Lisp HUG Maillist Archive

FLI performance

I'm curious as to the general overhead of a foreign function call. Currently, I have a program that is making - on average - about 50,000 FLI function calls every iteration of a tight loop. I've written the exact same program in C++ just for a simple comparison, and profiled the Lisp program to make sure I'm not doing something else bad.

In the C++ code, I can make the 50,000 DLL function calls in ~40 milliseconds. Considering what the function does internally, this is quite good. On the Lisp side, the same 50,000 calls take ~200 milliseconds. Now, while I expected there to be some overhead with the foreign call (type checking, setting up the stack, etc), I didn't expect it to be 5x that of comparable C++ code.

So, my question: are there any declarations I can do to increase this performance (turn off the type checking, making the foreign code __stdcall instead of __cdecl)? Also, another solution I have to the problem - since I have access to the DLL source - is to create a batch function that let's me accumulate all my data into a large buffer and then make the function call only once. But, before continue on this path, I'd like to make sure that a magical declare won't fix the problem for me, and that filling out the data in the buffer won't be equally (or more) slow than the foreign call.

Thanks for any help!

Jeff M.

Re: FLI performance

Just as a follow-up. Creating a batch function and making a foreign buffer that 
holds all the data sped the code to being only 2x instead of 5x. Much improved, 
but using the foreign buffer is a bit of a pain. It'd be much nicer if there 
was a way to improve the foreign call performance. ;-)

Another side question, while I was creating a batching structure, I came across 
define-foreign-pointer. I'm trying to understand what this is for and if it's 
can help simplify my code. For example, let's say this is my foreign buffer for 
batching:


(define-c-struct point3 (x :float) (y :float) (z :float))

(defun make-batch-buffer (size)
  (allocate-foreign-object :type `(:c-array point3 ,size)))


Trying to access members of any one of those point structures is a real 
frustration. I can't use the obvious foreign-aref or a simple dereference 
because I'm dealing with an aggregate type. So, getting an element in there is 
something like:


(foreign-slot-value 
 (dereference (foreign-array-pointer buf index) :copy-foreign-object nil)
 'x)


I was looking at define-foreign-pointer as a possible to this, since it allows 
defining slots in the structure. But I can't find any examples of where this is 
useful or how they are used. Does anyone have any?

Thanks!

Jeff M.
Updated at: 2020-12-10 08:41 UTC