Re: Performance of the Streams 'Read and 'Write

[Georg Bauhaus <rm-host.bauhaus@maps.futureapps.de>]

On 10/30/09 12:29 AM, Gautier write-only wrote:

>    procedure Write( b: in Buffer ) is
>      use Ada.Streams;
>      First     : constant Stream_Element_Offset:= Stream_Element_Offset
> (b'First);
>      Last      : constant Stream_Element_Offset:= Stream_Element_Offset
> (b'Last);
>      SE_Buffer : Stream_Element_Array (First..Last);
>    begin
>      for i in SE_Buffer'Range loop
>        SE_Buffer(i):= Stream_Element(b(Natural(i)));
>      end loop;
>      Write(Stream(f_out).all, SE_Buffer);
>    end Write;
>
> Naively, you would say it is even slower: you do even more by copying
> a buffer into another one, right ?
> Indeed, not at all, it is *lots* faster (on GNAT and ObjectAda)!


I finally thought that the above procedures are faster than 'Read
or 'Write because the latter are defined in terms of stream elements:
When there is a composite object like b : Buffer and you
'Write it, then for each component of b the corresponding 'Write
is called. This then writes stream elements, probably
calling Stream_IO.Write or some such in the end.
So Write from above appears closer to writing bulk loads
of stream elements than a bulk load of 'Writes can be.

Copying buffers does not matter in comparison to the needs
of I/O (on PCs).