use monotonic timer for time delta calculations

[RaduBerinde]

Things like timeouts or mutex held X for Yms should not be using time.Now which is dependent on the system clock. Most unix systems have support for a monotonic timer which can be reliably used for time period calculations.

Unfortunately there isn't a standard one in Go (golang/go#12914). But that discussion mentions a package which makes use of an internal runtime.nanotime fuction which uses a monotonic timer when available: https://github.com/aristanetworks/goarista/tree/master/monotime

Note that a monotonic timer can't be used for txn timestamps or things like that.

[tamird]

Well, I started looking into this a bit, and unfortunately it's not as easy as I'd hoped. We use time.Time for time delta calculations in all kinds of fun ways, including the return value of context.Context.Deadline(), which we then compare to the local clock reading.

Sad to say, I'm not seeing a nice way out of this one. Ideas?

[petermattis]

The easy solution is to wait for go1.9 and the time support for monotonic clocks.

Closing as I don't think there is anything easy to do on our end.

[RaduBerinde]

Once we are on 1.9, it might be a good idea to have a goroutine monitor the system clock periodically and panic if it sees the clock jumping back or forward by more than the allowed clock skew.

[tamird]

We already have this code in timeutil.Now, though it currently only logs a warning.

[RaduBerinde]

I think you mean hlc stuff not timeutil. It only detects backward jumps though, and not with any degree of reliability - if I turn the clock back 1s and then I wait 1s before doing anything that requires reading the clock, it won't get detected (I think).

[tamird]

You're right on both counts.

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On Mon, Mar 6, 2017 at 7:08 PM, RaduBerinde ***@***.***> wrote: I think you mean hlc stuff not timeutil. It only detects backward jumps though, and not with any degree of reliability - if I turn the clock back 1s and then I wait 1s before doing anything that requires reading the clock, it won't get detected (I think). — You are receiving this because you were assigned. Reply to this email directly, view it on GitHub <#10572 (comment)>, or mute the thread <https://github.com/notifications/unsubscribe-auth/ABdsPIx14l6TUSFhXvZCgwEvYIg5DtKHks5rjKAKgaJpZM4Kt7JX> .