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Entry  20 Sep 2006, Stefan Ritt, Suggestion, Increase of maximum event size 
    Reply  20 Sep 2006, Stefan Ritt, Suggestion, Increase of maximum event size 
    Reply  27 Sep 2006, Konstantin Olchanski, Suggestion, Increase of maximum event size 
       Reply  28 Sep 2006, Stefan Ritt, Suggestion, Increase of maximum event size 
Message ID: 308     Entry time: 27 Sep 2006     In reply to: 302     Reply to this: 309
Author: Konstantin Olchanski 
Topic: Suggestion 
Subject: Increase of maximum event size 
> The current event size in midas is limited to 512k (MAX_EVENT_SIZE in midas.h)

Yes, 512 kBytes is rather small. For the T2K prototype TPC DAQ, I built and ran
MIDAS with 4 MByte events, and it worked fine.

Now, we have per-buffer tunable size (see message
https://ladd00.triumf.ca/elog/Midas/283) and in the long run, I would prefer the
compiled-in limit to go away: already all memory is allocated dynamically and
the MAX_EVENT_SIZE is only useful as kind of a sanity check against frontend
misconfiguration or against malformed events.

If MAX_EVENT_SIZE goes away, the maximum event size becomes limited by the
largest SysV shared memory segment permitted by Linux (via sysctl kernel.shmmax).

To go beyound the limit on SysV shared memories, on can use mmap() based shared
memory: this is limited by available RAM+swap (and disk space for the
.SYSTEM.SHM file). Current MIDAS system.c has an experimental implementation of
mmap() shared memory, but AFAIK it has not been used in any production system, yet.

K.O.
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