Re: Thread-safe reference counting (modified dim-vector.h)

[Jarno Rajahalme]

On May 26, 2010, at 2:37 AM, ext Jaroslav Hajek wrote:

> 
> But, by returning a reference, you require the referenced object to be
> permanently stored somewhere. I think we absolutely do not want to
> make such a requirement for octave_value::dims, objects should be
> allowed to compute their dimensions on demand. Hence, making
> octave_base_value::dims return a const reference is out of question.
> If the need arises, we'll probably create an extra method for that.
> 

I agree, and I did not intend to propose that octave_base_value::dims () should 
be changed. My point was that when reference counting is made thread safe, the 
overheads become non-negligible.

> Interestingly enough, using the original dim_vector.h and stripping
> away the openmp stuff, the reference counting is so fast that there is
> no visible difference:
> 

I guess the back and forth changes stay mostly in the cache memory,  are the 
caches are not synchronized at every change, nor are the changes committed to 
the main memory very often.

> Another interesting point is that if
> you make dims() return a dim_vector, both versions generate exactly
> the same assembler output, suggesting that using a const reference
> brings no advantage if a value is returned.
> 

I would have expected this. But there is no disadvantage either?

> I'd say that the results of the test are not so bad. I expected worse.
> 

You mean the atomic ref counting?

 Jarno