Event Buffer: Difference between revisions

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(Created page with "By default the event buffer (used to contain the data from the event) is named SYSTEM. Its default size is 8MB. == Increase size of event buffer == In some experiments, this...")
 
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By default the event buffer (used to contain the data from the event) is named SYSTEM. Its default size is 8MB.
The midas documentation has moved. See [https://midas.triumf.ca/docs/develop/features/acquiring_data/event_buffers/configuring/#event-and-buffer-sizes Event and buffer sizes] on the Event buffers: configuring page for the new documentation about increasing event and buffer sizes.
 
== Increase size of event buffer ==
In some experiments, this may not be large enough. 
 
To increase the size, one must increase:
* the value of ODB /Experiment/MAX_EVENT_SIZE
* the size of the SYSTEM shared memory event buffer (and any buffers used by the event builder,
etc)
* max_event_size & co in the [[frontend]].
 
Actual limits on the bank size and event size are [[#Limits on the bank and event sizes|described below]].
 
 
Ultimately, the maximum event size is limited by the size of the SYSTEM buffer which is
limited by the physical memory of your computer. No recompilation of MIDAS necessary.
 
 
== Limits on the bank and event sizes ==
 
Limitations in order:
 
 
 
# bank32 size is limited to a 32 bit integer size (about 4000 Gbytes)
# bank size is limited by event size
# event size in a midas mfe.c based frontend is limited to the value of max_event_size set by the user
# maximum event size that can go through the MIDAS event buffer system is limited to ODB value /Experiment/MAX_EVENT_SIZE (MAX_EVENT_SIZE in midas.h does not do anything now)
# maximum event size is limited to *half* the size of the SYSTEM shared memory event buffer (or any other buffers that the event has to go through)
# default size of the SYSTEM buffer is 8 Mbytes set by ODB /Experiment/Buffer sizes/SYSTEM. This limits maximum event size to about 4 Mbytes.
# size of SYSTEM buffer can be increased to arbitrary size, but in practice no bigger than the amount of computer physical memory minus space needed for running the frontend program and the mlogger, which also allocate buffer space to hold 1 event of maximum size.
 
 
 
So for a computer with 8 Gbytes of RAM, you can make the SYSTEM buffer size 4
GBytes, set ODB MAX_EVENT_SIZE to 1 Gbyte, and in theory, you should be able to
write 1 Gbyte events from your frontend to disk.
 
In practice, I think the biggest events I have seen go through a MIDAS system are
non-compressed waveforms in the T2K/ND280 FGD and TPC detectors, about 4 Mbytes of
data per event.
 
Other considerations (rules of thumb):
 
# the SYSTEM event buffer should be big enough to hold 10-100 events.
# the SYSTEM event buffer should be big enough to hold about 5-10 seconds of data
:: i.e. if your event size is 1 Mbyte and data rate is 1 Hz, 10 seconds of data will be 1Mbyte*1Hz*10sec = 10 Mbytes.
 
This is because the SYSTEM buffer decouples the real-time activity of the frontend
program from non-real-time activity of writing data to storage devices.

Latest revision as of 22:00, 30 October 2026

The midas documentation has moved. See Event and buffer sizes on the Event buffers: configuring page for the new documentation about increasing event and buffer sizes.

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