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@163.com>: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.ThanksXuepeng
At 2014-07-04 07:55:39, "Michael Powell" <mwpowellhtx@gmail.com> wrote: >On Thu, Jul 3, 2014 at 4:10 PM, 饭桶 <athrun911500@163.com> 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 >> >> >> >> _______________________________________________ >> Boost-users mailing list >> Boost-users@lists.boost.org >> http://lists.boost.org/mailman/listinfo.cgi/boost-users >_______________________________________________ >Boost-users mailing list >Boost-users@lists.boost.org >http://lists.boost.org/mailman/listinfo.cgi/boost-users
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