Multiprocessing takes higher execution time

[Nick Craig-Wood]

Grant Edwards <invalid at invalid> wrote:
>  On 2009-01-07, Steve Holden <steve at holdenweb.com> wrote:
> 
> >> I use multiprocessing to compare more then one set of files.
> >> 
> >> For comparison each set of files (i.e. Old file1 Vs New file1)
> >> I create a process,
> >> 
> >> Process(target=compare, args=(oldFile, newFile)).start()
> >> 
> >> It takes 61 seconds execution time.
> >> 
> >> When I do the same comparison without implementing
> >> multiprocessing, it takes 52 seconds execution time.
> 
> > My first suggestion would be: show us some code. We aren't
> > psychic, you know.
> 
>  I am!
> 
>  He's only got one processor, and he's just been bit by Amdahl's
>  law when P<1 and S<1.
> 
>  There you have a perfectly "psychic" answer: an educated guess
>  camoflaged in plausible-sounding but mostly-bullshit buzzwords.
>  A better psychic would have avoided making that one falsifiable
>  statement (he's only got one processor).

;-)

My guess would be that the job is IO bound rather than CPU bound, but
that is covered by Amdahl's Law too where P is approx 0, N
irrelevant...

Being IO bound explains why it takes longer with multiprocessing - it
causes more disk seeks to run an IO bound algorithm in parallel than
running it sequentially.

-- 
Nick Craig-Wood <nick at craig-wood.com> -- http://www.craig-wood.com/nick