[Difx-users] Multi-thread 4-bit VDIF fails with 'vdifmux() failed with return code -2014' {External} {External}

[Hugh Garsden]

Hi all,

Regarding Jake's message below, difx is using multiplexing to combine multiple threads into a single thread. However it can only deal with 1 channel per input thread because that is hardwired in the code.  Here:   https://github.com/difx/difx/blob/c2ac995454fa96de38c651f6eea484b4a5db73ca/libraries/vdifio/src/vdifmux.c#L150. The way to change that is mentioned in comment, which is to call setvdifmuxinputchannels(). That's what the vmux program does, and it works ok. I'm wondering why difx doesn't do that, and maybe there should be a warning about it not doing that. Anyway, that line of code is in configurevdifmux() so I've added a call to setvdifmuxinputchannels() after configurevdifmux() is called in difx, and now the multiplexing is working with more than 1 channel.

          Hugh

________________________________
From: Difx-users <difx-users-bounces at listmgr.nrao.edu> on behalf of Jacob Brooks via Difx-users <difx-users at listmgr.nrao.edu>
Sent: 20 February 2025 12:12
To: difx-users at listmgr.nrao.edu <difx-users at listmgr.nrao.edu>
Subject: [Difx-users] Multi-thread 4-bit VDIF fails with 'vdifmux() failed with return code -2014' {External}

Hi all,

We're currently trying to get DiFX working with the following setup: 4-bit, 2 channel (L + R pols), 2 threads (2 subbands), delivered in real time via UDP. It has been working fine up until the point where we tried adding a second thread - single-thread VDIF has been working for us.

When running in multi-threaded mode, the datastream processes crash with the following message:

'vdifmux() failed with return code -2014, likely input buffer is too small!'

I have managed to trace the problem to a check in 'getLowestFrameNumber' in vdifio/src/vdifmux.c. The inequality check in that function:

'getVDIFNumChannels(vh) != vm->inputChannelsPerThread'

succeeds as the function returns a value of 2 while 'inputChannelsPerThread' has a value of 1. This happens for every iteration so vdifmux() fails and returns a -2014 error code.

I'm not sure where to go from here and was hoping someone more familiar with DiFX could help out. Hopefully I'm not missing something obvious! If you need any more information I haven't provided please do ask.

Cheers,
Jake
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