xinit/xserverrc
     - /usr/bin/dbus-launch --sh-syntax --exit-with-session
     - /usr/bin/dbus-daemon --fork --print-pid 6 --print-address 9 --session
     - /usr/bin/ssh-agent -- startxfce4
     - xscreensaver -no-splash
     - /usr/bin/xfce4-session
     - /usr/libexec/xfconfd
     - xfwm4 --sm-client-id 117f000001000115957674100000080570000 --display :0.0
     - xfsettingsd
     - /usr/bin/xfdesktop --sm-client-id 117f000001000121355315800000065980002 -
     - xfce4-panel --sm-client-id 20272b6bf-f534-4ae1-b5a5-49dce23346b1
     - xfce4-settings-helper --display :0.0 --sm-client-id 2e6187381-8fc5-4b9d-9
     - /usr/libexec/gam_server
     - /usr/lib64/mozilla-firefox/firefox --sm-config-prefix /firefox-P6PSAN/ --
     - /usr/lib64/mozilla-firefox/firefox --sm-config-prefix /firefox-P6PSAN/ --
     - /usr/lib64/mozilla-firefox/firefox --sm-config-prefix /firefox-P6PSAN/ --
     - /usr/lib64/mozilla-firefox/firefox --sm-config-prefix /firefox-P6PSAN/ --
     - /usr/lib64/mozilla-firefox/firefox --sm-config-prefix /firefox-P6PSAN/ --
     - /usr/lib64/mozilla-firefox/firefox --sm-config-prefix /firefox-P6PSAN/ --
     - /usr/lib64/mozilla-firefox/firefox --sm-config-prefix /firefox-P6PSAN/ --
     - /usr/lib64/mozilla-firefox/firefox --sm-config-prefix /firefox-P6PSAN/ --
     - /usr/lib64/mozilla-firefox/firefox --sm-config-prefix /firefox-P6PSAN/ --
     - /usr/lib64/mozilla-firefox/firefox --sm-config-prefix /firefox-P6PSAN/ --
     - /usr/libexec/gvfsd
     - /usr/libexec/xfce4/panel-plugins/xfce4-mixer-plugin socket_id 16777252 na
     - /usr/libexec/xfce4/panel-plugins/xfce4-mixer-plugin socket_id 16777252 na
     - /usr/libexec/xfce4/panel-plugins/xfce4-mixer-plugin socket_id 16777252 na
     - /usr/libexec/xfce4/panel-plugins/xfce4-sensors-plugin socket_id 16777253
     - /usr/libexec/notification-daemon
     - /usr/libexec/gconfd-2
     - /usr/bin/python2.6 /usr/bin/hp-systray --qt4
     - /usr/bin/python2.6 /usr/bin/hp-systray --qt4
     - /usr/bin/python2.6 /usr/bin/hp-systray --qt4
     - Terminal
     - gnome-pty-helper
     - bash
     - su - root
     - -su
     - su - jackd
     - -su
     - gmpc
     - bash
     - su - root
     - -su
     - transmission
     - transmission
     - transmission
     - /usr/bin/xfrun4 --daemon
     - /usr/libexec/gvfs-hal-volume-monitor
     - bash
     - su - root
     - -su
     - jackd -R -c h -P85 -d alsa -d hw:1,0 -r44100 -p256 -n3 -S -P
     - jackd -R -c h -P85 -d alsa -d hw:1,0 -r44100 -p256 -n3 -S -P
     - jackd -R -c h -P85 -d alsa -d hw:1,0 -r44100 -p256 -n3 -S -P
    95 jackd -R -c h -P85 -d alsa -d hw:1,0 -r44100 -p256 -n3 -S -P
    85 jackd -R -c h -P85 -d alsa -d hw:1,0 -r44100 -p256 -n3 -S -P
     - /usr/bin/mpd /etc/mpd.conf
     - /usr/bin/mpd /etc/mpd.conf
     - /usr/bin/mpd /etc/mpd.conf
     - /usr/bin/mpd /etc/mpd.conf
     - /usr/bin/mpd /etc/mpd.conf
     - /usr/bin/mpd /etc/mpd.conf
     - ps -Leo rtprio,cmd

- Grant