Re: [Discuss-gnuradio] RAM consumption

[David Knox]

Eric Blossom wrote:
> 
> 99% of the base code distributed with GNU Radio (runtime + blocks)
> allocates NO MEMORY once the flow graph has started (see below for
> exceptions).
> 
> DO NOT use a vector_sink for anything other than QA code that
> produces a small amount of output.  The vector_sink will allocate an
> unbounded amount of memory if you keep writing to it.
> 
> It is also possible to consume memory by sending an unbounded number
> of messages into a message queue (gr.msg_queue) that doesn't have a
> queue size limit and has a slow (or compute bound) reader.
> 
> If there's a GUI of any kind involved, or python code that's being
> executed beyond initialization time, look there for problems.
> 
> Eric
> 

Thanks for such a quick response.  I am modifying C++ code, rather than
Python code.  There is no GUI (just an XTERM accessed via std::cout
statements).  I am using std::queue data structures and the push and pop to
these queues occurs in the same routine (see below).  

I think that your vector sink comments refer to the Python construct, don't
they?  What is the C++ equivalent of your 'DO NOT DO THIS' statement?  Does
this mean I have to access the queue in a critical section or using explicit
thread-safe methods?  My output seems to be sequenced just fine and there
are no duplicates or the like.

My C++ implementation is:
    .....
    if ( (int)queue.size() >= MAX) { queue.pop() } 
    queue.push(value)  
    .....
It is possible that the CPU is not able to keep up with the emptying task,
but then I'd expect the queue size to be growing beyond MAX and it isn't...
based on std::cout output anyway.  I suppose that this output could be also
lagging further and further behind, but the output all appears to complete
properly as each of my relatively rare test events occur (~1 second apart).

Is there something specific that I must do inside the C++ code to avoid
GnuRadio run-time memory allocation issues?  I have not specifically added
any new malloc/dalloc/calloc statements to the existing code myself.  Could
declaring data structures (e.g. scalars, arrays, std::queue) inside or
external to subroutines have this kind of side effect?  Would I be better
off implementing the queue using an array/circular buffer or std::vector of
my own, rather than using the one from the std:: library?  

My overall objective is that I want to be able to capture and print off
samples from the ADC on a specific trigger characteristic of the sample
values (e.g. magnitude).  So, I need to remember (either print out to the
XTERM or store to a file) small subsets of contiguous ADC samples that
occurred just prior to my trigger condition.  Once they are printed, I want
to 'forget' them entirely and start looking for my trigger again.  In my
testing, this amounts to printing about 50 lines every second or so, but I
am consuming RAM at a steady rate of about 3 MB/s.

/ David Knox
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