0) Install this in admin/tool to make the testing easier: https://github.com/catalyst/moodle-tool_testtasks   Testing scenario 1. Test the case with a single task runner 1) Queue up 1000 one seconds tasks: php admin /tool/testtasks/cli/queue_adhoc_tasks .php -d=1 -n=1000 2) Behind this queue up a single 'another' type of adhoc task: php admin /tool/testtasks/cli/queue_adhoc_tasks .php -d=1 -n=1 --class= 'tool_testtasks\task\another_timed_adhoc_task' 3) Peek into the task queue by class: $ select count(*),classname from mdl_task_adhoc group by classname; count | classname -------+----------------------------------------------- 1000 | \tool_testtasks\task\timed_adhoc_task 1 | \tool_testtasks\task\another_timed_adhoc_task (2 rows) 4) Now start processing the queue: php admin /tool/task/cli/adhoc_task .php --execute  5) You should see that the first 2 tasks it picked off cycled through each type and very quickly you are left with a number lower than 1000 (i.e. 997): $ select count(*),classname from mdl_task_adhoc group by classname; count | classname -------+--------------------------------------- 997 | \tool_testtasks\task\timed_adhoc_task (1 row)   Testing scenario 2. A bunch of runners in parallel 1) First, allow this to work in config.php: $CFG ->task_adhoc_concurrency_limit = 1000; 2) Queue up 1000 of type A and then another 1000 of type B behind it: $ php admin /tool/testtasks/cli/clear_adhoc_task_queue .php $ php admin /tool/testtasks/cli/queue_adhoc_tasks .php -d=1 -n=1000 $ php admin /tool/testtasks/cli/queue_adhoc_tasks .php -d=1 -n=1000 --class= 'tool_testtasks\task\another_timed_adhoc_task'   3) Confirm the queues:  $ select count(*),classname from mdl_task_adhoc group by classname; count | classname -------+----------------------------------------------- 1000 | \tool_testtasks\task\timed_adhoc_task 1000 | \tool_testtasks\task\another_timed_adhoc_task ( 2 rows) 6) Now lets fire up several task runners, do this say 4 times (you may need to open several terminals in order to execute them): php admin /tool/task/cli/adhoc_task .php --execute & 7) Recheck the queues and confirm they are being processed evenly (so the count column number is lower than in step #3): $ select count(*),classname from mdl_task_adhoc group by classname; count | classname -------+----------------------------------------------- 944 | \tool_testtasks\task\timed_adhoc_task 944 | \tool_testtasks\task\another_timed_adhoc_task ( 2 rows)    Testing scenario 3. More cases [OPTIONAL] 8) Lastly, and most importantly, keep throwing more and more runners at it and confirm that the overall system is still scaling up linearly the more processes you throw at it.