Improving Linux sound latency
This document is an incomplete draft.
Various scheduling and sound low latency tweaks might be very
useful to avoid sound breaking up, some pointers:
- Using a real time priority
- It may be very helpful to run the sound processing
GnomeMeeting at a real time priority. To avoid
running them as
root, start them as a normal user,
then change their priority class to FIFO as
for example like this:
for p in $(pidof ohphone); do chrt -v -r -p 5 $p; done
or when starting the application, for example with
chrt -v -r -p 5 xmms
- Kernel latency improving patch collections for
- These two sites contain a set of low latency and
preemption patches, ready made and packaged for common
My impression is that preemption patches should
be only applied by the brave, as they can cause problems
with several drivers.
Also, there is
that in recent 2.6 series kernels, like 2.6.12, latency is
pretty low already and the preemption logic does not help.
- BIOS: ensure sound cards on interrups 9, 10 or 11
- These interrupt levels have the highest priority in case of
nearly simultaneous interrup events.
- X11: enable
- This helps ensure the graphics card does not use too many
PCI bus cycles, stalling the sound card.
- System: tweaking the
PCI latency timers
- PCI devices share the bus, and the “latency timer”
for each device determines for how many consecutive cycles the
device can use the bus. Devices with a high value for the
latency timer can and do cause gaps in sound. Often enough it is
video cards that have high default latency timer values, to look
better in benchmarks. It is possible to reset the default value
of the latency timer with the command:
setpci -v -s '*:*' latency_timer=20
setpci -v -s '0:0' latency_timer=0
which sets it to 32 cycles and resets the latency timer to 0 for
the host bridge, usually with id
prevent bad performance for DMA). It is possible and perhaps
useful to give a larger value to a sound device, for example
with one of:
setpci -v -d '1102:0002' latency_timer=80
setpci -v -s '00:09.0' latency_timer=80
where 1102:0002 is the manufacturer and model id of
your sound card, or 00:09.0 is the PCI bus id of the
same card. It will be different on your system, so use
lspci -n to discover
Usually it is better to use the manufacturer and model id
instead of the bus id, as moving the card from one slot to
another is more common than replacing the a sound card with
another in the same slot.
Note that sometimes setting too low a value for the latency
of the video card might make video card operations less visually
smooth; setting the latency timer of the host bridge to greater
than zero can significantly impact the transfer rate of DMA.
There is more information on this issue in this
general introduction to PCI latency,
PCI latency HOWTO,
Linux specific PCI latency article.
Audio Quality HOWTO
- In parts it is a bit dated, but the sections on
contain a number of very interesting points.
Low latency with ALSA mini HOWTO
- From the page:
This document is specifically aimed at those
interested in using 2.4.x with Andrew Morton's low-latency
patch for audio purposes.
It's not really hard when you know what to do, but
there's a lot of stuff to upgrade before you can compile
and install the kernel if you are moving from linux
2.2.x. So if you are, don't forget to read the section
on updating your system files.
We suggest scanning through this whole document to
get an overview before you start mucking with the
kernel. Everything described here requires kernel
configuration, and it's easier to do it all in one go
rather than rebuilding the kernel again for each feature
you want. You should also read through the necessary
upgrades to make sure you have all your software in
order before you start configuring. Once you have
things working you may want to run some benchmarks on
your system. Visit the LAD site for the latest info,
apps and test results.
Low latency for audio work on linux 2.6.x
- Information on latency issues and advice specific to the 2.6.x
kernel series, by Florian Schmidt.
- Daniel Robbins talk about IRQs and PCI latency
- Informative, even if slightly old (2001). But I rather
disagree with his conclusion to set very long PCI latencies to
optimize thruput. I think shortish burst while slightly less
efficient are more in tune with the relative slowness of the PCI
Linux latency resources
- Latency checking tools and info.
- Ardour hardware advice
- Interesting advice. Except for the blanket warning about all
VIA chipsets; many older motherboards with VIA chipsets do have
limitations and performance issues with DMA and PCI transactions,
but not all, and in particular recent (2004 and later) chipsets
seem to be rather OK.
- Ardour mailing list thread on latency
- This thread contains several interesting contributions,
especially as to IRQ priorities (highest to lowest: 0, 1, 8, 9,
10, 11, 12, 13, 14, 15, 3, 4, 5, 6, 7). I used to think that
these do not matter that much, but then I always had my sound
card on IRQs 9, 10 or 11, which are high priority ones. I also
agree with the warning about APIC interrupt routing.
posting on low latency,
advice on low latency
- Benno Sennoner's advice on low latency audio, a bit old
but still useful.
- From the page:
Planet CCRMA (CCRMA is pronounced ``karma'') at
Home is a collection of rpms (RPM stands for RedHat Package
Manager) that you can add to a computer running RedHat 7.3,
8.0, 9 or Fedora Core 1 to transform it into an audio
workstation with a low-latency kernel, current ALSA audio
drivers and a nice set of music, midi, audio and video
- The CCRMA project's advice on hardware tuning and latency
- Wise words from Planet CCRMA on realtime, and latency.