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Re: [Discuss-gnuradio] Buffer Overflow Debug

From: Richard Bell
Subject: Re: [Discuss-gnuradio] Buffer Overflow Debug
Date: Wed, 20 May 2015 11:02:39 -0700

Hi Martin,

Sorry for the delay in response. We have been able to put some time into debugging this issue and here is what we've found:

1) We have confirmed that the input stream to the HPD block is correctly tagged when the block freezes. The tags we set as "trigger tag" in the block are on the input stream.

2) At some point in the flowgraph operation, the HPD block gets stuck in the STATE_FIND_TRIGGER state (idle state). This is even though the trigger tags are present on the input, as confirmed in 1 above. We are observing that get_tags_in_range is failing to find the tags in the stream. We can't figure out what would cause this, or that this is even the issue. It's the best idea we have.

So we agree there is no buffering issue. The issue is with the HPD block not seeing tags that are confirmed to enter the input port. We would love some help on this issue.

The way we are debugging, is we copy and pasted the built in HPD block source to our own custom gr_modtool module and added cout debug statements there. 


On Thu, May 14, 2015 at 4:05 PM, Martin Braun <address@hidden> wrote:
On 14.05.2015 15:26, Richard Bell wrote:
> Hi all,
> I'm working on an incredibly annoying issue related to my use of the
> Header/Payload Demux (HPD) block. I think it's related to a buffer
> overflow at some point, but I'm having a really hard time coming up with
> a proper debug strategy to nail this down.
> What I'm seeing is my data streams freeze after the input to the HPD
> block, both on the header branch and the payload branch. Everything
> before the HPD block continues on without issue. The time it takes the
> streams to freeze is HIGHLY variable. I've watched it run for 30 minutes
> straight before a freeze and I've watched it freeze a few seconds after
> start. I'm using tags generated by the Correlation Estimator as the
> trigger for the HPD block.
> My question is this, if I suspect a buffer overflow is causing a freeze,
> how would I prove this to myself?


a "buffer overflow" wouldn't cause GR to freeze, rather, it would crash.
Going by your previous messages, I suspect what you're seeing is that
the HPD is starting to block, causing backpressure until that in turn
reaches the source. (Correct me if I'm wrong).

I remember you previously mentioning something similar. Did you confirm
the header parser is actually sending out a message for every data
packet it receives? This is a case where the HPD is actually designed in
a way that it'll fail.

As a debugging strategy, I would recommend printing out the state
changes inside the HPD state machine. If it freezes, it would be
interesting to see in which state that is.


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