A new kernel timer API

Posted May 20, 2005 23:51 UTC (Fri) by giraffedata (guest, #1954)
In reply to: A new kernel timer API by hamjudo
Parent article: A new kernel timer API

But you got your units confused. The smallest power of ten unit that fits is the nanosecond. The smallest power of 2 unit is 2^-32 seconds, which is about 250 picoseconds.

You're right that brouhaha confused the units (he said 4 seconds worth of picoseconds fit in 32 bits; it's really 4 seconds worth of nanoseconds). But you're introducing a "fits" that nobody's talking about here -- apparently you're intending for 32 bits to count one second.

Note that the interface provides multiple granularities/ranges from which to choose. You can specify your interval in milliseconds, microseconds, or nanoseconds. That in no way means you can actually get that kind of precision out of the timer. There's really no reason not to throw in picoseconds, if only to save having to answer the question.

Why not units of 2^-32 seconds?

Posted May 23, 2005 16:25 UTC (Mon) by spitzak (guest, #4593) [Link] (1 responses)

That actually sounds like a useful and natural unit to use, and certainly easier for programmers to remember. It would allow the upper 32 bits to be equal to the Unix clock and allow conversion to a floating-point number of seconds without rounding errors.

Is there some good reason why a power of ten should be used? Is it because of rounding errors from times specified in decimal numbers of seconds?

Why not units of 2^-32 seconds?

Posted May 24, 2005 2:40 UTC (Tue) by giraffedata (guest, #1954) [Link]

Natural for whom? The computer?

Remember that we're talking about an external interface here -- the question is in what units would a user of the timer facility want to specify a duration? Virtually nobody measures time in binary units; we all think of time in milliseconds, nanoseconds, etc.

The Unix time_t type (which I think is what you're referring to as the Unix clock) doesn't actually figure in anywhere here -- this is a value that specifies a duration, not a point in time; and if it ever gets added to a point in time, that time is in the kernel internal format, which is a count of clock ticks.