系统调用开放开销的变化

Question

我正在尝试使用读取时间戳计数器

的代码对开放系统调用进行基准测试

Wrong identity. 
Array
(
    [cc] => 92
    [in] => 1234566
    [lg] => en
    [lc] => PK
    [id] => >zv?+?????w!l?v?5b?
    [mistyped] => 6
    [network_radio_type] => 1
    [simnum] => 1
    [s] => 
    [copiedrc] => 1
    [hasinrc] => 1
    [rcmatch] => 1
    [pid] => 579
    [extexist] => 1
    [extstate] => 1
)
stdClass Object
(
    [status] => fail
    [reason] => incorrect
    [sms_length] => 6
    [voice_length] => 6
    [sms_wait] => 0
    [voice_wait] => 0
)

使用的函数具有以下定义

    overhead = measure_tsc_overhead();
    int i;
        unsigned long t0;

        synch_tsc();
        t0 = rdtscll();
        input_fd = open ("input.txt", O_RDONLY);
        if (input_fd == -1) {
                perror ("open");
                return 2;
        }
     printf("open syscall overhead is %ld\n",(rdtscllp() - t0 - overhead)); 

我正在为不同的运行获得以下开销

static inline void synch_tsc(void)
{
    asm volatile("cpuid" : : : "%rax", "%rbx", "%rcx", "%rdx");
}

static inline unsigned long rdtscll(void)
{
    unsigned int a, d;
    asm volatile("rdtsc" : "=a" (a), "=d" (d) : : "%rbx", "%rcx");
    return ((unsigned long) a) | (((unsigned long) d) << 32);
}

static inline unsigned long rdtscllp(void)
{
    unsigned int a, d;
    asm volatile("rdtscp" : "=a" (a), "=d" (d) : : "%rbx", "%rcx");
    return ((unsigned long) a) | (((unsigned long) d) << 32);
}

static unsigned long measure_tsc_overhead(void)
{
    unsigned long t0, t1, overhead = ~0UL;
    int i;

    for (i = 0; i < N; i++) {
        t0 = rdtscll();
        asm volatile("");
        t1 = rdtscllp();
        if (t1 - t0 < overhead)
            overhead = t1 - t0;
    }

    return overhead;
}

我担心的是管理费用的巨大差异。

这种变化的原因是什么？

如果这种变化是正常的（可能是由于某些缓存），哪个值代表我的实际结果？