[Bug 261496] sort -S size documentation is wrong

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=261496

            Bug ID: 261496
           Summary: sort -S size documentation is wrong
           Product: Base System
           Version: CURRENT
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Some People
          Priority: ---
         Component: bin
          Assignee: bugs@FreeBSD.org
          Reporter: wosch@FreeBSD.org

BSD sort has an option to configure the size of the memory buffer:

     -S size, --buffer-size=size
             Use size for the maximum size of the memory buffer.  Size
             modifiers %,b,K,M,G,T,P,E,Z,Y can be used.  If a memory limit is
             not explicitly specified, sort takes up to about 90% of available
             memory.  If the file size is too big to fit into the memory
             buffer, the temporary disk files are used to perform the sorting.

Looking at the source I see no reference to the default 90% except in the
manual page. It appears that sort(1) use by default 50% of main memory, which
is more reasonable for a server OS.

$ sysctl hw.physmem
hw.physmem: 68576686080

$ cat /dev/null  | sort --debug
Memory to be used for sorting: 34288343040

$ expr 34288343040 \* 2
68576686080

the 50% is apparently from  free_memory / 2;
sort.c: available_free_memory = free_memory / 2;

However, this is not right for percentage modifiers.

$ cat /dev/null  | sort -S 100% --debug
Memory to be used for sorting: 34288343040

so, 100% use only 32GB (50%) of main memory, and 

cat /dev/null  | sort -S 20% --debug 
Memory to be used for sorting: 6857668608
use only 6.5GB (10%) of main memory.

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