分散式系統的硬核：時間時鐘問題和演算法

/**
 * Returns the current time in milliseconds. Note that
 * while the unit of time of the return value is a millisecond,
 * the granularity of the value depends on the underlying
 * operating system and may be larger.  For example, many
 * operating systems measure time in units of tens of
 * milliseconds.
 *
 * ...
 *
 * @return  the difference, measured in milliseconds, between
 *          the current time and midnight, January 1, 1970 UTC.
 */
public static native long currentTimeMillis();

jlong os::javaTimeMillis() {
  timeval time;
  int status = gettimeofday(&time, NULL);
  assert(status != -1, "linux error");
  return jlong(time.tv_sec) * 1000  +  jlong(time.tv_usec / 1000);
}

jlong os::javaTimeMillis() {
  timeval time;
  int status = gettimeofday(&time, NULL);
  assert(status != -1, "bsd error");
  return jlong(time.tv_sec) * 1000  +  jlong(time.tv_usec / 1000);
}

NAME
       gettimeofday, settimeofday - get / set time

NOTES
       The time returned by gettimeofday() is affected by discontinuous
       jumps in the system time (e.g., if the system administrator manually
       changes the system time).  If you need a monotonically increasing
       clock, see clock_gettime(2).

jlong os::javaTimeNanos() {
  if (os::supports_monotonic_clock()) {
    struct timespec tp;
    int status = Linux::clock_gettime(CLOCK_MONOTONIC, &tp);
    assert(status == 0, "gettime error");
    jlong result = jlong(tp.tv_sec) * (1000 * 1000 * 1000) + jlong(tp.tv_nsec);
    return result;
  } else {
    timeval time;
    int status = gettimeofday(&time, NULL);
    assert(status != -1, "linux error");
    jlong usecs = jlong(time.tv_sec) * (1000 * 1000) + jlong(time.tv_usec);
    return 1000 * usecs;
  }
}

jlong os::javaTimeNanos() {
  const uint64_t tm = mach_absolute_time();
  const uint64_t now = (tm * Bsd::_timebase_info.numer) / Bsd::_timebase_info.denom;
  const uint64_t prev = Bsd::_max_abstime;
  if (now <= prev) {
    return prev;   // same or retrograde time;
  }
  const uint64_t obsv = Atomic::cmpxchg(now, &Bsd::_max_abstime, prev);
  assert(obsv >= prev, "invariant");   // Monotonicity
  // If the CAS succeeded then we're done and return "now".
  // If the CAS failed and the observed value "obsv" is >= now then
  // we should return "obsv".  If the CAS failed and now > obsv > prv then
  // some other thread raced this thread and installed a new value, in which case
  // we could either (a) retry the entire operation, (b) retry trying to install now
  // or (c) just return obsv.  We use (c).   No loop is required although in some cases
  // we might discard a higher "now" value in deference to a slightly lower but freshly
  // installed obsv value.   That's entirely benign -- it admits no new orderings compared
  // to (a) or (b) -- and greatly reduces coherence traffic.
  // We might also condition (c) on the magnitude of the delta between obsv and now.
  // Avoiding excessive CAS operations to hot RW locations is critical.
  // See https://blogs.oracle.com/dave/entry/cas_and_cache_trivia_invalidate
  return (prev == obsv) ? now : obsv;
}

jlong os::javaTimeMillis() {
  timeval time;
  int status = gettimeofday(&time, NULL);
  assert(status != -1, "linux error");
  return jlong(time.tv_sec) * 1000  +  jlong(time.tv_usec / 1000);
                                                      // ^^ precision loss
}

Instant instant = Clock.systemUTC().instant();
long epochSecond = instant.getEpochSecond();
int nanoSinceSecond = instant.getNano();

@Override
public long millis() {
    return System.currentTimeMillis();
}
@Override
public Instant instant() {
    return Instant.ofEpochMilli(millis());
}

void os::javaTimeSystemUTC(jlong &seconds, jlong &nanos) {
  timeval time;
  int status = gettimeofday(&time, NULL);
  assert(status != -1, "linux error");
  seconds = jlong(time.tv_sec);
  nanos = jlong(time.tv_usec) * 1000;
}

time = time + 1;
send(message, time);

(message, ts) = receive();
time = max(ts, time) + 1;

V[myId] = V[myId] + 1
send(message, V);

(message, Vr) = receive();
for (i, v) in Vr {
    V[i] = max(V[i], v);
}
V[myId] = V[myId] + 1;[/i][/i]