EWS Hates your patch! :-) > On Nov 4, 2016, at 10:01 AM, Filip Pizlo <fpi...@apple.com> wrote: > > Hi everyone! > > That last time we talked about this, there seemed to be a lot of agreement > that we should go with the Seconds/MonotonicTime/WallTime approach. > > I have implemented it: https://bugs.webkit.org/show_bug.cgi?id=152045 > <https://bugs.webkit.org/show_bug.cgi?id=152045> > > That patch just takes a subset of our time code - all of the stuff that > transitively touches ParkingLot - and converts it to use the new time > classes. Reviews welcome! > > -Filip > > > >> On May 22, 2016, at 6:41 PM, Filip Pizlo <fpi...@apple.com >> <mailto:fpi...@apple.com>> wrote: >> >> Hi everyone! >> >> I’d like us to stop using std::chrono and go back to using doubles for time. >> First I list the things that I think we wanted to get from std::chrono - >> the reasons why we started switching to it in the first place. Then I list >> some disadvantages of std::chrono that we've seen from fixing >> std::chrono-based code. Finally I propose some options for how to use >> doubles for time. >> >> Why we switched to std::chrono >> >> A year ago we started using std::chrono for measuring time. std::chrono has >> a rich typesystem for expressing many different kinds of time. For example, >> you can distinguish between an absolute point in time and a relative time. >> And you can distinguish between different units, like nanoseconds, >> milliseconds, etc. >> >> Before this, we used doubles for time. std::chrono’s advantages over >> doubles are: >> >> Easy to remember what unit is used: We sometimes used doubles for >> milliseconds and sometimes for seconds. std::chrono prevents you from >> getting the two confused. >> >> Easy to remember what kind of clock is used: We sometimes use the monotonic >> clock and sometimes the wall clock (aka the real time clock). Bad things >> would happen if we passed a time measured using the monotonic clock to >> functions that expected time measured using the wall clock, and vice-versa. >> I know that I’ve made this mistake in the past, and it can be painful to >> debug. >> >> In short, std::chrono uses compile-time type checking to catch some bugs. >> >> Disadvantages of using std::chrono >> >> We’ve seen some problems with std::chrono, and I think that the problems >> outweigh the advantages. std::chrono suffers from a heavily templatized API >> that results in template creep in our own internal APIs. std::chrono’s >> default of integers without overflow protection means that math involving >> std::chrono is inherently more dangerous than math involving double. This >> is particularly bad when we use time to speak about timeouts. >> >> Too many templates: std::chrono uses templates heavily. It’s overkill for >> measuring time. This leads to verbosity and template creep throughout >> common algorithms that take time as an argument. For example if we use >> doubles, a method for sleeping for a second might look like >> sleepForSeconds(double). This works even if someone wants to sleep for a >> nanoseconds, since 0.000001 is easy to represent using a double. Also, >> multiplying or dividing a double by a small constant factor (1,000,000,000 >> is small by double standards) is virtually guaranteed to avoid any loss of >> precision. But as soon as such a utility gets std::chronified, it becomes a >> template. This is because you cannot have sleepFor(std::chrono::seconds), >> since that wouldn’t allow you to represent fractions of seconds. This >> brings me to my next point. >> >> Overflow danger: std::chrono is based on integers and its math methods do >> not support overflow protection. This has led to serious bugs like >> https://bugs.webkit.org/show_bug.cgi?id=157924 >> <https://bugs.webkit.org/show_bug.cgi?id=157924>. This cancels out the >> “remember what unit is used” benefit cited above. It’s true that I know >> what type of time I have, but as soon as I duration_cast it to another unit, >> I may overflow. The type system does not help! This is insane: std::chrono >> requires you to do more work when writing multi-unit code, so that you >> satisfy the type checker, but you still have to be just as paranoid around >> multi-unit scenarios. Forgetting that you have milliseconds and using it as >> seconds is trivially fixable. But if std::chrono flags such an error and >> you fix it with a duration_cast (as any std::chrono tutorial will tell you >> to do), you’ve just introduced an unchecked overflow and such unchecked >> overflows are known to cause bugs that manifest as pages not working >> correctly. >> >> I think that doubles are better than std::chrono in multi-unit scenarios. >> It may be possible to have std::chrono work with doubles, but this probably >> implies us