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Re: [Discuss-gnuradio] Flowgraph running in "fits and starts"
From: |
Marcus D. Leech |
Subject: |
Re: [Discuss-gnuradio] Flowgraph running in "fits and starts" |
Date: |
Mon, 06 Sep 2010 20:51:17 -0400 |
User-agent: |
Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.9.1.11) Gecko/20100720 Fedora/3.0.6-1.fc12 Thunderbird/3.0.6 |
On 09/06/2010 05:03 PM, Tom Rondeau wrote:
> On Sat, Sep 4, 2010 at 8:47 PM, Eric Blossom <address@hidden> wrote:
> Marcus,
> Indeed, this could be something we want to talk more about. Kind of on
> the periphery of my vision, I can see a handful of applications where
> the large chunking issue could be a problem. If we can define enough
> need, then we can talk more about finding the right way about it.
>
> Eric's suggestion is a good start. Tell it how many items you want and
> then run the loop based off that number or the noutput_items,
> whichever is smaller. If this works well for you, we might want to
> find a way of integrating that concept as part of the
> scheduler/basic_block.
>
> Well, like I said, we can think this through more clearly if you come
> up with positive results with that hack.
>
> Tom
>
>
I hacked in a hard-coded value as a temporary test that amounts to
100msec worth of "super symbols" (the actual symbols are
di-bits, at a nominal 4800symbol/sec rate, but I send 1200 packed
bytes/second over the FIFO) from my external source.
Looking at the debug logging setup by the scheduler, the scheduler has
asked for 32767 noutput_items on the gr.file_source() in my
flow-graph, and I'm returning 100msec worth (which is 120 items in my
case).
The result is a flow-graph that runs with much less apparent latency,
depending on what blocks I pick. If I put in an interpolator block
as the last item in the graph before the FFT display, it becomes
"chunky" again, so I put in a rational resampler to resample up to
the final channel bandwidth (mininum 500KHz for a USRP2, if I've done
my math correctly :-) ).
But the result is quite a bit more CPU hungry than the previous "fits
and starts" version of the flow-graph. So this little hack is
instructive, but not in and of itself any kind of path forward.
It seems like some kind of global approach to the latency issue for
narrow-bandwidth/low-event-rate applications is definitely
worth discussing. Likely much careful treading required :-)
--
Marcus Leech
Principal Investigator
Shirleys Bay Radio Astronomy Consortium
http://www.sbrac.org