LFS and Fortran in Scyld Beowulf

Donald Becker becker at scyld.com
Tue Mar 26 17:11:17 EST 2002


On Tue, 26 Mar 102, Mikhail Kuzminsky wrote:

> According to Joe Griffin
> >
> > g77 may be recompiled for large files.
> > We use: -D_FILE_OFFSET_BITS=64 -D_LARGEFILE_SOURCE
..
> > Unformatted/Sequential < 4 Gb
>   I think that like restrictions are very importatn for many
> "beowulfers". By my opinion, large unformatted files are more
> important than formatted, and 4 Gbytes restriction is inappropriate.

LFS support is much more important for Beowulf systems than the average
workstation user.

> a) use special subset of system calls to open/read/write
..
> b) Use modern features of kernels 2.4.x and ext3fs

The reason that this is brought up for Scyld is that we added LFS to our
releases about two years ago.  That meant the addition of LFS support in
the 2.2 kernel, C library (kudos to Matti Aarnio for the early basis for
this) and enhancing some utilities.

The LFS kernel support has some limitations, many of which remain true
for 2.4 kernels.  First, the offset is really only 40/41/42 bits, not 64
bits, because we still use 32 bit block offsets with 512/1K blocks.
Very few places have a single file larger than 4TB, so this isn't a
current problem.

Second, only a subset of filesystem types can support 40/64 bit
offsets.  An example of one that can not is NFSv2, where the network
protocol defines 32 bit byte offsets.  The common Linux filesystems,
including ext2/3, do support full 64 bit offsets.

The standard GNU utilities were surprisingly well written with LFS
system calls in mind.  Most of the work was in running a test suite
against the utilities, not in direct code modifications.  There were a
few fixes required.  The representative example was adding long offsets
as parameters to 'split'.

> > The CURRENT Intel compiler - 5.0.1 has a
> > 2 Gb limit.  The next release to come out
> > at the end of this month (6.0) will have
> > the limit removed.


-- 
Donald Becker				becker at scyld.com
Scyld Computing Corporation		http://www.scyld.com
410 Severn Ave. Suite 210		Second Generation Beowulf Clusters
Annapolis MD 21403			410-990-9993

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