Re: [Xenomai-core] More on rt pipes usage

[Philippe Gerum <rpm@xenomai.org>]

Ignacio García Pérez wrote:
> Hi,
> 
> Suppose I have a kernel rt task that samples data at a certain rate and
> writes it as messages into a rt pipe, from which it is read by a user
> space non rt program.
> 
> I want to limit the number of messages that are put into the pipe,
> because otherwise if the user space program dies, it will grow endlessly
> till it exausts the rt heap.
> 
> What I want to do is to have a pipe that can hold a limited number of
> messages such that rt_pipe_write will fail if it is full.
> 
> Is there a way to know how many messages are there in the pipe?
> 
> Even if there is a way, to prevent a (harmless) race condition, I would
> need to lock the pipe between checking the number of messages and
> calling rt_pipe_write. As far as I know, pipe locking belongs to the
> nucleus and I'd like to stay in the native skin as much as possible.
> 
> Another method would be to count how many messages I write, but then I'd
> need some hook that notifies me when the user space program reads a
> message so I can decrement the count.
> 
> Any ideas?

The plan is to be able to tell the pipe manager to use a user-provided heap 
instead of the system one; this way, exhausting the local heap (*) in RT space 
would be a clear sign that non-RT must be allowed to consume the pending data 
first. A simple interface to do that is missing, but the pipe manager already 
works with a variable heap pointer.

(*) Using the overall available memory seems a better metric than the number of 
pending messages, since it may often happen that messages have different sizes.

-- 

Philippe.