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Re: [Discuss-gnuradio] Losing samples in the flowgraph

From: Martin Braun
Subject: Re: [Discuss-gnuradio] Losing samples in the flowgraph
Date: Tue, 26 Jul 2011 09:33:33 +0200
User-agent: Mutt/1.5.20 (2009-06-14)

On Sun, Jul 24, 2011 at 01:07:36PM +0200, Michael Höin wrote:
> I think my program loses samples. If I choose for the input of my
> flowgraph a file source with a throttle (Rate = 2 MSamples) and for the
> output a vector sink, I see in the output vector that samples are loss
> (every run differnet datas were loss). The CPU usage is only 40 Percent.
> If I reduce the throttle rate to 30 percent. Less or no samples are
> loss.
> [...]
> I tested the graph with different computers (Ubuntu 10.10) and with the
> latest git code. The problem occurs always.
> I have no more ideas what the problem could be. Had somebody the same
> problem?

Hi all,

I think there *might* be a fundamental problem with GNU Radio. I've seen
something like this myself:
When using a file_source w/o throttle as an input, the output can be
inconsistent. I saw this a while ago when demodulating pre-recorded
signals. I knew a file contained N bursts, but the number of bursts
found was random (and they were not always the same!).

My explanation was, simply put, that in the no-throttle mode, when every
block tries to do everything as quickly as possible, there are some
hiccups with the memory allocation, some race condition or something
occurs and samples are lost *between blocks*. If throttling is active,
the scheduler is forced to distribute processor time more evenly between
blocks, and this problem vanishes.

From one of Michael's previous posts, I might be able to cook up a flow
graph which demonstrates this error.


Karlsruhe Institute of Technology (KIT)
Communications Engineering Lab (CEL)

Dipl.-Ing. Martin Braun
Research Associate

Kaiserstraße 12
Building 05.01
76131 Karlsruhe

Phone: +49 721 608-43790
Fax: +49 721 608-46071

KIT -- University of the State of Baden-Württemberg and
National Laboratory of the Helmholtz Association

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