[Sbs_daq] Big endian raw data?

[Andrew Puckett]

Interesting. So perhaps I’m being naïve here, but other than the byte-swapping inefficiency Ole pointed out in processing the raw data on the compute farm nodes, is there an actual problem here? Do we need to check/care about this in the software in writing our raw data decoders?

The cause of Bradley’s crash while processing GRINCH data doesn’t necessarily seem related to this…

Andrew

From: Robert Michaels <rom at jlab.org>
Date: Sunday, October 3, 2021 at 1:21 PM
To: Ole Hansen <ole at jlab.org>, Andrew Puckett <puckett at jlab.org>, sbs_daq at jlab.org <sbs_daq at jlab.org>
Subject: Re: [Sbs_daq] Big endian raw data?
I believe there are byte-swapping routines available in the DAQ libraries which allow to put the bytes in the right state and be consistent.  But the DAQ expert needs to make this happen.  Below is a snippet of an email from Dave Abbott about a year ago when I was having some trouble, which I think is relevant..  Dave is a good person to ask.  Can ask Bryan Moffit or Alexandre, too.

----------------------  snippet of email from Dave Abbott ------------------------

The CODA data files are written from a Java Event Builder. JAVA is inherently Big Endian. The EVIO
files will be by default in big endian.

However, ALL Banks of User data - created in your readout list - will NOT be swapped. They will stay
whatever Endian it was when it was written.

Typically the ROC will run in Linux on Intel which is Little Endian. Therefore the Data banks you create will stay
little endian. However the Bank headers will be swapped to be compatible with the rest of the CODA file.

An even more confusing possibility is that you might do a DMA from the VME bus into a CODA data Bank.
The VME bus is Big endian. Therefore the data from the VME bus will stay Big endian in this bank.

Our general rule for CODA 3 is that for purposes of DAQ we will not touch (or modify) the User's data in any way.
We will only modify the EVIO headers to match the endianess of whatever System writes the file.




________________________________
From: Sbs_daq <sbs_daq-bounces at jlab.org> on behalf of Ole Hansen <ole at jlab.org>
Sent: Sunday, October 3, 2021 1:06 PM
To: Andrew Puckett <puckett at jlab.org>; sbs_daq at jlab.org <sbs_daq at jlab.org>
Subject: Re: [Sbs_daq] Big endian raw data?

Maybe our various front-ends differ in endianness, so we write mixed-endian data?!? That would be disastrous since it is not supported by EVIO. A file can only be one or the other—a very binary view. (I guess EVIO was written before we became diversity-aware ;) ).

Ole
On 3.10.21 at 13:03, Andrew Puckett wrote:

Hi Ole,



This is interesting. The GRINCH data are being read out by the new VETROC modules, I don’t know if they differ from the other modules in terms of “endian-ness”. Maybe a DAQ expert can weigh in here?



Andrew



From: Sbs_daq <sbs_daq-bounces at jlab.org><mailto:sbs_daq-bounces at jlab.org> on behalf of Ole Hansen <ole at jlab.org><mailto:ole at jlab.org>
Date: Sunday, October 3, 2021 at 1:00 PM
To: sbs_daq at jlab.org<mailto:sbs_daq at jlab.org> <sbs_daq at jlab.org><mailto:sbs_daq at jlab.org>
Subject: [Sbs_daq] Big endian raw data?

Hi guys,

Bradley reported a crash of the replay (actually in EVIO) with /adaq1/data1/sbs/grinch_72.evio.0 (see https://logbooks.jlab.org/entry/3916105).

When digging into the cause of this crash, I discovered that these raw data are written in big-endian format. How can this be? I thought the front-ends are Intel processors. Are we taking data with ARM chips that are configured for big-endian mode? Is this a mistake, or is there some plan to it?

These big-endian data have to be byte-swapped when processing them on x86, which is what all our compute nodes run. That's a LOT of work. It leads to significant and seemingly completely unnecessary overhead. I.e. we're burning CPU cycles for nothing good, it seems.

Please explain.

Ole

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