Re: [chrony-dev] New online/offline command

On Thu, May 24, 2018 at 11:54 AM, Miroslav Lichvar <mlichvar@xxxxxxxxxx> wrote:

On Wed, May 23, 2018 at 04:44:30PM +0200, Christian Ehrhardt wrote:
> No I'm fine with a single command.
> If it is low on cpu consumption and fast.
> We have to consider that in some environments you might have plenty
> (really a lot) of changes ongoing.
> And you might get a notification for each of them.

How much is plenty?

On auto-deployed config changes of virt systems (with plenty of switch interfaces for example) I've seen three digits per second (rare, but existing).

I guess we could use a delay and batch all
requests in some interval (e.g. one second), but I think I'd prefer if
it was executed synchronously. It's simpler and there will be no
surprises if someone runs the new command and then follows it with the
old online command, which might have no effect.

This has two things in it:

a) make it synchronous - ack for all the reasons you outlined

b) need batch/coalesce - this still is true as the async nature can already come from the network event handling

So imagine a timeline (seconds at the top) with the update process taking 6 seconds

second event

1 net event 1 triggers "probe I"

2 Events 2-20 trigger 19 more

...

7 probe I completes and returns to caller "event 1"

8 the formerly waiting probe II (just one) runs now

....

14 probe II completes and returns to all callers Events 2-20

This is synchronous + batching as I'd think of it

- We can't skip probe II as network might have changed in a way "probe I" was unable to pick up

- Running one full probe per event can become a hue backlog and a thundering herd problem

> With just one command neither Stephens nor my suggestion make sense.
> How about just "probe" then?

I'm not sure. What exactly are we probing here? There are no packets
exchanged. It's just a local check of the routing configuration.

You wrote later:
"How about "onoffline" or "maybeonline"?"

I'd like auto-onoffline or something like it more than any of the two above

--
Miroslav Lichvar

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Christian Ehrhardt

Software Engineer, Ubuntu Server

Canonical Ltd