Re: Performance (was Re: Selenophobia)

[Dirk Laurie <dirk.laurie@...>]

2017-03-25 17:30 GMT+02:00 Frank Kastenholz <fkastenholz@verizon.net>:
> On 3/25/17 1:30 AM, Alex Larsen wrote:
>>
>> Was this with the PUC-Rio Lua?
>
>
> The work I did was on PUC-Rio Lua 5.3.
>
> The tests that we used were simple "for i=0 to 100,000,000" loops
> with the body of the loop simply adding a value. In Lua:
>   local i=0
>   local j=0
>   for i=0,100000000,1 do
>      j = j + i
>   end
>
> I just redid the tests on my home computer (imac with 3.1ghz core i5)
> In Lua the loop took 1.289 seconds to run
> In C
>    with normal optimization .227s (Lua takes 5.7x longer to run)
>    with -O0 optimization    .276s            4.7x
> We also coded the C version so that the "j=j+i" operation was not
> done in the loop --- rather it was in a separate function, which was
> called from within the loop. the notion was to eliminate some of the
> locality of reference; the C code probably all fit in a single cache
> line whereas Lua would not. The times were
>    with normal optimization .255s (Lua takes 5.1x longer to run)
>    with -O0 optimization    .391s            3.3x
>
> This is why we felt comfortable saying that if the Lua version
> of the application was running 50x slower than the C version, there
> were probably issues in the Lua version's implementation that were
> not good. The biggest problem was that the application had a loop
> of the form
>   for i = 0 to a very big number
>       if flag==true
>          do stuff
>       else
>          do different stuff
>       end
>    end
> the loop did not change flag.  We recoded it as
>
>   if flag==true
>       for i = 0 to a very big number
>          do stuff
>       end
>    else
>       for i = 0 to a very big number
>          do different stuff
>       end
>    end
> And got the 10x or so improvement in performance.
> Any reasonable C computer would have optimized the C
> code like this.  Lua didn't.

It can't and shouldn't. The static analysis that enables the C compiler to
know that 'flag' cannot be changed by 'do stuff' or by 'do different stuff'
is simply not valid in Lua.