writing our own clocks. std::chrono’s default clocks use >> integers, not doubles. It also may be possible to teach std::chrono to do >> overflow protection, but that would make me so sad since using double means >> not having to worry about overflow at all. >> >> The overflow issue is interesting because of its implications for how we do >> timeouts. The way to have a method with an optional timeout is to do one of >> these things: >> >> - Use 0 to mean no timeout. >> - Have one function for timeout and one for no timeout. >> - Have some form of +Inf or INT_MAX to mean no timeout. This makes so much >> mathematical sense. >> >> WebKit takes the +Inf/INT_MAX approach. I like this approach the best >> because it makes the most mathematical sense: not giving a timeout is >> exactly like asking for a timeout at time-like infinity. When used with >> doubles, this Just Works. +Inf is greater than any value and it gets >> preserved properly in math (+Inf * real = +Inf, so it survives gracefully in >> unit conversions; +Inf + real = +Inf, so it also survives >> absolute-to-relative conversions). >> >> But this doesn’t work with std::chrono. The closest thing to +Inf is >> duration::max(), i.e. some kind of UINT_MAX, but this is guaranteed to >> overflow anytime it’s converted to a more precise unit of time >> (nanoseconds::max() converted to milliseconds is something bogus). It >> appears that std::chrono doesn’t have a good story for infinite timeout, >> which means that anyone writing a function that can optionally have a >> timeout is going to have a bad time. We have plenty of such functions in >> WebKit. For example, I’m not sure how to come up with a feel-good solution >> to https://bugs.webkit.org/show_bug.cgi?id=157937 >> <https://bugs.webkit.org/show_bug.cgi?id=157937> so long as we use >> std::chrono. >> >> Going back to doubles >> >> Considering these facts, I propose that we switch back to using doubles for >> time. We can either simply revert to the way we used doubles before, or we >> can come up with some more sophisticated approach that blends the best of >> both worlds. I prefer plain doubles because I love simplicity. >> >> Simply revert to our old ways: I like this approach the best because it >> involves only very simple changes. Prior to std::chrono, we used a double >> to measure time in seconds. It was understood that seconds was the default >> unit. We would use both monotonic and wall clocks, and we used double for >> both of them. >> >> Come up with a new type system: Having learned from std::chrono and doubles, >> it seems that the best typesystem for time would comprise three classes: >> Seconds, WallTime, and MonotonicTime. Seconds would be a class that holds a >> double and supports +/+=/-/-=/</<=/>/>=/==/!= operations, as well as >> conversions to a raw double for when you really need it. WallTime and >> MonotonicTime would be wrappers for Seconds with a more limited set of >> available operations. You can convert WallTime or MonotonicTime to Seconds >> and vice-versa, but some operators are overloaded to make casts unnecessary >> in most cases (WallTime + Seconds = WallTime, WallTime - WallTime = Seconds, >> etc). This would save us from forgetting the unit or the clock. The name >> of the Seconds class is a dead give-away, and WallTime and MonotonicTime >> will not yield you a value that is unit-sensitive unless you say something >> like WallTime::toSeconds(). There will be no easy way to convert WallTime >> to MonotonicTime and vice-versa, since we want to discourage such >> conversions. >> >> Personally I feel very comfortable with doubles for time. I like to put the >> word “Seconds” into variable names and function names >> (waitForSeconds(double) is a dead give-away). On the other hand, I sort of >> like the idea of a type system to protect clock mix-ups. I think that’s the >> biggest benefit we got from std::chrono. >> >> If it was entirely up to me, I’d go for doubles. I think that there needs >> to be a high burden of proof for using types to catch semantic bugs. A type >> system *will* slow you down when writing code, so the EV (expected value) of >> the time savings from bugs caught early needs to be greater than the EV of >> the time lost due to spoonfeeding the compiler or having to remember how to >> use those classes. Although I know that using doubles sometimes meant we >> had bugs, I don’t think they were frequent or severe enough for the odds to >> be good for the Seconds/WallTime/MonotonicTime solution. >> >> Thoughts? >> >> -Filip >> > > _______________________________________________ > webkit-dev mailing list > webkit-dev@lists.webkit.org > https://lists.webkit.org/mailman/listinfo/webkit-dev
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