Do not list CronJob unmet starting times beyond deadline

[peay]

What this PR does / why we need it:

See #36311. getRecentUnmetScheduleTimes gives up after 100 unmet times to avoid wasting too much CPU or memory generating all the times, as it generates them sequentially.

When concurrency is forbidden, this is conceptually un-necessary: we only need the last unmet start time. This suggests that when concurrency is forbidden, we could generate times by going backward in time from now. This is not very practical as CronJob currently relies on a package that only provides Next and no Prev. Hand-cooking a Prev does not seem like a good idea. I could submit a PR to the cron library to add a Prev method, and use that when concurrency is forbidden through something like getLastUnmetScheduleTime. This would be O(1) and there would be no limit involved.

(edit: actually, even for the other concurrency settings, we only start the last unmet start times -- there is a TODO in the controller to actually start all of them, but that is not implemented at the moment. This means the solution would apply, at least temporarily, to all concurrency settings).

cc @soltysh what do you think?

In the meantime, I would suggest to do something simple. Currently, the user has no way to configure anything to ensure that his CronJob will not get stuck if one job takes more that 100 unmet times.

getRecentUnmetScheduleTimes starts with an initial time corresponding to the last start (or to the creation of the CronJob, if nothing has started yet). However, when StartingDeadlineSeconds is set, the controller will not start anything that is older than the deadline, so if the last start is way beyond the deadline, we are generating potentially lots of unmet start times that will not be considered by the scheduler for scheduling anyway.

Consider a job running every minute, where the last instance has taken 120 minutes. This means there are more than 100 unmet times when we start counting from the last start time.

The PR makes getRecentUnmetScheduleTimes only consider times that do not fall beyond the deadline. Here, the CronJob can be configured with a StartingDeadlineSeconds of, say, 10 minutes. After the 120min job has run, getRecentUnmetScheduleTimes will only consider the times in the last 10 minutes from now, and will not get stuck.

As a side note on the max. number of unmet times to use as limits in terms of CPU used by the controller: I have run a quick benchmark on my i7 mac. Schedules corresponding to "once a week" tend to be more expensive to generate unmet times for. Just FYI.

+--------------+---------------+--------------+
|   SCHEDULE   | MISSED STARTS |    TIMING    |
+--------------+---------------+--------------+
| */1 * * * ?  |           100 | 383.645µs    |
| */30 * * * ? |           100 | 354.765µs    |
| 30 1 * * ?   |           100 | 1.065124ms   |
| 30 1 * * 0   |           100 | 1.80034ms    |
| */1 * * * ?  |           500 | 1.341365ms   |
| */30 * * * ? |           500 | 1.814441ms   |
| 30 1 * * ?   |           500 | 8.475012ms   |
| 30 1 * * 0   |           500 | 10.020613ms  |
| */1 * * * ?  |          1000 | 2.551697ms   |
| */30 * * * ? |          1000 | 4.075813ms   |
| 30 1 * * ?   |          1000 | 17.674945ms  |
| 30 1 * * 0   |          1000 | 19.149324ms  |
| */1 * * * ?  |         10000 | 25.725531ms  |
| */30 * * * ? |         10000 | 87.520022ms  |
| 30 1 * * ?   |         10000 | 174.29216ms  |
| 30 1 * * 0   |         10000 | 196.565748ms |
+--------------+---------------+--------------+

using

package main

import (
    "fmt"
    "time"
    "os"
    "strconv"

    "github.com/robfig/cron"
    "github.com/olekukonko/tablewriter"
)

func timeSchedule(schedule string, iterations int) (time.Duration) {
    sched, err := cron.ParseStandard(schedule)

    if err != nil {
        panic(fmt.Sprintf("Unparseable schedule: %s", err))
    }

    start := time.Now()
    t := time.Now()

    for i := 1; i <= iterations; i++ {
        t = sched.Next(t)
    }

    return time.Since(start)
}

func main() {
    table := tablewriter.NewWriter(os.Stdout)
    table.SetHeader([]string{"Schedule", "Missed starts", "Timing"})

    schedules := []string{"*/1 * * * ?", "*/30 * * * ?", "30 1 * * ?", "30 1 * * 0"}
    iteration_nums := []int{100, 500, 1000, 10000}

    for _, iterations := range iteration_nums {
        for _, schedule := range schedules {
            table.Append([]string{schedule,
                                  strconv.Itoa(iterations),
                                  timeSchedule(schedule, iterations).String()})
        }
    }
    table.Render()
}

Which issue this PR fixes: fixes #36311

Special notes for your reviewer:

Release note:

[k8s-ci-robot]

Hi @peay. Thanks for your PR.

I'm waiting for a kubernetes member to verify that this patch is reasonable to test. If it is, they should reply with @k8s-bot ok to test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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[k8s-reviewable]

This change is 

[soltysh]

@peay sorry for the long waiting time. I like your idea, esp. that we explicitly say that anything above startingDeadlineSeconds is considered as failed.

[soltysh]

Can you prepare test cases covering this new flow?

[peay]

Yes, will do

[peay]

I have updated the PR with tests.

[soltysh]

If you're touching this I'd suggest something like:

Too many missed start time (> 100). Set or decrease .spec.startingDeadlineSeconds or check clock skew.

[soltysh]

@kubernetes/sig-apps-misc fyi

[soltysh]

This LGTM, I've updated the title, to drop the WIP prefix.

@k8s-bot ok to test

[soltysh]

@k8s-bot kubemark e2e test this

[k8s-ci-robot]

Jenkins Kubemark GCE e2e failed for commit d141a43. Full PR test history. cc @peay

The magic incantation to run this job again is @k8s-bot kubemark e2e test this. Please help us cut down flakes by linking to an open flake issue when you hit one in your PR.

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes/test-infra repository. I understand the commands that are listed here.

[soltysh]

@k8s-bot kubemark e2e test this

[k8s-github-robot]

@k8s-bot test this [submit-queue is verifying that this PR is safe to merge]

[k8s-github-robot]

Automatic merge from submit-queue