Re: Optimizing Float Ref's (Yaron Minsky)

[Bertrand Jeannet <Bertrand.Jeannet@inrialpes.fr>]

The problem may not only be unboxing, but also allocation.

In your first code, you allocate two new cells each time you enter in 
the j-loop (one for the boxed float, and one for the reference cell 
"sum" pointing to it).
You should first move the line "let sum = ref 0.0" before the i-loop and 
just write at the same place "sum := 0".

I will not be as efficient as your second version (because you still 
have boxing), but it should still be much better.

Bertrand

> On Aug 28, 2009, at 4:32 PM, Will M Farr <farr@MIT.EDU> wrote:
> 
>> Hello all,
>>
>> I'm running OCaml 3.11.1, and I noticed something strange in some  
>> native code for matrix multiply today.  The code was
>>
>> let mmmul store m1 m2 =
>>  let (ni,nk) = dims m1 and
>>      (nk2,nj) = dims m2 and
>>      (sni,snj) = dims store in
>>  assert(nk=nk2);
>>  assert(ni=sni);
>>  assert(nj=snj);
>>  for i = 0 to ni - 1 do
>>    let row1 = m1.(i) and
>>        srow = store.(i) in
>>    for j = 0 to nj - 1 do
>>      let sum = ref 0.0 in   (* Un-boxed float ref? *)
>>      for k = 0 to nk - 1 do
>>        let row2 = Array.unsafe_get m2 k in
>>        let x = Array.unsafe_get row1 k and
>>            y = Array.unsafe_get row2 j in
>>        sum := !sum +. x*.y
>>      done;
>>      Array.unsafe_set srow j !sum
>>    done
>>  done;
>>  store
>>
>> (I compiled with ocamlopt.)  It multiplies the matrices (represented  
>> as arrays of arrays of floats) m1 and m2 together and puts the  result 
>> into the matrix store.  Profiling the code, I noticed a call  to 
>> caml_modify during the execution of this function!  Turns out  that 
>> the culprit was the float ref "sum".  Changing to the following  code 
>> (which eliminates the float ref, and uses the <- and .( )  operators 
>> instead of unsafe_set and unsafe_get) eliminated that  call, and sped 
>> things up tremendously:
>>
>> let mmmul store m1 m2 =
>>  let (ni,nk) = dims m1 and
>>      (nk2,nj) = dims m2 in
>>  for i = 0 to ni - 1 do
>>    let row1 = m1.(i) and
>>        srow = store.(i) in
>>    for j = 0 to nj - 1 do
>>      srow.(j) <- 0.0;
>>      for k = 0 to nk - 1 do
>>        let row2 = Array.unsafe_get m2 k in
>>        let x = row1.(k) and
>>            y = row2.(j) in
>>        srow.(j) <- srow.(j) +. x*.y
>>      done
>>    done
>>  done;
>>  store
>>
>> But, I thought that float ref's were automatically unboxed by the  
>> compiler when they didn't escape the local context.  Is this a  
>> complier bug, is there a bad interaction with unsafe_get and  
>> unsafe_set, or is there something else going on that I don't  
>> understand?  Any enlightenment would be appreciated.
>>
>> Thanks!
>> Will
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> 
> 
> 
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