Donald,
This topic is very timely. I am just configuring a new machine and I am trying
to establish 3M of shared memory from 125M to 128M. I would appreciate it
if you could e-mail the link to the buzz code or maybe send a snipit of how
you setup shared memory.
Thanks,
Rich
> ----------
> From: Donald Gaffney[SMTP:[EMAIL PROTECTED]]
> Sent: Sunday, April 04, 1999 2:09 PM
> To: Rob Butera
> Cc: Realtime Linux
> Subject: Re: [rtl] Shared Memory Applications
>
>
> On Sun, 4 Apr 1999, Rob Butera wrote:
>
> >
> > #define BASE_ADDRESS base_address_of_shared_memory
> > void *shmem ;
> > ...
> > shmem = BASE_ADDRESS ;
> >
> > I find it necessary to use "vremap" to assigned shmem for the
> > runtime process. I have emailed others who have as well, and
> > others who have not found this necessary. Can anyone suggest
> > why the conflicting results?
> >
>
> I also found vremap was necessary (but I had your buzz code to look at, so
> the memory mapping stuff was a snap - thanks for making it available) when
> I used memory above 32 MB (this was on a Pentium 166); upto 32 MB the
> straight pointer worked fine. I never investigated this further.
>
> I'm impressed that you can run a coherent closed loop at 70kHz. At this
> rate it seems like the jitter would, at least in the limit, be on par with
> the task period.
>
> I have also found memory mapping useful, in a similar application, and
> also use the FIFO trick to alert the Linux process that the data has
> changed (actually I use an array of data buckets, when one fills the user
> process is alerted via a FIFO). I'll admit that I never tried jumbo FIFOs
> to see if they would work; I guess I assumed the memcpys would be
> expensive, it's interesting to hear that is not the case.
>
> This thread has been excellent. My only (other) comment would be that
> perhaps a pointer to Rob Butera's buzz code should be added to the
> RT-Linux homepage near the link to Fred Proctor's page (which was also
> *very* useful) dealing with shared memory.
>
>
> -Don
>
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