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2012-08-10 »

py-monotime, CLOCK_MONOTONIC, and time.monotonic()

An increasingly common problem nowadays is caused by the fact that the time() or gettimeofday() system calls do not always return monotonically increasing values. That is, sometimes they go backwards, and sometimes they take giant leaps.

In Unix, thankfully, timezones don't affect this: you don't have to worry about, say, daylight savings time messing up your time calculations, as long as you use time_t or struct timeval instead of struct tm for any long-lived storage. (Example of not doing it right: if your log file includes the current local time of each message, then every autumn when daylight savings causes a one-hour backwards jump, you will have log messages that appear out of order in the file, and if you sort them by date, it'll be a lie.)

However, there are other things that do make a mess, because Unix time was not entirely well thought out when it was invented. The most commonly known problem is NTP, which jumps your clock around sometimes. People mostly try to ignore this by just assuming that after boot, NTP won't jump the time, it'll only slew the time (which it tries very hard to do), and this mostly works, so people get away with this assumption 99.9% of the time, and their program goes slightly bananas the other 0.1% of the time, after which they typically reboot and are happy.

There's another annoyingly niggly one though, which is virtually impossible to work around: leap seconds. As implemented in Unix, a leap second literally causes the same time_t to occur twice. That turns out to be most definitely the wrong answer; the right answer would have been to include leap seconds in the localtime() calculation, not in the kernel's implementation of time. But sadly, mistakes were made, and we have what we have. There are many stories in computer lore of programs that work great except at the exact moment of a leap second, at which time all sorts of things (notably multimedia timing algorithms) go completely wrong. Once again, though, rebooting fixes it.

Anyway, it turns out there is already a solution for all these problems, and it's been out there for a long time, but not well standardized. The solution is called a "monotonic clock." In a monotonic clock, absolute times aren't very meaningful (they are usually "seconds since the system booted" or something like that) but relative times, which are almost always what you care about, always do what you want. So if you say, "give me an event 60 seconds from now" then you schedule it for time monotonic() + 60, and life is simple and good, and you don't need any crazy hackery to make sure you deal correctly with backwards-flowing time.

If you want access to this lovely thing on a system compliant with POSIX.1-2001, such as Linux, what you want is the clock_gettime(CLOCK_MONOTONIC) system call. Unfortunately, non-Linux systems seem to largely not support it. On MacOS, you can use this advice from Apple instead.

What's worse, if you're writing in python, there is no access to monotonic time even on systems that do support it. Well, there is, starting in python 3.3 apparently (which adds a time.monotonic() function), but nobody uses python 3, so that doesn't really help most of us. For the rest of the world, I just made a new python module called monotime. If you import it, then time.monotonic() suddenly appears, and you can write your program to use it, just like if it were supported internally in your copy of python. It's licensed under the Apache 2.0 license. You can get it through pypi and I also added Debian packaging scripts.

I should also mention python-monotonic-time, which I didn't use for two reasons. First, it's under the GPLv3 (not the LGPL), which would infect any program that uses it. Secondly, it's written using python's ctypes, which is great for hackery, but is very brittle (it depends tightly on system-dependent library names and struct formats, without actually including the system header files during build) and much slower than a C module, which is what my monotime module uses. Programs that need monotonic time typically need a lot of monotonic time, and you want it to be fast.

For more time-related trivia than you can possibly imagine, check out python's PEP 418, which introduced the time.monotonic() function and a few other things.

By the way, getting approval to release this code through Google's super-streamlined open source releasing process took only 47 (monotonic) minutes and almost no CPU time.

August 10, 2012 22:27
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