[patch, libfortran] Adjust block size for libgfortran for unformatted reads

[Janne Blomqvist]

On Mon, Jul 8, 2019 at 11:18 AM Manfred Schwarb <manfred99@gmx.ch> wrote:
>
> Am 07.07.19 um 22:13 schrieb Thomas Koenig:
> > Hello world,
> >
> > the attached patch sets the I/O block size for unformatted files to
> > 2**17 and makes this, and the block size for formatted files,
> > adjustable via environment variables.
> >
> > The main reason is that -fconvert=big-endian was quite slow on
> > some HPC systems. A bigger buffer should eliminate that.  Also,
> > People who use unformatted files are likely to write large amounts
> > of data, so this seems like a good fit.  Finally, some benchmarking
> > showed that 131072 seemed like a good value to use. Thanks to Jerry
> > for support.
> >
> > I didn't change the value for formatted files because, frankly, we are
> > using a lot of CPU for converting numbers there, so any gain
> > negligible (unless somebody comes up with a benchmark which says
> > otherwise).
>
> formatted write: Did you try writing to an USB stick or similar? I guess
> for flash based devices anything below 64k will slow down operation.
> Computers like Raspberry Pi and the like often have flash based storage,
> and it is not extremely unrealistic to run fortran programs on them.

Good point. If you happen to have a USB stick handy, can you try the
simple C benchmark program at
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91030#c38 ?

(the kernel will coalesce IO's by itself, so the granularity of IO
syscalls is not necessarily the same as the actual IO to devices.
Network filesystems like NFS/Lustre/GPFS may have less latitude here
due to coherency requirements etc.)


--
Janne Blomqvist