[plug] upper RAM size limits in Linux

[Denis Brown]

Dear PLUG list members,

I have a "nice" problem :-)  Am about to order a couple of gritty
workstations and servers for a medical imaging laboratory.  On the server
side we're talking dual Xeon 2.8GHz, hardware RAID, initially 0.4TByte
disk space across three spindles, U320 SCSI and now the question... Budget
will probably allow up to 4 GByte RAM, possibly more, but isn't 4GByte the
upper addressable limit anyway given the bus width?   Too late in the
night for my math cells to function :-(

On the workstation side, single Xeon 2.8GHz, single SCSI320 drives and ???
RAM with gigabit links between server(s) and WS(s).   Since this is
imaging, the more RAM the better but is there a practical upper bound in
Linux?   I've seen reference to having to specify memory size (in
lilo.conf???) but I think that was in the Bad Old Days (tm)

As an example of the sort of memory requirements there was a message on
one of the imaging lists tonight suggesting that 3GByte swap would be
appropriate for some functional MRI work - file sizes for a single study
500MByte, and we generally average 100 of those from a population to
create normalised templates for subsequent analysis.   (Do not try this on
your abacus!)

I did some Googling but haven't found much on the topic since about Feb
2000, when a discussion on the kernel archive talked of brk() having an
upper bound of 900MByte and mmap() up to 3GByte.   There was some talk in
the same thread about getting the glibc people to opt for the 3GByte limit
by default.

TIA,
Denis