Re: [Boost-users] boost::function performance issues with C++11

Subject: Re: [Boost-users] boost::function performance issues with C++11
From: Michael Powell (mwpowellhtx_at_[hidden])
Date: 2014-07-03 19:55:39

On Thu, Jul 3, 2014 at 4:10 PM, 饭桶 <athrun911500_at_[hidden]> wrote:
> Hi,
>
> Does anyone know why -std=c++11 causes so much difference on
> boost::function?
>
> I was planed to understand if there any performance issues with big size of
> parameters.
> So I wrote a function that takes a vector as parameter, like func2 shows. I
> know it's better to use a pointer or reference as function parameter. I just
> want to evaluate the performance, so let's keep the vector as parameter.
>
> However, I found that it's quite slow when compiled with -std=c++11. In
> detail, it takes 173874 milliseconds with C++11, while it takes 3676 seconds
> without C++11.
>
> About 50 times slower!! How can that be?
>
> In my opinion, I thought boost::function should had the same performance
> with std::function. So I decided to try std::function in C++11 directly,
> Finally, it takes about 29233 milliseconds. That's till 8 times slows!
>
> Can anyone tell me what happend here?

I don't know the inner workings of either boost::function or
std::function. It's not boost's fault per se, but there are a couple
of things you could do differently.

> int func2(std::vector<int> i){
>     total += i.size();
>     return i.size();
> }

Pass a reference or even a pointer instead of the whole vector. You
are copying the vector every time.

>     const int T = 1000000;
>     s = boost::chrono::system_clock::now();
>     for (int i = 0; i < T; ++i)
>         call(boost::bind(&func2, v));
>     e = boost::chrono::system_clock::now();

You are also binding func2 every time; not sure if that's getting
optimized or not.

You likely want to bind with a placeholder instead of the vector
itself, then call the binding itself. i.e.

auto binding = boost::bind(&func2, _1);
binding(v);

I do that frequently enough; and with more event-driven systems,
boost::signals, etc, it is unavoidable. You want the placeholder
instead, once-bound/later-called.

> In case you need to know my enviorment, my OS is Arch, compiler is gcc
> 4.9.0, and optimizations are default.

Default can mean a lot of things. Debug or release mode? Beyond those
two broad categories, do verify your settings and build for release
mode.

HTH

> The execution time (ms) of three versions I tried:
> boost::function with C++11 : 173874
> boost::function without C++11 : 3676
> std::function in C++11 : 29233
>
> Any thoughts are appreciated!
> Thanks,
> Athrun
>
>
>
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