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 <anl...@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
>
> ---------
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