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Comment by dheera | Hacker News Reader

Why?

    low bits + CPU cycle counter

is enough. No need of the hash()

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You don't know precisely at which frequency the cycle counter runs. Depending on the system load it might either run faster or slower than the lowest bits the HPTC. For what it's worth this part is more or less nondeterministic, so the sane thing to do, is spread out the information as much as possible (maximize entropy), in order to minimize the probability of collisions.

That's ok, the bits past the low bits are just there to avoid collisions, not an actual measure of high precision time beyond the low bits.

It's not worse than the hash solution, I'm just saying it's not necessary to hash it if the only objective is to reduce collisions.

In fact the hashing solution, if it is replacing the low bits with a hash of low bits plus something else, is actually destroying valuable time information.

That only works, if you know exactly, that the low bits are constant. Otherwise you may run into the issue that due to unsteady rate of RDTSC between two processes/threads that may be preemptively unscheduled between reading the HPTC and the RDTSC you might again end up with colliding time stamps. And if you took the derivative between timestamps taken in succession, you might even find is to be non-monotonic in places.

The combination of multiple counters incremented by individual unsteady clocks used to be a source for pseudo random scrambler sequences; these days we prefer LFSRs, but overall this is something that can be weird.

Hence my recommendation: Just throw xxHash32 on concatenation of the HPTC's low bits and the CPU clock cycle counter, and forgo any pretense of monotony in the low bits (because very likely you don't have it anyway).

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