Ugh! - Software, Music and more…

Introduction

The 2.6.x kernels series is said to be performing worse (wrt. low latency operation) than 2.4.x with lowlatency and/or preemptible patches applied. For some people it is performing ok. For others the performance is unsatisfactory. Especially when working with very small periodsizes like 128 or 64 frames for RT operation.

This might be true for vanilla 2.6.x (even with kernel preemption) but this is not true anymore for 2.6.x kernels patched with Ingo Molnars realtime preemption patches. They are still work in progress but allow the maintainer of this site (me) to run jackd with 2*32 frames and basically xrun free (which results in a roundtrip latency of 64 frames. At a samplerate of 48000hz this is about 1.3ms).

Update: It might sound unbelievable for some people but given the right hardware (i.e. M-Audio Delta-66 or similar) it is possible for me to use periodsizes down to 16 or 8 frames Xrun-free :) Oh joy! This is submillisecond roundtrip latency. Sure, context switching overhead gets a little heavy but it’s very much ok for playing i.e. guitar practically latency-free.

Have a look at my Ramblings on linux audio/midi setup page. While it is still incomplete it tells you how i setup my system to reach these results.

Also note that there is some “official” realtime preemption documentation now available on this Linux Realtime Wiki

Besides kernel-latencies, there are some more issues to watch out for. See the pages menu on the left for some reading..

If you are looking for information on lowlatency for linux 2.4.x you have come to the wrong place. The jack FAQ might have infos for you..

The RP patches are still in flux! So all this is not for the faint of heart. There are no prepackaged kernels with the RP patches afaik. So you gotta build your own. Please make sure you know what you are doing! If in doubt about infos provided on these pages, please check ingo’s announcements on lkml.org.

If you get absolutely nonsatisfactory results with a 2.6.x kernel, there’s a pretty big probability that you have been bitten by the NPTL hell. So make sure you read the NPTL-hell page.

Another issue might be tmpfs related.

The VP kernels based on 2.6.9-rc3-mm2 are broken as rc3-mm2 is broken (USB suibsystem mainly and ACPI)