Re: [Discuss-gnuradio] what drives noutput_items passed to forecast?

[George Nychis]

Eric Blossom wrote:
> For blocks derived from gr_sync_block, gr_sync_interpolator and
> gr_sync_interpolator the scheduler knows how to compute noutputs from
> ninputs and vice versa, and thus chooses noutputs so as to maximize
> the amount of data given to work subject to all other constraints.
>
> For other blocks, there isn't a mechanism for mapping from ninputs to
> noutputs, so it does a binary search calling forecast until it finds
> the value that processes the most input subject to all other constraints.
>
> The details are in gr_single_threaded_scheduler.cc
>
> The two basic constraints are:
>
>   how much input is available
>   how much output space is available
>
> There are additional constraints based on output_multiple, etc.

I'm trying to emulate buffers and call general_work() for 
gr_clock_recovery_mm_ff.  I'm 99.99% sure that its output_multiple is 1.

I have a certain number of input items, and I start probing for 
noutput_items by starting at 8192 and cutting it in half each time 
forecast() tells me I need more input than I actually have.

Once I find an appropriate noutput_items, I pass the input with the 
total number of input items and noutput_items.

Instead of returning noutput_items from the block, I return 'ii', which 
is the value that would have been passed to consume().  I then remove 
this number of items from my input and use whats left to probe again 
until I reach noutput_items=1 and the total required input is greater 
than the input I have left.

But after a short while 'ii' becomes some really odd number such as 
-2147483646.... this obviously kills everything because consuming a 
negative number is bad.

Is there some other constraint that is less obvious?  I'm using 16-byte 
aligned buffers, calling forecast() to ensure that what it returns is <= 
to the total input I have, and then call general_work.

I know I'm totally hacking around the system, but I'd appreciate any 
help if there's something obvious I'm missing.

Thanks!
George