[R] enableJIT(2) causes major slow-up in rpart

[Uwe Ligges]

On 12.04.2012 23:15, Tal Galili wrote:
> Hello,
>
> Due to exploration of the JIT capabilities offered through the {compiler}
> package, I came by the fact that using enableJIT(2) can *slow* the rpart
> function (from the {rpart} package) by a magnitude of about 10 times.
>
> Here is an example code to run:
>
> library(rpart)
> require(compiler)
>
> enableJIT(0) # just making sure that JIT is off # We could also use
> enableJIT(1) and it would be fine
> fo<- function() {rpart(Kyphosis ~ Age + Number + Start, data=kyphosis)}
> system.time(fo())
> #   user  system elapsed
> #      0       0       0   # this can also be 0.01 sometimes.
>
> enableJIT(2)  # also happens for enableJIT(3)
> system.time(fo())
> #   user  system elapsed
> #   0.12    0.00    0.12
>

Because the overhead for JIT is considerable for this example and the 
time critical parts of rpart are written in compiled code already.

JIT compilers are good for inefficiently written R code such as loops 
that could be avoided by vectorized operations. JIT cannot optimize 
runtime for code already written in C (or written in excellent R code).

Uwe Ligges






>
> Which brings me to my *questions*:
> 1) Is this a bug or a feature?
> 2) If this is a feature, what is causing it? (or put another way, can one
> predict ahead of time the implications of using enableJIT(2) or
> enableJIT(3) on his code?)
>
>
> *Links*:
> A post I recently wrote about my exploration of JIT -
> www.r-statistics.com/2012/04/speed-up-your-r-code-using-a-just-in-time-jit-compiler/
> The question asked on SO regarding the limitations of JIT:
> http://stackoverflow.com/questions/10106736/possible-shortcomings-for-using-jit-with-r
>
> Thanks,
> Tal
>
>
>
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