Improve timeout calculations with more accurate start time and by including all tasks

[kkmuffme]

I'd like to suggest a few small improvements and hope to gather some feedback on them:

1. $this->created_time = microtime( true ); in QueueRunner should be changed to:

$this->created_time = isset( $_SERVER['REQUEST_TIME_FLOAT'] ) && is_float( $_SERVER['REQUEST_TIME_FLOAT'] ) ? $_SERVER['REQUEST_TIME_FLOAT'] : microtime( true );

Since the action scheduler might only be loaded after a significant amount of time has passed already (especially in wp-admin with the async request, there are loads of incredibly slow sites) thus distorting the calculations

2. time_likely_to_be_exceeded only takes into account the $processed_actions - however this can be off signifcantly if the time between the time the request started (even worse now with the change of 1) and the time "run" was called - either bc some unrelated stuff beforehand was slow (see 1) OR there were lots of clean up tasks that took a while.

I suggest to create an additional property which holds microtime( true ) immediately before the do/while loop in run starts and use only that to calculate the average action time.

3. $estimated_time = $execution_time + ( $time_per_action * 3 );
   should be changed to

$estimated_time        = $execution_time + ( $time_per_action * apply_filters( 'action_scheduler_queue_runner_batch_size', 25 ) );

This will massively reduce DB load and speed up execution because:

• at the moment it claims a full batch (= DB writes = bad, bc we need to go to DB master) even if it knows it won't be able to complete a full batch but only at least 3 actions (instead of 25 or whatever it is filtered too). The bigger the batch size is, the worse the problem is
• once it's claimed, we have to write again to unclaim it - the claims created 2 unnecessary writes since we already knew that we won't have enough time
• the task is blocked and it's not immediately freed when the request times out, thus delaying the execution of tasks

Perhaps even *1.5 it to be on the save side (should be apply filterable too)

EDIT: for 3) where batch_limits_exceeded is called in do_batch - this needs to be handled separately too, since there it should not use the batch size or hardcoded 3, but it should use only 1, bc we only need to break when the next iteration of the loop will put us over the limit.

[lsinger]

Thanks @kkmuffme, those sound like plausible improvements. If you want to submit a PR we'd be happy to take a look.

We'd especially interested in some sort of performance test that shows under which situations performance would improve. If you have any ideas for something like that we'd also be happy to discuss.