[Sbs_daq] [EXTERNAL] Re: Big endian raw data?

[Andrew Puckett]

To qualify my earlier statement, it would seem that from a software POV, I don’t need to care about the endian-ness of the data, provided my software decodes the raw data correctly… So far I am not aware of any system in the GMN DAQ that is experiencing issues with decoding its raw data correctly… So then I agree with Ole that what we mainly want is consistency, and if little-endian data is more efficient to decode on the jlab batch farm than big-endian data, then that is what we should prefer.

Andrew

From: Alexandre Camsonne <camsonne at jlab.org>
Date: Sunday, October 3, 2021 at 2:22 PM
To: Paul King <pking at jlab.org>
Cc: Andrew Puckett <puckett at jlab.org>, Robert Michaels <rom at jlab.org>, Ole Hansen <ole at jlab.org>, sbs_daq at jlab.org <sbs_daq at jlab.org>
Subject: [EXTERNAL] Re: [Sbs_daq] Big endian raw data?
Everything is intel besided VTP. Though Dave mentionned VME was big endian.

Alexandre
On Sun, Oct 3, 2021, 13:56 Paul King <pking at jlab.org<mailto:pking at jlab.org>> wrote:
I can comment that when I wrote the helicity scaler library, I found that I needed to byte swap the data words (module data and diagnostic counters) on the crate in order to be decoded correctly.
I'm not sure if halladaq8 is an Intel or arm cpu.
Does Podd or evio2xml do a dynamic check of endianness and then byteswap, or is that explicitly enabled?

Sent from my mobile device.
Get Outlook for Android<https://urldefense.proofpoint.com/v2/url?u=https-3A__aka.ms_ghei36&d=DwMFaQ&c=CJqEzB1piLOyyvZjb8YUQw&r=UewsPL6Z06cTByi2Ms7DKw&m=ecRb_ZFdjO1bDouAI3PD56oNk9UndHy7kNrc0zOsuv4&s=ewgHMESoHSzQrk7jFRhEIxfyUyalWocksFcCBuIfhjw&e=>
________________________________
From: Sbs_daq <sbs_daq-bounces at jlab.org<mailto:sbs_daq-bounces at jlab.org>> on behalf of Andrew Puckett <puckett at jlab.org<mailto:puckett at jlab.org>>
Sent: Sunday, October 3, 2021 1:31:05 PM
To: Robert Michaels <rom at jlab.org<mailto:rom at jlab.org>>; Ole Hansen <ole at jlab.org<mailto:ole at jlab.org>>; sbs_daq at jlab.org<mailto:sbs_daq at jlab.org> <sbs_daq at jlab.org<mailto:sbs_daq at jlab.org>>
Subject: Re: [Sbs_daq] Big endian raw data?


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<mailto:rom at jlab.org>>
Date: Sunday, October 3, 2021 at 1:21 PM
To: Ole Hansen <ole at jlab.org<mailto:ole at jlab.org>>, Andrew Puckett <puckett at jlab.org<mailto:puckett at jlab.org>>, sbs_daq at jlab.org<mailto:sbs_daq at jlab.org> <sbs_daq at jlab.org<mailto: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<mailto:sbs_daq-bounces at jlab.org>> on behalf of Ole Hansen <ole at jlab.org<mailto:ole at jlab.org>>
Sent: Sunday, October 3, 2021 1:06 PM
To: Andrew Puckett <puckett at jlab.org<mailto:puckett at jlab.org>>; sbs_daq at jlab.org<mailto:sbs_daq at jlab.org> <sbs_daq at jlab.org<mailto: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


_______________________________________________
Sbs_daq mailing list
Sbs_daq at jlab.org<mailto:Sbs_daq at jlab.org>
https://mailman.jlab.org/mailman/listinfo/sbs_daq
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://mailman.jlab.org/pipermail/sbs_daq/attachments/20211003/f439dadd/attachment.html>