[fpc-devel] Benchmark for FreePascal

[Michael Van Canneyt]

On Wed, 14 Dec 2005, Jonas Maebe wrote:

> 
> On 10 dec 2005, at 22:09, darekM wrote:
> 
> > I've see that reverse-complement benchmark <http://
> > shootout.alioth.debian.org/benchmark.php?test=revcomp&lang=all> for FPC
> > is very slow. I discover, that problem is with readln, that function
> > consume about 90% of time.
> 
> In my test (Mac OS X/PPC, rtl and program compiled with register variables,
> text buffer of 64kb) readln's overheid is negligible. The large cost comes
> from writeln. The reason the C version is so much faster is because its output
> is buffered as well. We automatically flush the output after each writeln
> (i.e. after each writeln of 60 characters), while in C this only happens at
> the very end of the program for 128kb of characters at a time.

Normally, You can speed that up considerably by  doing the following:

program testp;

uses dos; // Needed for TextRec...

Var
  F : Text;
  Buf : Array[1..100000] of char;
  I : Integer;

begin
  Assign(F,'');
  Rewrite(F);
  SetTextBuf(F,Buf); // Allocate HUGE buffer
  TextRec(F).FlushFunc:=Nil; // Disable flushing after each writeln().
  For I:=1 to 10000 do  
    Writeln(F,'teststring');  
  Close(F); // Will flush.
end.

If you strace that, you'll see that the whole buffer is written to screen in one 
big gulp, instead of 256 chars per write.

In fact, I think maybe we should do the 

  TextRec(F).FlushFunc:=Nil; // Disable flushing after each writeln().

In SetTextBuf if the file is open for writing, as otherwise the settextbuf 
is meaningless. Or add a DisableFlushText() call to the system unit.
(to avoid dragging the Dos unit in if you want this)

Michael.