Clock Skew Exists(jvns.ca)
jvns.ca
Clock Skew Exists
http://jvns.ca/blog/2016/02/09/til-clock-skew-exists/
3 comments
"If you restrict the failures to this class of issues" is exactly where the assumption that clocks behave is hiding in this paragraph. It's a bit like saying "as long as you ignore false, all booleans can be assumed to be true".
He's talking about literally restricting the failures to a specific class by preventing errors outside that class, not just restricting consideration of errors to that class.
If you use the monotonic time API the issues are restricted to this subclass of hardware failures. All the others will be irrelevant, like ntpd setting a random time or alike.
how does using the monotonic api protect against time discontinuity as seen when a vm is migrated from one hypervisor to another for example?
In this case, even in a broken implementation, the time freezes during the migration and is restored immediately after. This is bad since it makes even softwares using the monotonic API not able to count time reliably, however the result is that the time is over-counted, and never under-counted. If the algorithm is designed so that we want to count "at least 5 seconds" it is safe even during VM migrations. This is true for Redlock for example, but in general if one wants to exploit this system model, and can setup the algorithm so that over counting is harmless (if not for liveness), is a good idea.
Btw this problems could be mitigated by changing the majority. For instance in the case of a distributed lock, it's possible to acquire the lock in 50% + 2 instances in order to consider the lock valid, so that in one instance the clock can misbehave safely.
Btw this problems could be mitigated by changing the majority. For instance in the case of a distributed lock, it's possible to acquire the lock in 50% + 2 instances in order to consider the lock valid, so that in one instance the clock can misbehave safely.
Monotonic clocks are guaranteed never to go backwards, for one. In POSIX it would be a major violation of the API, although implementations are free to substitute clock sources if the monotonic source is not provided.
Or, to put it more bluntly: a hypervisor that allows its VMs to be paused SHOULD NOT expose a monotonic time source to said VMs. (In the RFC meaning of "SHOULD NOT".) Enabling monotonic time should disable the ability to pause/migrate/snapshot the VMs, and vice-versa.
...as long as they continue to count the time
approximately at the same speed.
If you can characterize the latency involved in messaging between domains, it stands to reason that you can provide a reasonable bound on how close both systems are to an ideal shared clock source. If it's good enough, who cares if it's perfect?
This is a really important point that a lot of people seem to have missed. A common assumption that stems from monotonously increasing clocks is that: all computers can guarantee the causality of their own internal events. Which is a critical assumption for a wide variety of distributed algorithms.
I'm wondering if you read the article. Mainly because she only mentions an NTP paper about local clock skew causing servers to serve incorrect times. I don't know why you don't want to admit the existence of certain failure modes, it's probably a pride thing.
Clock skew is a bad enough thing that it's accepted wisdom to not run timing-sensitive authentication authorities (read: Kerberos, mostly) in a hypervisor or VM container.
This has a few of reasons:
1) all abstraction necessarily adds a performance penalty.
2) Hypervisors or VMs have core affinity, but, often, multiple VMs and Hypervisors are pinned to the same core. Since one cpu cannot expose time to two different vm requests at precisely the same time, there is always some variance.
3) Process isolation is pretty good, but total performance isolation is not quite the same.
There are a TON of papers written about VM and hypervisor time management. It's all fascinating and hard.
1) all abstraction necessarily adds a performance penalty.
2) Hypervisors or VMs have core affinity, but, often, multiple VMs and Hypervisors are pinned to the same core. Since one cpu cannot expose time to two different vm requests at precisely the same time, there is always some variance.
3) Process isolation is pretty good, but total performance isolation is not quite the same.
There are a TON of papers written about VM and hypervisor time management. It's all fascinating and hard.
Absolutely. I'm just surprised that this many people seem to be in the dark about clock synchronicity (or lack thereof) effects on distributed systems.
Servers, even ones that run NTP, aren't always on time. I had one the other day that somehow got 30 minutes behind.
It's the more subtle instances of this that always are the most worrying to me, to be honest.
Imagine you have a data pipeline that spans multiple hosts, and you want telemetry on when an entity passes through each step? Suddenly you have to start caring about clock alignment in an order of magnitude comparable with network transit/computation time (microseconds) and that level of skew happens VERY often. There are certainly ways around this in how you build your system, but it's something that _must_ be considered and I find often isn't.
Imagine you have a data pipeline that spans multiple hosts, and you want telemetry on when an entity passes through each step? Suddenly you have to start caring about clock alignment in an order of magnitude comparable with network transit/computation time (microseconds) and that level of skew happens VERY often. There are certainly ways around this in how you build your system, but it's something that _must_ be considered and I find often isn't.
If you're monitoring other system metrics, and your application is time-dependent, you should be monitoring system time and ntpd stats as well.
/u/devnull42 is doing it right: https://news.ycombinator.com/item?id=11075883
/u/devnull42 is doing it right: https://news.ycombinator.com/item?id=11075883
For critical time dependent systems, I absolutely agree. I was unclear in that I was referring to far more mundane issues where slight skew can be problematic. I've seen telemetry systems in which the aformentioned deltas would be computed by doing a timedelta() on the recorded timestamp from each machine. With even subsecond skew, you can get some really interesting artifacts; negative durations, that sort of thing.
I run some pretty large NTP deployments and our NTP drift is never really over 1 second. Now the difference between NTP serverside drift and client server-side "skew" isn’t something I have looks at but for the most part I get a nagios alert on anything over 1 second of drift with 5 seconds being the most I have ever seen.
Can you share more detail as to how you would alert on drift? What acts as the canonical source?
Not sure how you'd configure it, but you don't actually need a canonical source. You just compare it against all/many other servers and look at the difference. If there's any real skew you need to look at all of them that had a difference then but you don't need a canonical source anymore.
Hello Julia, the kind of skew that is a problem for Redlock, or other algorithms that would rely on the same clock assumptions, is not when the computer "logical" clocks skew, for example by getting a wrong time from NTP. By using the monotonically increasing clock API we only need to care about failures that cause the clock to go faster / slower even if never modified. Also note that there is no issues with clocks of different processes to be at a totally different absolute times, as long as they continue to count the time approximately at the same speed. If you restrict the failures to this class of issues, then to see it in the practice becomes not impossible but extremely rare. Most real world systems out there using proven algorithms, have to do the same assumptions on the fact that the system works in a mostly expected way. For example disk corruptions or network packets corruptions can cause the same issues, even if they are rarely checksummed at application level, or checked with a checksum length that does not make collisions impossible in the long run. Btw kudos for trying to reason with your head instead of trusting other people's or my words.