[linux-audio-dev] XRuns and Low-Latency

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Subject: [linux-audio-dev] XRuns and Low-Latency
From: René Rebe (rene.rebe_AT_gmx.net)
Date: Mon Aug 06 2001 - 14:55:21 EEST

Hi all.

I have a problem during 10 channel, full-duplex IO using ALSA 0.9-beta6 and a Midimann Delta 1010. (Linux 2.4.7-low-latency, AMD Athlon 600Mhz, 256MB RAM)

I use my own program (which BTW will be releases in a .01v in some weeks) which does this (+some other bits +the same for capture - C++ methos from my ALSA-wrapper ...):

Open (cname, SND_PCM_STREAM_PLAYBACK, SND_PCM_NONBLOCK)
SetAccess (SND_PCM_ACCESS_RW_INTERLEAVED)
SetFormat (SND_PCM_FORMAT_S16_LE)
SetBufferTimeNear (400000)
SetPeriodTimeNear (400000 / 4)

Using the plug layer I get this setup:
min_ch: 10
max_ch: 10
min_rate: 44100
max_rate: 44100
period_size: 2730
buffer_size: 6553
periods: 2

Then I use normal ReadI / WriteI to pass the data to ALSA.

I works pretty well, except after 20-30 seconds I get many XRuns every 1 - 2 seconds. They happen everytime Linux decides to flush the data in the disc-cache. (I have a IBM 30gig IDE disk (yes in UDMA66 mode ...) with an ReiserFS on it.)

To check if this guess was right I used my NFS mounted Server (100MBit, K6-2 200, 32MB Ram, slower IDE drives ...) to store the data - and I nearly never got XRUns (even having a laptop logged in, GNOME 2x, Konqui 2x, XEmacs 2x and a GCC compiling) ... .

Also this hack reduces the XRUNs to zero:
while true; do echo "syncing ..."; sync; sleep 1; done

So: Has someone some advices what to do better (or even to set via sysctl) ...

I know that using hw:0 and SND_PCM_FORMAT_S32_LE would same some converting time ... . Are there some tipps using ALSA and or multi-channles cards better ?? (MMap wouldn't save much here, would it??)

And another question to ALSA:
Would it be possible to return better values for:

snd_pcm_hw_params_get_channels_min / max
snd_pcm_hw_params_get_rate_min / max

when the plug layer is used?:
min_ch: 1
max_ch: 1073741823
min_rate: 1
max_rate: -1

This way the users has to enter how many channels he want's to have. And I have no possibility to check the count or provide meaningfull defaults or selectable ranges for the card ... - and using the hw:x directly I have to provide all format convertions myself ...

Thanks a lot
René 

-- 
René Rebe (Registered Linux user: #127875)

eMail:    rene.rene_AT_gmx.net
          rene.rebe_AT_rocklinux.org

Homepage: http://www.rene.rebe.myokay.net/

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