Почему таймер libuv дрейфует?

Пытаясь реализовать простой таймер с использованием libuv, я заметил, что обратный вызов «дрейфует». Я делаю что-то не так здесь?

Код:

uv_timer_t timer;
uv_timer_init(uv_default_loop(), &timer);

double ts = std::chrono::high_resolution_clock::now().time_since_epoch().count();
timer.data = &ts;

uv_timer_start(&timer,
[](uv_timer_t* handle) {
double* before = (double*)handle->data;
double now =
std::chrono::high_resolution_clock::now().time_since_epoch().count();
auto diff = now - (*before);
is::log::info("tick... diff={}ns", diff);
*before = now;
},
1000, 1000);

uv_run(uv_default_loop(), UV_RUN_DEFAULT);

Выходные данные [libuv]: (разница всегда больше 1,0 с)

[info][10:48:26:636439] tick... diff=1.00083e+09ns
[info][10:48:27:637071] tick... diff=1.00063e+09ns
[info][10:48:28:638155] tick... diff=1.00108e+09ns
[info][10:48:29:639219] tick... diff=1.00106e+09ns
[info][10:48:30:640291] tick... diff=1.00107e+09ns
[info][10:48:31:641364] tick... diff=1.00107e+09ns
[info][10:48:32:641457] tick... diff=1.00009e+09ns
[info][10:48:33:642468] tick... diff=1.00101e+09ns
[info][10:48:34:643621] tick... diff=1.00115e+09ns
[info][10:48:35:644701] tick... diff=1.00108e+09ns

Эквивалентный код libev работает отлично.

Код:

ev_timer timer;

double ts = std::chrono::high_resolution_clock::now().time_since_epoch().count();
timer.data = &ts;

ev_timer_init(&timer,
[](struct ev_loop*, ev_timer* handle, int) {
double* before = (double*)handle->data;
double now =
std::chrono::high_resolution_clock::now().time_since_epoch().count();
auto diff = now - (*before);
is::log::info("tick... diff={}ns", diff);
*before = now;
},
1.0, 1.0);

ev_timer_start(EV_DEFAULT, &timer);
ev_run(EV_DEFAULT, 0);

Выходные данные [libev]:

[info][10:54:01:624788] tick... diff=1.00015e+09ns
[info][10:54:02:624943] tick... diff=1.00016e+09ns
[info][10:54:03:625035] tick... diff=1.00009e+09ns
[info][10:54:04:625177] tick... diff=1.00014e+09ns
[info][10:54:05:624284] tick... diff=9.99106e+08ns
[info][10:54:06:624415] tick... diff=1.00013e+09ns
[info][10:54:07:624533] tick... diff=1.00012e+09ns
[info][10:54:08:624592] tick... diff=1.00006e+09ns
[info][10:54:09:625245] tick... diff=1.00065e+09ns
[info][10:54:10:624331] tick... diff=9.99086e+08ns

Решение

Реализована собственная логика компенсации таймера, которая запускает новый таймер с «правильным» таймаутом на каждой итерации.

using namespace std::chrono;

struct Timer {
uv_loop_t* loop;
uv_timer_t timer;
uint64_t period;
time_point<high_resolution_clock> ref;

Timer(uv_loop_t* loop, uint64_t period)
: loop(loop), period(period), ref(high_resolution_clock::now() + milliseconds(period)) {
uv_timer_init(loop, &timer);
timer.data = this;
uv_timer_start(&timer, callback, period, 0);
}

static void callback(uv_timer_t* handle) {
auto self = (Timer*)handle->data;
{
auto delta = self->ref - high_resolution_clock::now();
auto delta_count = duration_cast<milliseconds>(delta).count();
is::log::info("tick... delta={}ms", delta_count);
}

{
self->ref = self->ref + milliseconds(self->period);
auto delta = self->ref - high_resolution_clock::now();
auto delta_count = duration_cast<milliseconds>(delta).count();
uv_timer_start(&self->timer, self->callback, delta_count, 0);
}
}
};

Timer timer(uv_default_loop(), 1000);
uv_run(uv_default_loop(), UV_RUN_DEFAULT);

Выход:

[info][14:20:19:772354] tick... delta=-1ms
[info][14:20:20:771120] tick... delta=0ms
[info][14:20:21:772235] tick... delta=-1ms
[info][14:20:22:771040] tick... delta=0ms
[info][14:20:23:772174] tick... delta=0ms
[info][14:20:24:771302] tick... delta=0ms
[info][14:20:25:771448] tick... delta=0ms
[info][14:20:26:771568] tick... delta=0ms
[info][14:20:27:771117] tick... delta=0ms
[info][14:20:28:772250] tick... delta=-1ms
[info][14:20:29:771374] tick... delta=0ms
[info][14:20:30:771495] tick... delta=0ms
[info][14:20:31:771608] tick... delta=0ms
[info][14:20:32:771691] tick... delta=0ms