var ticks ticksType
type ticksType struct {
- lock mutex
- val atomic.Int64
+ // lock protects access to start* and val.
+ lock mutex
+ startTicks int64
+ startTime int64
+ val atomic.Int64
}
-// Note: Called by runtime/pprof in addition to runtime code.
-func tickspersecond() int64 {
+// init initializes ticks to maximize the chance that we have a good ticksPerSecond reference.
+//
+// Must not run concurrently with ticksPerSecond.
+func (t *ticksType) init() {
+ lock(&ticks.lock)
+ t.startTime = nanotime()
+ t.startTicks = cputicks()
+ unlock(&ticks.lock)
+}
+
+// minTimeForTicksPerSecond is the minimum elapsed time we require to consider our ticksPerSecond
+// measurement to be of decent enough quality for profiling.
+//
+// There's a linear relationship here between minimum time and error from the true value.
+// The error from the true ticks-per-second in a linux/amd64 VM seems to be:
+// - 1 ms -> ~0.02% error
+// - 5 ms -> ~0.004% error
+// - 10 ms -> ~0.002% error
+// - 50 ms -> ~0.0003% error
+// - 100 ms -> ~0.0001% error
+//
+// We're willing to take 0.004% error here, because ticksPerSecond is intended to be used for
+// converting durations, not timestamps. Durations are usually going to be much larger, and so
+// the tiny error doesn't matter. The error is definitely going to be a problem when trying to
+// use this for timestamps, as it'll make those timestamps much less likely to line up.
+const minTimeForTicksPerSecond = 5_000_000*(1-osHasLowResClockInt) + 100_000_000*osHasLowResClockInt
+
+// ticksPerSecond returns a conversion rate between the cputicks clock and the nanotime clock.
+//
+// Note: Clocks are hard. Using this as an actual conversion rate for timestamps is ill-advised
+// and should be avoided when possible. Use only for durations, where a tiny error term isn't going
+// to make a meaningful difference in even a 1ms duration. If an accurate timestamp is needed,
+// use nanotime instead. (The entire Windows platform is a broad exception to this rule, where nanotime
+// produces timestamps on such a coarse granularity that the error from this conversion is actually
+// preferable.)
+//
+// The strategy for computing the conversion rate is to write down nanotime and cputicks as
+// early in process startup as possible. From then, we just need to wait until we get values
+// from nanotime that we can use (some platforms have a really coarse system time granularity).
+// We require some amount of time to pass to ensure that the conversion rate is fairly accurate
+// in aggregate. But because we compute this rate lazily, there's a pretty good chance a decent
+// amount of time has passed by the time we get here.
+//
+// Must be called from a normal goroutine context (running regular goroutine with a P).
+//
+// Called by runtime/pprof in addition to runtime code.
+//
+// TODO(mknyszek): This doesn't account for things like CPU frequency scaling. Consider
+// a more sophisticated and general approach in the future.
+func ticksPerSecond() int64 {
+ // Get the conversion rate if we've already computed it.
r := ticks.val.Load()
if r != 0 {
return r
}
- lock(&ticks.lock)
- r = ticks.val.Load()
- if r == 0 {
- t0 := nanotime()
- c0 := cputicks()
- usleep(100 * 1000)
- t1 := nanotime()
- c1 := cputicks()
- if t1 == t0 {
- t1++
+
+ // Compute the conversion rate.
+ for {
+ lock(&ticks.lock)
+ r = ticks.val.Load()
+ if r != 0 {
+ unlock(&ticks.lock)
+ return r
}
- r = (c1 - c0) * 1000 * 1000 * 1000 / (t1 - t0)
- if r == 0 {
- r++
+
+ // Grab the current time in both clocks.
+ nowTime := nanotime()
+ nowTicks := cputicks()
+
+ // See if we can use these times.
+ if nowTicks > ticks.startTicks && nowTime-ticks.startTime > minTimeForTicksPerSecond {
+ // Perform the calculation with floats. We don't want to risk overflow.
+ r = int64(float64(nowTicks-ticks.startTicks) * 1e9 / float64(nowTime-ticks.startTime))
+ if r == 0 {
+ // Zero is both a sentinel value and it would be bad if callers used this as
+ // a divisor. We tried out best, so just make it 1.
+ r++
+ }
+ ticks.val.Store(r)
+ unlock(&ticks.lock)
+ break
}
- ticks.val.Store(r)
+ unlock(&ticks.lock)
+
+ // Sleep in one millisecond increments until we have a reliable time.
+ timeSleep(1_000_000)
}
- unlock(&ticks.lock)
return r
}