分配和释放内存 VS。先分配，每次只处理

Question

我正在尝试编写机器人硬件的串行通信部分。我有一个台式电脑主板，该程序没有内存限制 (8GB RAM, i3 cpu etc.)，该程序通过 USB(Fullspeed) 或串口以 115200 波特率与微控制器通信。我对我的问题很小感到困惑。我有 20 到 30 种方法，他们正在使用这种通信功能。

哪个处理速度快更有效？此函数只有一个实例同时运行。

先定义，每次使用；

...
private:
    struct timespec ctv1, ctv2;
    double time_diff;
    int serial_write_ret;
    int ret_val;
...
int MSerial::genAndSend_setInt32Command() 
{
    genSum ( stm_buf_t );
    sem_wait ( &serial_mutex );
    // someFunctions();
    sem_post ( &serial_mutex );
    return ret_val;
}

或者每次分配和释放；

int MSerial::genAndSend_setInt32Command()
{
    genSum ( stm_buf_t );
    struct timespec ctv1, ctv2;
    double time_diff = .0;
    int serial_write_ret;
    int ret_val = TIMEOUT_ERROR_IN_SERIAL;

    sem_wait ( &serial_mutex );
    // someFunction();
    sem_post ( &serial_mutex );
    return ret_val;
}

这种差异真的很重要吗？

Answer 1

我的懒惰...

这是 80 * 60 * 5 (Hz x 秒 x 分钟):

周期的结果

Process only:: mean: 0.00356445 in seconds
Alloc Dealloc:: mean: 0.0743379 in seconds

这是代码和冗余输出：

分配 - 每次都取消分配

class AllocEvery
{
public:

    int doSomething()
    {
        double pi, gold, ogh;
        std::string den_rit, jobs, bill;
        char c_str[64];

        pi   = 3.1415926535;
        gold = 1.6180339887;
        ogh  = 0.0000000033;
        ogh += pi;
        ogh += gold;
        jobs = "Being the richest man in the cemetery doesn't matter to me. Going to bed at night saying we've done something wonderful, that's what matters to me.";
        bill = "Your most unhappy customers are your greatest source of learning.";
        den_rit = "UNIX is basically a simple operating system, but you have to be a genius to understand the simplicity.";
    }
};

仅处理：

class ProcessOnly
{
public:

    double pi, gold, ogh;
    std::string den_rit, jobs, bill;
    char c_str[64];

    int doSomething()
    {
        pi   = 3.1415926535;
        gold = 1.6180339887;
        ogh  = 0.0000000033;
        ogh += pi;
        ogh += gold;
        jobs = "Being the richest man in the cemetery doesn't matter to me. Going to bed at night saying we've done something wonderful, that's what matters to me.";
        bill = "Your most unhappy customers are your greatest source of learning.";
        den_rit = "UNIX is basically a simple operating system, but you have to be a genius to understand the simplicity.";
    }
};

和主要

int main ( int argc, char **argv )
{
    int max = 80 * 60 * 5; // Rate * Seconds * Minutes
    struct timespec time1, time2;
    double time_diff = .0;
    AllocEvery obj; /// ProcessOnly obj;

    clock_gettime ( CLOCK_MONOTONIC, &time1 );

    for (int i = 0; i < max; i++)
    {
        obj.doSomething();
    }

    clock_gettime ( CLOCK_MONOTONIC, &time2 );

    time_diff = time2.tv_sec - time1.tv_sec + ( time2.tv_nsec - time1.tv_nsec ) / BILLION;

    std::cout << "Process only:: Elapsed time: " << time_diff << std::endl;

return 0;
}

以及不同运行的输出：

ogh@ubuntu:~/C_Cpp_Examples/tryspeed_cpp$ ./a.out
------------> Alloc - Dealloc Everytime:: Elapsed time: 0.075384
ogh@ubuntu:~/C_Cpp_Examples/tryspeed_cpp$ ./a.out
------------> Alloc - Dealloc Everytime:: Elapsed time: 0.0741677
ogh@ubuntu:~/C_Cpp_Examples/tryspeed_cpp$ ./a.out
------------> Alloc - Dealloc Everytime:: Elapsed time: 0.074426
ogh@ubuntu:~/C_Cpp_Examples/tryspeed_cpp$ ./a.out
------------> Alloc - Dealloc Everytime:: Elapsed time: 0.0740817
ogh@ubuntu:~/C_Cpp_Examples/tryspeed_cpp$ ./a.out
------------> Alloc - Dealloc Everytime:: Elapsed time: 0.0734898
ogh@ubuntu:~/C_Cpp_Examples/tryspeed_cpp$ ./a.out
------------> Alloc - Dealloc Everytime:: Elapsed time: 0.0747045
ogh@ubuntu:~/C_Cpp_Examples/tryspeed_cpp$ ./a.out
------------> Alloc - Dealloc Everytime:: Elapsed time: 0.0727975
ogh@ubuntu:~/C_Cpp_Examples/tryspeed_cpp$ ./a.out
------------> Alloc - Dealloc Everytime:: Elapsed time: 0.0772903
ogh@ubuntu:~/C_Cpp_Examples/tryspeed_cpp$ ./a.out
------------> Alloc - Dealloc Everytime:: Elapsed time: 0.0726992
ogh@ubuntu:~/C_Cpp_Examples/tryspeed_cpp$ ./p.out
------------>  Process only:: Elapsed time: 0.00806864
ogh@ubuntu:~/C_Cpp_Examples/tryspeed_cpp$ ./p.out
------------>  Process only:: Elapsed time: 0.00727956
ogh@ubuntu:~/C_Cpp_Examples/tryspeed_cpp$ ./p.out
------------>  Process only:: Elapsed time: 0.00202144
ogh@ubuntu:~/C_Cpp_Examples/tryspeed_cpp$ ./p.out
------------>  Process only:: Elapsed time: 0.00195636
ogh@ubuntu:~/C_Cpp_Examples/tryspeed_cpp$ ./p.out
------------>  Process only:: Elapsed time: 0.00203696
ogh@ubuntu:~/C_Cpp_Examples/tryspeed_cpp$ ./p.out
------------>  Process only:: Elapsed time: 0.00387936
ogh@ubuntu:~/C_Cpp_Examples/tryspeed_cpp$ ./p.out
------------>  Process only:: Elapsed time: 0.00276425
ogh@ubuntu:~/C_Cpp_Examples/tryspeed_cpp$ ./p.out
------------>  Process only:: Elapsed time: 0.00200299
ogh@ubuntu:~/C_Cpp_Examples/tryspeed_cpp$ ./p.out
------------>  Process only:: Elapsed time: 0.00207049

分配和释放内存 VS。先分配，每次只处理

Allocate and free memory VS. allocate first, process only for each time

performance

dynamic-memory-allocation

serial-communication

usbserial