Is clock_gettime() adequate for submicrosecond timing?

Question

I need a high-resolution timer for the embedded profiler in the Linux build of our application. Our profiler measures scopes as small as individual functions, so it needs a timer precision of better than 25 nanoseconds.

Previously our implementation used inline assembly and the rdtsc operation to query the high-frequency timer from the CPU directly, but this is problematic and requires frequent recalibration.

So I tried using the clock_gettime function instead to query CLOCK_PROCESS_CPUTIME_ID. The docs allege this gives me nanosecond timing, but I found that the overhead of a single call to clock_gettime() was over 250ns. That makes it impossible to time events 100ns long, and having such high overhead on the timer function seriously drags down app performance, distorting the profiles beyond value. (We have hundreds of thousands of profiling nodes per second.)

Is there a way to call clock_gettime() that has less than ¼μs overhead? Or is there some other way that I can reliably get the timestamp counter with <25ns overhead? Or am I stuck with using rdtsc?

Below is the code I used to time clock_gettime().

// calls gettimeofday() to return wall-clock time in seconds:
extern double Get_FloatTime();
enum { TESTRUNS = 1024*1024*4 };

// time the high-frequency timer against the wall clock
{
    double fa = Get_FloatTime();
    timespec spec; 
    clock_getres( CLOCK_PROCESS_CPUTIME_ID, &spec );
    printf("CLOCK_PROCESS_CPUTIME_ID resolution: %ld sec %ld nano\n", 
            spec.tv_sec, spec.tv_nsec );
    for ( int i = 0 ; i < TESTRUNS ; ++ i )
    {
        clock_gettime( CLOCK_PROCESS_CPUTIME_ID, &spec );
    }
    double fb = Get_FloatTime();
    printf( "clock_gettime %d iterations : %.6f msec %.3f microsec / call\n",
        TESTRUNS, ( fb - fa ) * 1000.0, (( fb - fa ) * 1000000.0) / TESTRUNS );
}
// and so on for CLOCK_MONOTONIC, CLOCK_REALTIME, CLOCK_THREAD_CPUTIME_ID.

Results:

CLOCK_PROCESS_CPUTIME_ID resolution: 0 sec 1 nano
clock_gettime 8388608 iterations : 3115.784947 msec 0.371 microsec / call
CLOCK_MONOTONIC resolution: 0 sec 1 nano
clock_gettime 8388608 iterations : 2505.122119 msec 0.299 microsec / call
CLOCK_REALTIME resolution: 0 sec 1 nano
clock_gettime 8388608 iterations : 2456.186031 msec 0.293 microsec / call
CLOCK_THREAD_CPUTIME_ID resolution: 0 sec 1 nano
clock_gettime 8388608 iterations : 2956.633930 msec 0.352 microsec / call

This is on a standard Ubuntu kernel. The app is a port of a Windows app (where our rdtsc inline assembly works just fine).

Addendum:

Does x86-64 GCC have some intrinsic equivalent to __rdtsc(), so I can at least avoid inline assembly?

Answer 1

No. You'll have to use platform-specific code to do it. On x86 and x86-64, you can use 'rdtsc' to read the Time Stamp Counter.

Just port the rdtsc assembly you're using.

__inline__ uint64_t rdtsc(void) {
  uint32_t lo, hi;
  __asm__ __volatile__ (      // serialize
  "xorl %%eax,%%eax \n        cpuid"
  ::: "%rax", "%rbx", "%rcx", "%rdx");
  /* We cannot use "=A", since this would use %rax on x86_64 and return only the lower 32bits of the TSC */
  __asm__ __volatile__ ("rdtsc" : "=a" (lo), "=d" (hi));
  return (uint64_t)hi << 32 | lo;
}

Answer 2

It's hard to give a globally applicable answer because the hardware and software implementation will vary widely.

However, yes, most modern platforms will have a suitable clock_gettime call that is implemented purely in user-space using the VDSO mechanism, and will in my experience take 20 to 30 nanoseconds to complete (but see Wojciech's comment below about contention).

Internally, this is using rdtsc or rdtscp for the fine-grained portion of the time-keeping, plus adjustments to keep this in sync with wall-clock time (depending on the clock you choose) and a multiplication to convert from whatever units rdtsc has on your platform to nanoseconds.

