[boost] Re: Performance: shared_ptr

From: Matthew Wilson (stlsoft_at_[hidden])
Date: 2003-11-15 20:10:21

"Joe Gottman" <jgottman_at_[hidden]> wrote in message
news:bp42l6$kia$1_at_sea.gmane.org...
>
> "Matthew Wilson" <stlsoft_at_[hidden]> wrote in message
> news:bovl1d$juh$1_at_sea.gmane.org...
> > Hi Boosters
> >
> > I'm just doing some research on reference-counting, and contrasting
> > self-counting types with externally counted types, the latter effected
> with
> > the rather excellent shared pointer technique.
> >
> > [As a side question, is it true that Nicolai Josuttis was the inventor
of
> > the technique?]
> >
> > Naturally, one of the shared pointer's I've used is boost::shared_ptr.
The
> > measurements have demonstrated some surprising performance
characteristics
> > for shared_ptr in particular, and I want to solicit the opinion of the
> > expert's here before I make any bold claims, as some results are just
too
> > surprising for me to trust them.
> >
> > There are five types tested.
> >
> > 1. A self-counting type, using an ancient Synesis (my company) template
> > ref-counting bolt-in class
> > 2. A self-counting type, using a custom-written (for this test) template
> > ref-counting bolt-in class
> > 3. Externally counting, using boost::shared_ptr
> > 4. Externally counting, using a custom-written (for this test) shared
> > pointer type
> > 5. Externally counting, using a custom-written (for this test) shared
> > pointer type, which has a custom allocator for the shared count members
> >
> >
>
>   results snipped.  Shared_ptr did not do well.
>
> > Naturally, the very big question is: what's happening to
> boost::shared_ptr?
> >
> > In thread times, it performs 2-3 times worse than in single threaded
> > scenarios (irrespective of whether they are built for single or multiple
> > threads). However, in total time, it performs extremely poorly, up to
100
> > times slower than some of the others! Clearly, there is some serious
> > contention involved.
> >
> > There are three possibilities I have thought of:
> >
> > (i) It performs very poorly in multi-threaded sharing scenarios. If this
> is
> > the case, can any people more expert in Boost than me proffer an
> explanation
> > as to precisely the problem, and is any such person motivated to try and
> > address the issue?
> > (ii) The particular sharing scenario is well away from any real-world
> > scenario in which boost::shared_ptr is likely to be used. If this is the
> > case, why do four different implementations, representing two very
> different
> > reference-counting strategies, all maintain a consistent relative
> > performance? What are the scenarios for which boost::shared_ptr is
> > optimised?
> > (iii) There are environment settings for which boost::shared_ptr will
> > perform much better. If that's so, can someone help me out with
> suggestions
> > of what defines to make? Why are these settings not the default.
> >
> > I'd be very interested in your thoughts.
> >
> > Many thanks in advance.
> >
>
>     Have you tried getting the latest shared_ptr.hpp and
> detail/shared_count.hpp from the CVS files?  The version of shared_ptr in
> release 1.30.2  has a lot of extra overhead due to weak_ptr support.
Every
> time a shared_ptr is copied there is an extra comparison and increment
(and
> later an extra decrement).  The version in the CVS avoids almost all of
this
> extra work.

I've not. That sounds worth trying.

<blush>How would one go about getting the CVS version</blush>

Cheers

Matthew