[linux-audio-user] Jack and SHM

Robert Jonsson robert.jonsson at dataductus.se
Thu Jan 22 04:48:36 EST 2004

On Thursday 22 January 2004 09.45, Christophe Vescovi wrote:
> Since there has been a lot of post concerning jack usage and
> optimisation recently it has remind me a problem I got that I never been
> able to understand.
> I have never been able to run jackstart with -R option when compiling
> jack with Sys V shm support. jackd -R as root work with SysV shm but if
> I want to use jackstart I have to use POSIX shm.
> The error message I am getting with SysV shm is the classical "cannot
> lock down memory for RT thread (Cannot allocate memory)".
> I am running a Mandrake 9.2 multimedia kernel (2.4.22, low latency,
> preemption and capability patches).
> Since most of jack users seem to use SysV shm I don't understand why I
> cannot (I am the only exception ....)
> Christophe

Hi Christophe,

I'm glad you brought it up, I have this problem too (actually with an Mdk9.2 
system but I don't think it's related) and have forgotten to report it back 

I've done a fair bit of investigation in this area. The problem seems to be 
that when locking the program tries to lock _all_ memory that the program 
If you have DRI running in your X configuration (as I do) and utilize a QT 
based program (as I do) there is a major problem. (It probably is similar 
with other toolkits.)
I'm not sure of this but evidence suggest that QT maps the graphic memory 
(available through DRI) into it's memory space.
What this means in reality is that with my 128mb graphic card QT apps occupy 
300mb of memory(!). With 512mb physical memory I sometimes manage to lock ONE 
application, but never two.

My "solution" has been to limit the amount of graphic-memory the x-server is 
allowed to use by adding this line to the XF86Config-4 in the Section 
VideoRam   32768

thus limiting it to 32mb memory, which is quite enough for me.
Another solution is to remove DRI, but I use it so it can't be removed.

That there is a difference if you build with POSIX shm vs SysV shm I didn't 
know. I guess there are still other problems with POSIX shm that keep you 
from using that ?


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