[erlang-questions] Speeding up text file I/O

[Dmitrii 'Mamut' Dimandt]

Per Gustafsson wrote:
> Christian S wrote:
>> Do you know if this has been benchmarked against the more attractive 
>> code
>> that R12B makes more efficient? Because those functions make me want to
>> change profession to something where you get to hurt people.
>>
>
>
>
> I found that the following function is about 10% faster then the 
> unrolled function when using BEAM R12B:
>
> find_8(Buffer, Char, Pos) ->
>   case Buffer of
>     << Char, _/bits >> -> Pos;
>     << _, Rest/bits >> ->
>       find_8(Rest, Char, Pos+1);
>     _ ->
>       not_found
>   end.
>
> It might depend a little on the input though and when both functions 
> were native compiled there was no major difference between them.
>
> Per

Here's the reply:

"""start quote""

This is incorrect. My unrolled function is twice as fast as the 
one-liner when it is compiled with the native flag (I didn't translate 
the flag part - my bad, D.) The number of lines has been carefully 
measured through various tests.

Without the native flag there shouldn't be a significant difference in 
speed. That's because unrolling only helps the machine code for 
superpiplined processors. Since the VM isn't superpiplined there's not 
much point in unrolling the loop

The only simplification that can be done for R12 is:

find_8( Buffer, Char, Pos ) ->
case Buffer of
<< Char:8, _/bytes >> -> Pos;
<< _:1/bytes, Char:8, _/bytes >> -> Pos + 1;
<< _:2/bytes, Char:8, _/bytes >> -> Pos + 2;
...
<< _:32/bytes, Rest/bytes >> -> find_8( Rest, Char, Pos + 32 );
_ -> not_found
end.

That's it.


"""end quote"""

Sorry for missing the "compile with the native flag" in my translation