On 05Apr2012 13:39, Zooko Wilcox-O'Hearn <
[hidden email]> wrote:
| Good job, Victor Stinner on baking the accumulated knowledge of this
| thread into PEP 418. Even though I'm very interested in the topic, I
| haven't been able to digest the whole thread(s) on the list and
| understand what the current collective understanding is.
There isn't a collective understanding :-) That's why all the noise!
| The detailed
| PEP document helps a lot.
Yes indeed, though like all of us I think it could (always) use more
detail on my pet concerns.
| I think there are still some mistakes, either in our collective
| understanding as reflected by the PEP, or in my own head.
|
| For starters, I still don't understand the first, most basic thing:
| what do people mean when they say "monotonic clock"? I don't
| understand the current text of PEP 418 with regard to the definition
| of that word.
A monotonic clock never returns t0 > t1 for t0, t1 being two adjacent
polls of the clock. On its own it says nothing about steadiness or
correlation with real world time.
_Quality of implementation_ says that the montonic() call should try to
return a close that is monotonic and _also_ steady and preferably
precise. How these things are balancer is a matter of policy.
| Allow me to resort to an analogy. There is an infinitely long,
| perfectly straight and flat racetrack. There is a flag that gets
| dragged along it at a constant rate, with the label "REAL TIME" on the
| flag. There are some runners, each with a different label on their
| chest:
|
| Runner A: a helicopter hovers over Runner A. Occasionally it picks him
| up and plops him down right next to the flag. Also, he wears a headset
| and listens to instructions from his coach to run a little faster or
| slower, as necessary, to remain abreast of the flag.
If he always runs forwards, it is montonic. Not very steady when the
helicopter comes to play.
| Runner B: a helicopter hovers over Runner B. If he is behind the flag,
| it will pick him up and plop him down right next to the flag. However,
| if he is ahead of the flag it will not pick him up.
Seems like runner A without instruction. Monotonic. Not very steady.
| Runner C: no helicopter ever picks up Runner C, but he does wear a
| headset and listens to instructions from his coach to run a little
| faster or a little slower. His coach tells him to run a little faster
| if he is behind the flag or run a little slower if he is in front of
| the flag, with the goal of eventually having him right next to the
| flag.
If he always runs forward, monotonic. And steady.
| Runner D: like Runner C, he never gets picked up, but he listens to
| instructions to run a little faster or a little slower. However,
| instead of telling him to run faster in order to catch up to the flag,
| or to run slower in order to "catch down" to the flag, his coach
| instead tells him to run a little faster if he is moving slower than
| the flag is moving, and to run a little slower if he is moving faster
| than the flag is moving. Note that this is very different from Runner
| C, in that it is not intended to cause him to eventually be right next
| to the flag, and indeed if it is done right it guarantees that he will
| *never* be right next to the flag, although he will be moving just as
| fast as the flag is moving.
|
| Runner E: no helicopter, no headset. He just proceeds at his own pace,
| blissfully unaware of the exhortations of others.
|
| Now: which ones of these five runners do you call "monotonic"? Which
| ones do you call "steady"?
If they all run forwards, they're all monotonic.
If their coach or helicopter can move then _backwards_ they're not
monotonic. If the helicopter can move them an arbitrary (but matching
the game plan) distance, they're not steady. Otherwise they are steady,
if the runner's speed is always sufficiently close to the flag speed
(this threshold and the criteria for measuring it as subject to debate,
forming policy).
And "high resolution" has its own flavours, though generally less
contentious.
Cheers,
--
Cameron Simpson <
[hidden email]> DoD#743
http://www.cskk.ezoshosting.com/cs/If you don't shoot the fish in your barrel, your barrel will soon be
full of fish. - Tim Mefford
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