Re: [Caml-list] Performance questions, -inline, ...

[Jon Harrop <jon@ffconsultancy.com>]

Optimizing numerical code is discussed in detail in my book OCaml for 
Scientists. You may also be interested in a very similar thread where I 
optimized someone's almost identical code on the beginners list recently. 
There is also this relevant blog post of mine:

  http://ocamlnews.blogspot.com/2007/09/spectral-norm.html

Essentially, your benchmark has rediscovered the fact that abstractions (HOFs, 
polymorphism etc.) are prohibitively slow for high-performance numerics and 
must be avoided. There are also some minor boxing issues at play.

On Thursday 03 January 2008 16:28:30 Kuba Ober wrote:
> I haven't looked at assembly output yet, but I've run into some unexpected
> behavior in my benchmarks.

Your benchmarks aren't sufficiently well defined to convey information about 
anything specific, so you're highly likely to misinterpret what you see.

> This was compiled by ocamlopt -inline 100 -unsafe,

You should use Array.unsafe_get and _set rather than the command-line 
option -unsafe because the latter breaks correct code.

> What I wonder is why vector-to-vector add is so much faster than (constant)
> scalar to vector add. Vectors are preinitialized each time with a 1.0000,
> 1.0001, ... sequence.

This is also highly likely to be platform and architecture dependent.

> Also, the very bad performance from generic vector-to-vector *with*
> inlining is another puzzler, whereas generic add of scalar-to-scalar
> performs similarly to straight-coded one.
>
> Cheers, Kuba
>
> * add1: add scalar to scalar   120 MIPS
> * add3: add scalar to vector   250 MIPS
> * add5: add vector to vector   320 MIPS
> * add2: generic add scalar to scalar   100 MIPS
> * add4: generic add vector to vector   38 MIPS
>
> let start = 1.3
>
> (* generic scalar operation *)
> let op1 op const nloop =
> 	let accum = ref start in
> 	for i = 1 to nloop do
> 		accum := op !accum const
> 	done

You probably meant to return "!accum".

> (* generic vector operation *)
> let op2 op const a b (nloop : int) =
> 	let len = Array.length a in
> 	for j = 0 to len-1 do
> 		for i = 0 to len-1 do
> 			b.(i) <- op a.(i) b.(i)
> 		done;
> 	done
>
> (** addition **)
> let add1 nloop =
> 	let accum = ref start in
> 	for i = 1 to nloop do
> 		accum := !accum +. addconst
> 	done

Again, should probably return "!accum". However, you can encourage OCaml to 
unbox the float by returning "!accum + 0.0" instead.

> let add2 = op1 ( +. ) addconst

This should be slower than "add1".

> let add3 a b nloop =
> 	let len = Array.length a in
> 	for j = 0 to len-1 do
> 		for i = 0 to len-1 do
> 			b.(i) <- a.(i) +. addconst
> 		done;
> 	done

The loop over "j" can be removed.

> let add4 = op2 ( +. ) addconst

This will be slow because "op2" is polymorphic and "+." will not be inlined.

> let add5 a b nloop =
> 	let len = Array.length a in
> 	for j = 0 to len-1 do
> 		for i = 0 to len-1 do
> 			b.(i) <- a.(i) +. b.(i)
> 		done;
> 	done

This increments "b" by "a" many times. Replace the repeated adds with a single 
multiply for each element.

-- 
Dr Jon D Harrop, Flying Frog Consultancy Ltd.
http://www.ffconsultancy.com/products/?e