Hi,
I am possibly to work on a data acquisition system (to be used in physics)
where I expect huge amounts of data. The idea is to get 8 PCs running
Linux (RTLinux ?) each responsible for a "part" of data. These PCs are
then connected to a giga-switch (each PC running 100Mbs ethernet), and on
the "other" side there is 1 PC (or 2 PCs) again running Linux (RTLinux ?)
with 1G ethernet. This "special" PC (or 2 PCs) is "collecting" data from
these 8 "acquisition" PCs.
As I expect huge amount of data, I would like to limit the number the data
are copied in memory. So, the idea it to run a real-time task (driven by
an interrupt) which would store data in a shared memory and a linux task
which would send this data directly from this shared memory via ethernet
(I can also consider the "ethernet-transport" task to be real-time instead
of user's process - if this gives me performance improvement).
Unfortunately, looking at the shmem HOWTO I have found that the maximum
size of shmem segment is limited to 4MB (older machines to 1MB). Do I
understand right that even if I have 215MB RAM I cannot have 100MB shared
memory segment ? If yes, is there any way to overcome this limitation ?
This reminds me another question - what is the maximum time that I can
spend in an interrupt (1ms , 10ms, 100ms), so that the system does not
become instable ? On principle I expect a structure of about 4s when I
need to collect events as fast as possible (I mean there can be interrupt
after interrupt coming) followed by some 4s of quiet (I can send the
collected data in that time via ethernet).
If I decide to use FIFOs - is there any size limit for the real-time
part to store the data before it is read by the user's program ? I mean
can I "buffer" 100MB of data in the FIFO and then "start" the user's
process to send the data via ethernet ?
How much time do I lose using a FIFO and storing, in the real time part,
byte after byte (in a loop - I read a byte from a VME module, then I
store it in the FIFO) as compared to storing the same amount of bytes in a
shared memory (also in a loop) ? I can't use memcpy - I need to build a
loop which reads acquisition modules and stores the read data in the
"buffer" (the "ethernet-transport" task can read "blocks" of data, of
course).
Any hints welcome.
Thanks in advance,
Jacek.
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