测试mktime和localtime_r性能及优化方法

// 测试mktime和localtime_r性能及优化方法
//
// 编译方法：g++ -g -o x x.cpp或g++ -O2 -o x x.cpp，两种编译方式性能基本相同。
//
// 结论：
// 1) 环境变量TZ和isdst均不影响localtime_r的性能（第一次调用了除外）
// 2) 环境变量TZ严重影响localtime的性能
// 3) 环境变量TZ和isdst均会严重影响mktime的性能
// *4) 注意mktime的参数即是输入参数也是输出参数，它会修改isdst值
// *5) 另外需要注意localtime_r为非信号安全函数，
//     不能在信号处理过程中调用，否则可能发生死锁等问题
//
// 64位机器性能数据（与32位CPU不同）：
/*
$ ./x 1000000
test: localtime ...
TZ is NULL: 2457ms
TZ is empty: 172ms
TZ is Asia/Shanghai: 173ms

test: localtime_r ...
TZ is NULL and isdst=1: 125ms
TZ is NULL and isdst=0: 125ms
TZ is NULL and isdst=-1: 125ms
TZ is NULL and isdst undefined: 125ms
TZ is empty and isdst=1: 125ms
TZ is empty and isdst=0: 125ms
TZ is empty and isdst=-1: 125ms
TZ is empty and isdst undefined: 127ms
TZ is Asia/Shanghai and isdst=1: 126ms
TZ is Asia/Shanghai and isdst=0: 125ms
TZ is Asia/Shanghai and isdst=-1: 125ms
TZ is Asia/Shanghai and isdst undefined: 125ms

test: mktime ...
TZ is NULL and isdst=1: 635841ms
TZ is NULL and isdst=0: 2583ms
TZ is NULL and isdst=-1: 2596ms
TZ is NULL and isdst undefined: 2579ms
TZ is empty and isdst=1: 122377ms
TZ is empty and isdst=0: 229ms
TZ is empty and isdst=-1: 230ms
TZ is empty and isdst undefined: 229ms
TZ is Asia/Shanghai and isdst=1: 122536ms
TZ is Asia/Shanghai and isdst=0: 228ms
TZ is Asia/Shanghai and isdst=-1: 230ms
TZ is Asia/Shanghai and isdst undefined: 228ms
*/

// 32位机器性能数据（与64位CPU不同）：
/*
$ ./x 1000000
test: localtime ...
TZ is NULL: 1445ms
TZ is empty: 252ms
TZ is Asia/Shanghai: 252ms

test: localtime_r ...
TZ is NULL and isdst=1: 161ms
TZ is NULL and isdst=0: 160ms
TZ is NULL and isdst=-1: 161ms
TZ is NULL and isdst undefined: 161ms
TZ is empty and isdst=1: 160ms
TZ is empty and isdst=0: 161ms
TZ is empty and isdst=-1: 161ms
TZ is empty and isdst undefined: 161ms
TZ is Asia/Shanghai and isdst=1: 161ms
TZ is Asia/Shanghai and isdst=0: 161ms
TZ is Asia/Shanghai and isdst=-1: 161ms
TZ is Asia/Shanghai and isdst undefined: 161ms

test: mktime ...
TZ is NULL and isdst=1: 199375ms
TZ is NULL and isdst=0: 1488ms
TZ is NULL and isdst=-1: 1483ms
TZ is NULL and isdst undefined: 1497ms
TZ is empty and isdst=1: 161057ms
TZ is empty and isdst=0: 325ms
TZ is empty and isdst=-1: 328ms
TZ is empty and isdst undefined: 326ms
TZ is Asia/Shanghai and isdst=1: 161558ms
TZ is Asia/Shanghai and isdst=0: 321ms
TZ is Asia/Shanghai and isdst=-1: 335ms
TZ is Asia/Shanghai and isdst undefined: 328ms
*/
// localtime_r相关源代码：
/*
// The C Standard says that localtime and gmtime return the same pointer.
struct tm _tmbuf; // 全局变量

struct tm * __localtime_r (t, tp)
     const time_t *t;
     struct tm *tp;
{
  return __tz_convert (t, 1, tp);
}

