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

Subject: Re: [Boost-users] boost::function performance issues with C++11
From: ·¹Í° (athrun911500_at_[hidden])
Date: 2014-07-04 19:28:03

Right, there are 1000 integers in the vector. If something, say STL, is bottleneck for boost::bind with C++ 11, it should also be a bottleneck without C++11. And they should have the same performance. However, the fact shows that C++11 mode has some additional overhead. I wanna figure that out. 

Thanks
Athrun

At 2014-07-04 11:10:28, "Michael Powell" <mwpowellhtx_at_[hidden]> wrote:
>On Thu, Jul 3, 2014 at 7:23 PM, ·¹Í° <athrun911500_at_[hidden]> wrote:
>> Hi Michael,
>>
>> Thanks for your suggestion. I understand that using a reference or a pointer
>> will achieve better performance, and I really appreciate you pointed out
>> that for me.
>> But I intend to do this to highlight my point, that the performance are
>> quite different with or without -std=c++11. And I'm really curious about
>> that.
>>
>> Consider a situiation, that I'm using boost::function. However, the
>> performance could be seriously affected by using a compiling flag
>> (-std=c++11). That will be very weired to me, unless someone can tell me
>> what happens there.
>
>Well, you're testing potentially two things: STL performance, or
>std::bind performance. Couldn't tell you which is the bottleneck, but
>neither in my mind are trivial depending how many elements are in your
>vector.
>
>> BTW, when I say default, I mean release mode.
>> Thanks
>> Xuepeng
>>
>>
>>
>>
>>
>>
>> At 2014-07-04 07:55:39, "Michael Powell" <mwpowellhtx_at_[hidden]> wrote:
>>>On Thu, Jul 3, 2014 at 4:10 PM, ·¹Í° <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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>>
>>
>>
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