Not all of the clocks offered by clock_gettime will implement this fast method, and it's not always obvious which ones do. Usually CLOCK_MONOTONIC is a good option, but you should test this on your own system.

Answer 3

This is what happens when you call clock_gettime() function.

Based on the clock you choose it will call the respective function. (from vclock_gettime.c file from kernel)

int clock_gettime(clockid_t, struct __kernel_old_timespec *)
    __attribute__((weak, alias("__vdso_clock_gettime")));

notrace int
__vdso_clock_gettime_stick(clockid_t clock, struct __kernel_old_timespec *ts)
{
    struct vvar_data *vvd = get_vvar_data();

switch (clock) {
case CLOCK_REALTIME:
    if (unlikely(vvd->vclock_mode == VCLOCK_NONE))
        break;
    return do_realtime_stick(vvd, ts);
case CLOCK_MONOTONIC:
    if (unlikely(vvd->vclock_mode == VCLOCK_NONE))
        break;
    return do_monotonic_stick(vvd, ts);
case CLOCK_REALTIME_COARSE:
    return do_realtime_coarse(vvd, ts);
case CLOCK_MONOTONIC_COARSE:
    return do_monotonic_coarse(vvd, ts);
}
/*
 * Unknown clock ID ? Fall back to the syscall.
 */
    return vdso_fallback_gettime(clock, ts);
}

CLOCK_MONITONIC better (though I use CLOCK_MONOTONIC_RAW) since it is not affected from NTP time adjustment.

This is how the do_monotonic_stick is implemented inside kernel:

notrace static __always_inline int do_monotonic_stick(struct vvar_data *vvar,
                              struct __kernel_old_timespec *ts)
{
    unsigned long seq;
    u64 ns;

    do {
        seq = vvar_read_begin(vvar);
        ts->tv_sec = vvar->monotonic_time_sec;
        ns = vvar->monotonic_time_snsec;
        ns += vgetsns_stick(vvar);
        ns >>= vvar->clock.shift;
    } while (unlikely(vvar_read_retry(vvar, seq)));

    ts->tv_sec += __iter_div_u64_rem(ns, NSEC_PER_SEC, &ns);
    ts->tv_nsec = ns;

    return 0;
}

And the vgetsns_stick() function which provides nano seconds resolution is implemented as:

notrace static __always_inline u64 vgetsns(struct vvar_data *vvar)
{
    u64 v;
    u64 cycles;

    cycles = vread_tick();
    v = (cycles - vvar->clock.cycle_last) & vvar->clock.mask;
    return v * vvar->clock.mult;
}

Where the function vread_tick() reads the cycles from register based on the CPU:

notrace static __always_inline u64 vread_tick(void)
{
    register unsigned long long ret asm("o4");

    __asm__ __volatile__("rd %%tick, %L0\n\t"
                 "srlx %L0, 32, %H0"
                 : "=r" (ret));
    return ret;
}

A single call to clock_gettime() takes around 20 to 100 nano seconds. reading the rdtsc register and converting the cycles to time is always faster.

I have done some experiment with CLOCK_MONOTONIC_RAW here: Unexpected periodic behaviour of an ultra low latency hard real time multi threaded x86 code

Answer 4

Give clockid_t CLOCK_MONOTONIC_RAW a try?

CLOCK_MONOTONIC_RAW (since Linux 2.6.28; Linux-specific) Similar to CLOCK_MONOTONIC, but provides access to a raw hardware-based time that is not subject to NTP adjustments or the incremental adjustments performed by adjtime(3).

From Man7.org

Answer 5

I need a high-resolution timer for the embedded profiler in the Linux build of our application. Our profiler measures scopes as small as individual functions, so it needs a timer precision of better than 25 nanoseconds.

Have you considered oprofile or perf? You can use the performance counter hardware on your CPU to get profiling data without adding instrumentation to the code itself. You can see data per-function, or even per-line-of-code. The "only" drawback is that it won't measure wall clock time consumed, it will measure CPU time consumed, so it's not appropriate for all investigations.

Answer 6

You are calling clock_getttime with control parameter which means the api is branching through if-else tree to see what kind of time you want. I know you cant't avoid that with this call, but see if you can dig into the system code and call what the kernal is eventually calling directly. Also, I note that you are including the loop time (i++, and conditional branch).