// 非线程安全版本，用到了全局变量_tmbuf
struct tm * localtime(t)
     const time_t *t;
{
  return __tz_convert (t, 1, &_tmbuf);
}

struct tm * __tz_convert (const time_t *timer, int use_localtime, struct tm *tp)
{
  。。。
  // 信号处理函数中调用非信号安全函数，可能造成死锁的地方
  __libc_lock_lock (tzset_lock);
  
  // localtime_r未用到_tmbuf，只是localtime使用它！！！
  // 因此对于localtime_r，传递给tzset_internal的第一个参数总是为0（tp != &_tmpbuf），
  // 而对于localtime，它传递给tzset_internal的第一个参数总是为1
  tzset_internal (tp == &_tmbuf && use_localtime, 1);
  。。。
}

// 决定性能的函数，原因是可能涉及文件操作，
// 因此要想提升性能，则应当想办法避免操作文件！！！
static void internal_function
tzset_internal (always, explicit)
     int always;
     int explicit;
{
  static int is_initialized; // 静态变量
  const char *tz;

  // 对于mktime，参数always值总是为1
  // 对于localtime，参数always值总是为1
  // 对于localtime_r，参数always值总是为0    
  if (is_initialized && !always)
    return; // 对于localtime_r第一次调用后，后续都在这里直接返回！
  is_initialized = 1;

  tz = getenv ("TZ");
  if (tz == NULL && !explicit)
    tz = TZDEFAULT;
  if (tz && *tz == '\0')
    tz = "Universal";
  if (tz && *tz == ':')
    ++tz;

  // 如果不设置环境变量TZ，则下面这个if语句总是不成立！！！
  // 因此只有设置了环境变量TZ，才有可能在这里直接返回而不进入读文件操作__tzfile_read
  if (old_tz != NULL && tz != NULL && strcmp (tz, old_tz) == 0)
    return; // 在这里返回则可以避免走到文件操作__tzfile_read
  if (tz == NULL)
    tz = TZDEFAULT;

  tz_rules[0].name = NULL;
  tz_rules[1].name = NULL;

  // Save the value of `tz'.
  free (old_tz);
  old_tz = tz ? __strdup (tz) : NULL;

  // 读文件，性能慢的原因  
  __tzfile_read (tz, 0, NULL); // Try to read a data file.
  if (__use_tzfile)
    return;
  。。。
}
*/
// mktime相关源代码：
/*
time_t mktime (struct tm *tp)
{
#ifdef _LIBC
  // POSIX.1 8.1.1 requires that whenever mktime() is called, the
  //   time zone names contained in the external variable 'tzname' shall
  //   be set as if the tzset() function had been called.
  __tzset ();
#endif

  // __mktime_internal会调用localtime_r，
  // isdst的取值在这里会严重影响到mktime的性能
  return __mktime_internal (tp, __localtime_r, &localtime_offset);
}

void __tzset (void)
{
  __libc_lock_lock (tzset_lock);

  // 和localtime_r一样也会调用tzset_internal
  tzset_internal (1, 1);

  if (!__use_tzfile)
    {
      // Set `tzname'.
      __tzname[0] = (char *) tz_rules[0].name;
      __tzname[1] = (char *) tz_rules[1].name;
    }

  __libc_lock_unlock (tzset_lock);
}
*/
#include 
#include 
#include 
#include 
#include 

static void test_localtime(int M); // 测试localtime性能
static void test_localtime_r(int M); // 测试localtime_r性能
static void test_mktime(int M); // 测试mktime性能

int main(int argc, char* argv[])
{
    const int M = (argc<2)? 1000000: atoi(argv[1]);

    test_localtime(M);
    printf("\n");
    test_localtime_r(M);
    printf("\n");
    test_mktime(M);

    return 0;
}

// test_localtime
void test_localtime(int M)
{
    int i;
    time_t now = time(NULL);
    struct timeval tv1, tv2;

    printf("test: localtime ...\n");
    unsetenv("TZ");

    // test1
    {
        struct tm* result1;
        gettimeofday(&tv1, NULL);
        for (i=0; i