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

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 :

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" wrote:

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On Thu, Jul 3, 2014 at 4:10 PM, 饭桶 wrote:

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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.

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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.

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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.

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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

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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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