Re: [linux-audio-dev] Re: Low Latency Kernel Combos

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Subject: Re: [linux-audio-dev] Re: Low Latency Kernel Combos
From: Benno Senoner (sbenno_AT_gardena.net)
Date: Fri Jun 01 2001 - 16:07:58 EEST


On Fri, 01 Jun 2001, Josh Green wrote:
> I finally found a satisfactory Linux lowlatency kernel combo. I was able
> to make a patch for Linux 2.4.5 from Andrew Mortin's lowlatency patch
> for Linux 2.4.4 (to be honest I really didn't have to do that much :).
> This patch is now on his site:
> http://www.uow.edu.au/~andrewm/linux/schedlat.html
>
> I posted results of Benno's latencytest program on my machine with and
> without the lowlatency stuff enabled
> (http://www.gardena.net/benno/linux/audio/) at the following site:
> http://www.resonance.org/~josh/lowlat/

>
> I'm pretty impressed with how the Linux kernel is performing without the
> lowlatency patch. It appears that probably a good portion of the low
> latency work being done has been incorporated into the official source.

Indeed the unpatched 2.4.5 is performing very well.
As for the 2.4.4-lowlatency: have you tried 3x128 buffers ?
If you have time can you post them too ?
3.8ms is good but it would be better being able to deliver rock-solid sub-3ms
latencies.
See my old 2.2.x test .. 2.1ms rock solid ... 2.4 should be able to do this
too.

For example the phpnuke portal would be ideal to announch test results like
yours ecc.
(My friend Udo has already agreed that he will produce RPM packages of new
lowlat kernels as soon as they will be available and that he will publish them
on the phpnuke portal. This is one of the reason why I'm advocating this
"community driven" portal)

> Good to know :) And of course I'm really impressed with the lowlatency
> patch enabled! 3.8 ms max latency with tons of disk access, etc! WOW! I
> think that is probably pretty good :) Something I'm fairly curious about
> is how non SCHED_FIFO programs are affected by all this low latency
> work. Do any programs that want to take advantage of low latency have to
> be run as root to modify scheduling etc. If anybody wants to enlighten
> me on this I would appreciate it.

Yes in order to set SCHED_FIFO you do need root privileges, but perhaps future
kernels (the famous capabilities concept) will allow some users/apps to set
this without the need of being root.
Non SCHED_FIFO programs take advantage of a lowlat kernel as well (much lower
 latency peaks) but the peaks will still be much higher than using SCHED_FIFO
scheduling especially when the machine is heavily loaded.

So if you want to be dropout free and need latencies in the sub 10-20ms range
then SCHED_FIFO is a must.

Benno.

http://www.linuxaudiodev.com The Home of Linux Audio Development


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