Add custom configuration for Action Scheduler

[rmccue]

Optimises Action Scheduler to always use Cavalcade, instead of falling back to async requests. In addition, tweaks some numbers to ensure queues are processed efficiently.

Fixes #780

See also, https://github.com/woocommerce/action-scheduler/blob/trunk/docs/perf.md

@joehoyle to tweak and test these numbers further.

[joehoyle]

We pushed this to one customer, and things look to be operating as expected. I don't have data on the exact numbers, but it seems to be ok as-is.

[rmccue]

Is it better than before?

[johnbillion]

I'd love to see the reason for the changes documented more clearly. On the rs-dms project we tweaked these settings quite a lot and made sure to document the changes clearly.

See:

• https://github.com/humanmade/rs-dms/blob/master/client-mu-plugins/rs-bulk-management/inc/functions.php#L74-L79
• https://github.com/humanmade/rs-dms/blob/master/client-mu-plugins/rs-bulk-management/inc/scheduler.php

[joehoyle]

For documentation, I think we could elaborate on the in-code docs in https://github.com/humanmade/altis-core/pull/779/files#diff-66101e5c9a2ddcfa6f9bfa5ec8c36099c49038ceffc4182eede19db6f9406990R17-R25. @johnbillion is that along the lines you're thinking?

[joehoyle]

Is it better than before?

The issue we're seeing in this case is that the fallback exists at all. The numbers on how many jobs to do at once etc is not really that important. The larger issue is that Action Scheduler will by default start making very slow HTTP requests (blocking PHP workers) if the cron jobs get slightly backed up. This PR changes that behavior so it's more binary in that sense. Yes, it's better, because we now don't use the fallback behavior and block the PHP workers doing so.

[joehoyle]

@rmccue I added the above suggestion for document this further.

[svandragt]

Hey is there anything that can be done to progress this @rmccue ? We're seeing a lot of missed action scheduler quee items (couple dozen)

[joehoyle]

@rmccue added your comment suggestion. Can you re-review please.

[joehoyle]

Oh wait you opened the PR