Re: Problem setting buffer size for gfortran ( v 11.2)

["Bertini, Denis Dr." <D.Bertini@gsi.de>]

Hi Harald,

In attachment you will find a typical strace output when running with

gfortran8 on lustre filsystem.

As i said for gfortran version 8.x, 10.x, 11.2 setting the runtime

env. variable to change buffer size did not work for me as you could

see in the strace output.

You could also see that "mmap" is used by the lustre client.

The optimal blocksize given back by the STAT() routine indeed

favor a 4 MiB buffer size.


Best

Denis


________________________________
From: Harald Anlauf <anlauf@gmx.de>
Sent: Wednesday, February 23, 2022 7:26 PM
To: Bertini, Denis Dr.; fortran@gcc.gnu.org
Subject: Re: Problem setting buffer size for gfortran ( v 11.2)

Hi Denis,

Am 23.02.22 um 17:10 schrieb Bertini, Denis Dr. via Fortran:
> Hi
> Investigating some sporadical performance drop when using Fortran IO on our Lustre
> shared file system, we realized that the buffer size used as default by the gnu fortran
> compiler is 8 kiB only.
> We wanted to change this default and according to the documentation, one has just to set
> the environment variables:
>
>   GFORTRAN_UNFORMATTED_BUFFER_SIZE
>   GFORTRAN_FORMATTED_BUFFER_SIZE=
>   GFORTRAN_UNBUFFERED_ALL=n
>
> to increase the buffer size  during runtime.

this works for me on gcc-12 development for the following
test program on Linux/x86:

open(10,file="bigfile")
write(10,*) "data"
close(10)
end

Compile, link, and run under strace with

GFORTRAN_FORMATTED_BUFFER_SIZE=4194304 strace ./a.out

[...]
openat(AT_FDCWD, "bigfile", O_RDWR|O_CREAT|O_CLOEXEC, 0666) = 3
fstat(3, {st_mode=S_IFREG|0644, st_size=6, ...}) = 0
mmap(NULL, 4198400, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1,
0) = 0x146b69b1c000
write(3, " data\n", 6)                  = 6
ftruncate(3, 6)                         = 0
munmap(0x146b69b1c000, 4198400)         = 0
close(3)                                = 0

> Unfortunately isetting these variables  does not  change the default buffer size.
> I tried with gfortran 8, 10, and the new 11.2.
> Looking a the code, i found in
>   io/unix.c
>   that the default is set to the following value,
>
> static const int FORMATTED_BUFFER_SIZE_DEFAULT = 8192;
> static const int UNFORMATTED_BUFFER_SIZE_DEFAULT = 128*1024;
>
> Of course, changing the default value and recompiling works and change accordingly the buffer size
> It should nevertheless be possible to change these values without recompiling.
> Is there something one should be aware of when setting these variables?

You said you're using Lustre.  I guess you use the default
blocksize (4MB).  Does your Lustre client support mmap?
What does strace report?

I remember I once saw a system where we had to disable the use
of mmap in a library.

Cheers,
Harald

> Best regards,
> Denis
>
> ---------
> Denis Bertini
> Abteilung: CIT
> Ort: SB3 2.265a
>
> Tel: +49 6159 71 2240
> Fax: +49 6159 71 2986
> E-Mail: d.bertini@gsi.de
>
> GSI Helmholtzzentrum für Schwerionenforschung GmbH
> Planckstraße 1, 64291 Darmstadt, Germany, www.gsi.de
>
> Commercial Register / Handelsregister: Amtsgericht Darmstadt, HRB 1528
> Managing Directors / Geschäftsführung:
> Professor Dr. Paolo Giubellino, Dr. Ulrich Breuer, Jörg Blaurock
> Chairman of the GSI Supervisory Board / Vorsitzender des GSI-Aufsichtsrats:
> Ministerialdirigent Dr. Volkmar Dietz
>