Reliability of Beowulf (memory limits)

Huntsinger, Reid reid_huntsinger at merck.com
Mon Sep 16 18:01:15 EDT 2002


PC's memory limit is a function of how many slots on the mainboard and
what "chipset" is used. Higher-end Serverworks chipsets for example can
support
very large amounts of RAM.

To access more memory than 32-bit unsigned integers can address (4GB), Linux
(and any other OS) uses an Intel hardware provision called Physical Address
Extension (PAE), which does some sleight-of-hand with 4 extra bits to make
use of the available RAM up to 64 GB.

Regardless of the amount of physical RAM, Linux allows a process to address
3 GB of memory (maybe up to .5 GB more if kernel parameters are modified).
So the extra RAM is only helpful if there are several memory-hungry
processes. (Some information is available at
http://www.spack.org/index.cgi/LinuxRamLimits)

If you want a single process to address more memory than this, you really
need a 64-bit machine (not necessarily a supercomputer). But you may be able
to rewrite your applications as several processes, each of which needs a
smaller (say < 3GB) amount of RAM. This can then be distributed across a
Beowulf, for example. Thankfully, the tools you need are all free. 

If you're thinking of making a network of PCs into a machine with a unified
> 4GB virtual address space, this is as far as I know a research topic.

Reid Huntsinger




Date: Mon, 16 Sep 2002 05:48:36 -0700 (PDT)
From: Toof Chanon <tooff2 at yahoo.com>
Subject: Reliability of Beowulf
To: beowulf at beowulf.org

Hello,

        I am a newbie to Beowulf and Computer
Architecture context. I don't understand how Beowulf 
PC cluster could manage huge memory usage.  Please
correct if I am wrong.  For each PC, a Beowulf's node
could provide only 32-bit address array A(n) ; n <
(2**31-1)/nb; nb = number of byte, e.g. real*8 array, 
A(268435455), which depend on (fortran) compilers.
Currently, 2-3 GB physical ram is PC limit. Suppose
that I have 2 nodes, then 4 GB is all share physical
ram. Ok, I knew that there is linux kernel which
supports > 2 GB ram. Do I need a special compiler
which supports > 2GB address also? If yes, I guess
such a compiler may be too expensive. No advantage for
going Beowulf. I believe that Beowulf is a set of
cheap and robust thing. My point is,

How could Beowulf cluster manage its huge memory > 2GB
by using a common 32-bit extent
system/compiler(linux/fortran)? 

Is this true that Beowulf provides only parallel
processing jobs but it does not tackle a huge problem
such a big array requirement/management? We have to
turn to supercomputer again. 

I have a stupid question because I am a student who is
seeking for a high performance machine for doing
research. Beowulf is only one worth, I go. 

So I would like to give your suggestion to
convince my supervisor go Beowulf for a big problem. 

Any suggestion will be appreciated

Regards,

Toof Chanon


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