[Haskell-cafe] FFI: Overhead of foreign unsafe imports

[Sanket Agrawal]

On Sun, Feb 26, 2012 at 1:36 PM, Yves Parès <yves.pares at gmail.com> wrote:

> Hello,
> When I was using C code from Python, the overhead put on calling C code by
> Python was significant.
> To simplify, say I have on C-side two procedures f and g, I do all the
> stuff to call them in a row from Python, well I'm better off factorizing:
> adding on C side a wrapper h procedure that calls them both, and then call
> h from Python, because then I will have half as much overhead:
>
> Instead of SwitchToC -> call f -> SwitchToPython -> SwitchToC -> call g ->
> SwitchToPython,
> the factorization leads to SwitchToC -> call f -> call g -> SwitchToPython,
> which gives the same result yet is different performance-wise because each
> switching has a cost.
>
> This is painful, because if another time I have to call f and j (another
> function), then I have to make another wrapper.
>
> In Haskell world, now, given that my functions f and g would have been
> imported using *unsafe*:
>
> foreign import unsafe "f" f :: Thing -> Foo -> IO ()
> foreign import unsafe "g" g :: Stuff -> Bar -> IO ()
> foreign import unsafe "h" h :: Thing -> Foo -> Stuff -> Bar -> IO ()
>
> Are
> doStuff = f x y >> g z w
> and
> doStuff = h x y z w
> equivalent, or is there an overhead (e.g. due to IO monad, or due to the
> way the FFI does the calls) when compiled (if need be with optimizations)
> with GHC?
>

Each unsafe call to FFI should be pretty fast - I have measured it to be
about 15ns on my computer (~30-50 clock cycles). Assuming C implementation
of (f;g) and h take about the same time in C, first version of doStuff
would likely be a bit slower than second version because of one additional
FFI call - I would expect it to take ~15ns more on my computer. From what I
have seen in my code, GHC optimizes away IO monad when compiled with -O2
flag. So, impact of IO monad on performance should usually be
negligible/close to zero.

Keep in mind that unsafe call to FFI will block the OS capability used up
for that FFI thread until that FFI call returns (more here:
http://blog.melding-monads.com/category/haskell/). So, if it takes long
time to execute, you might want to use the safe version instead. I have
seen safe version take about ~150ns in my tests.

Like Jason said, you could create a micro-benchmark to see the difference.
For example, like below:

Haskell code:

import Control.Monad (forM_)import Foreign.C.Types (CInt)import
Data.Time.Clock (diffUTCTime, getCurrentTime)import Foreign.C

foreign import ccall safe "print"
  printsafe :: CInt -> IO ()foreign import ccall unsafe "print"
  printunsafe :: CInt -> IO ()

main = do
  let  l = 50000
  a <- getCurrentTime
  forM_ [1..l] $ \x -> printsafe x
  b <- getCurrentTime
  forM_ [1..l] $ \x -> printunsafe x
  c <- getCurrentTime

  print $ "safe call average overhead " ++ show ((diffUTCTime b
a)/fromIntegral l)
  print $ "unsafe call average overhead " ++ show ((diffUTCTime c
b)/fromIntegral l)

C code:
#include <stdio.h>
void print(int x){
  return;}
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