Re: [boost] shared_ptr: policy for usage_count lock

From: Slawomir Lisznianski (slisznianski_at_[hidden])
Date: 2004-02-10 12:10:22

Peter Dimov wrote:
> Slawomir Lisznianski wrote:
> 
> [...]
> 
> 
> Please note that identifiers that start with _M (underscore followed by an
> uppercase letter) are reserved by the implementation, as are identifiers
> containing a douible underscore.
> 

Thanks for noting :-) I will refrain from using them.

> Thank you for the test. I was able to confirm your results on an AMD Athlon
> 1.4. However, you have to agree that your test code isn't very realistic, or
> to be precise, it's very unrealistic. ;-) 

Quite right, However, performance test scenarios are usually an 
exaggeration of reality ;-) Library code will be re-used by developers 
of various skills, and unintentional misuses are possible. Tests, such 
as above, are demonstrating the "worst case".

> I was able to cut both single- and
> multithreaded times to 20ms by replacing shared_count::operator= as shown
> below:
> 
>     shared_count & operator= (shared_count const & r) // nothrow
>     {
>         sp_counted_base * tmp = r.pi_;
> 
>         if(tmp != pi_)
>         {
>             if(tmp != 0) tmp->add_ref_copy();
>             if(pi_ != 0) pi_->release();
>             pi_ = tmp;
>         }
> 
>         return *this;
>     }

I see. Thanks for the tip. I will use it.

> That said, your test, when rerun with the "next release shared_count" (proof
> of concept available at
> 
>     http://www.pdimov.com/cpp/shared_count_x86_exp2.hpp
> 

Do we sacrifice platforms that do not support atomic instructions then? 
;-) I was faintly aiming to show a bigger picture, hence my proposal for 
locking policy. IMHO, overriding operator= is a valuable alternative, 
but puts an ado on users.

> I find this (~3x instead of 10x) slightly less alarming. ;-)

I agree ;-)

Slawomir Lisznianski; [ www.rhapsodia.org ]