Re: [pdm-devel] [PATCH proxmox-datacenter-manager 10/25] metric collection: collect overdue metrics on startup/timer change

[Lukas Wagner <l.wagner@proxmox.com>]

On  2025-02-13 15:19, Wolfgang Bumiller wrote:
> On Thu, Feb 13, 2025 at 02:50:32PM +0100, Lukas Wagner wrote:
>>
>>
>> On  2025-02-13 09:55, Wolfgang Bumiller wrote:
>>>>          loop {
>>>>              let old_settings = self.settings.clone();
>>>>              tokio::select! {
>>>> @@ -124,7 +132,12 @@ impl MetricCollectionTask {
>>>>                      "metric collection interval changed to {} seconds, reloading timer",
>>>>                      interval
>>>>                  );
>>>> -                timer = Self::setup_timer(interval);
>>>> +                (timer, next_run) = Self::setup_timer(interval);
>>>> +                // If change (and therefore reset) our timer right before it fires,
>>>> +                // we could potentially miss one collection event.
>>>
>>> Couldn't we instead just pass `next_run` through to `setup_timer` and
>>> call `reset_at(next_run)` on it? (`first_run` would only be used in the
>>> initial setup, so `next_run` could either be an `Option`, or the setup
>>> code does the `next_aligned_instant` call...
>>>
>>> This should be much less code by making the new
>>> `fetch_overdue{,_and_save_sate}()` functions unnecessary, or am I
>>> missing something?
>>>
>>
>> I guess the question is, do we want nicely aligned timer ticks?
>>
>> e.g. 14:01:00, 14:02:00, 14:03:00 ... for 60 second interval
>> or   14:00:00, 14:05:00, 14:10:00 ... for a 5 minute interval?
>>
>> Because that was the main intention behind using the 'collection-interval' as
>> a base for calculating the aligned instant for the first timer reset.
>> If we reuse the 'old' `next_run` when the interval is changed, we
>> also reuse the old alignment. 
>>
>> For instance, when changing from initially 1 minute to 5 minutes, the
>> timer ticks might come at 
>>   14:01:00, 14:06:00, 14:11:00
>>
>> Technically, the naming for the `next_run` variable is not the best,
>> since it just contains the Instant when the timer *first* fires, but
>> this is then never updated to the *next* time the timer will fire...
>> So that means that when changing the interval with your suggested change,
>> you'd pass an Instant to `reset_at` that is already in the past,
>> causing the timer to fire immediately.
>>
>> If we *don't* care about the aligned ticks as described above, we could
>> just use a static alignment boundary, e.g. 60 seconds.
>> In this case we can also get rid of the fetch_overdue stuff, since
>> at worst case we have 60 seconds until the next tick on startup or timer change,
>> which should be good enough to prevent any significant gaps in the data.
> 
> What about setting a flag - if the current next tick was earlier than
> the new next tick - to tell tick() to re-align the timer when it is next
> triggered?

The problem is that tokio::time::Interval doesn't give you a way to query when
the next expected tick will be. We can only approximate it by recalculating
a new aligned instant with the same interval, but I guess this might behave 
unpredictably in edge cases.

> 
> So when going from 1 to 5 minutes at 14:01:50, we `.reset_at(14:02:00)`
> and also set `realign = true`, and at 14:02, tick() should
> `.reset_at(14:05:00)`.
> 
> I just feel like the logic in the "fetch_overdue" code should not be
> necessary to have, but if it's too awkward to handle via the tick timer,
> it's fine to keep it in v2.

fetch_overdue is also called when the daemon starts up. If we align to the collection interval
and the daemon was down for a while, we otherwise might end up with gaps in the data.

Remotes keep metric history for 30 minutes. If PDM is down for, say, 29 minutes and
we are aligning to 15min boundaries, in the worst case we might have to wait for
another 15min to fetch metrics, resulting in a gap.

Of course we could just unconditionally force collection after startup, but I think the
fetch_overdue solution solves this and the timer change issue quite okayish.

In v2 I got rid of the fetch_overdue_and_save_state wrapper by putting the state.save()
that we already had in the main loop at the end of the loop, so it's a bit less code now.
The remaining code is not really that complex, I think I'd prefer to keep it
for now.

-- 
- Lukas



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