[Sbs_daq] Big endian raw data?

Sun Oct 3 19:56:59 EDT 2021

It sounds like from David's message, that the evio data file headers are written as bigendian, and then from what Ole says the evio reader interprets that as the whole data file needs to be byteswapped.

That would maybe explain why I needed to byteswap the helicity scaler data words.

Does that make sense?

Sent from my mobile device.
Get Outlook for Android<https://aka.ms/ghei36>
________________________________
From: Ole Hansen <ole at jlab.org>
Sent: Sunday, October 3, 2021 3:06:20 PM
To: Alexandre Camsonne <camsonne at jlab.org>; Paul King <pking at jlab.org>
Cc: Andrew Puckett <puckett at jlab.org>; Robert Michaels <rom at jlab.org>; sbs_daq at jlab.org <sbs_daq at jlab.org>
Subject: Re: Big endian raw data?

When EVIO opens a file, it looks for a "magic" word in a certain place early in the file. If it isn't found, it byte-swaps that word and checks if it matches the magic value then. If it does, it sets a flag that this file needs byte-swapping, and all header and data words, except "unknown" data (see the swap_data routine in evioswap.c), are then swapped. There is no (easy) way to enable/disable this flag outside of the library; it's hardcoded, pre-determined behavior.

I am also not sure if there are any flags in any of the headers to indicate the endianness of the data. It would just require a single bit. The headers are not user-accessible anyway, BTW, so it looks like you're on your own to find out whether you need swapping, on a per-module or per-ROC basis, I guess. Like I said, it's not very clear. What does "VME is big endian" really mean? An Intel CPU in a VME crate produces what endianness ... 🤔

Ole

On 3.10.21 at 14:21, Alexandre Camsonne wrote:
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=FxpSg1qAggzMg8YCjcvm4w&m=W_Py47zEQFMF4YLSxKlQ6YvCZkjxlCgMIaViW0IvmiA&s=HtjtqwZf9TY9s3jftpebziiNiD97Y3BAaURjA_dYHjc&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/e9614255/attachment-0001.html>