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

Subject: Re: [Boost-users] boost::function performance issues with C++11
From: 饭桶 (athrun911500_at_[hidden])
Date: 2014-07-03 21:22:56

Thanks, that helps. But still there are only about 7 times slower.
Specifically, the performance difference only happens when passing a vector as parameter. I tried passing reference and shared pointer, they are kind of similar to boost::function.

So here is my guess according to your suggestion. Maybe std::function uses copying code that can be easier optimized. But that's just my guess, still need proof.

-Athrun

At 2014-07-04 08:50:00, "Angelo Mondaini" <oangelo_at_[hidden]> wrote:

I guess that with -std=c++11 the compiler may not do all the possible optimizations, since c++11 is "new".
How about go wild and use -O3 as compilation flag?
This will give more freedom to the compiler optimization, and maybe more velocity.

2014-07-03 21:23 GMT-03:00 饭桶 <athrun911500_at_[hidden]>:

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.

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, 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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