Once that’s complete, the next run will be scheduled with DAS, which puts that activity back into its scheduling list. Those are combined to produce a score: when that exceeds a threshold, DAS decides to run that activity, and will in turn tell Centralized Task Scheduling (CTS) via XPC to run that activity. It does that by scoring them, taking into account the window during which the activity should occur, its priority expressed in terms of Quality of Service (QoS), the current load on the CPU, and environmental conditions. DAS maintains a list of scheduled activities, and frequently evaluates those activities to determine which should be run next. This is related to CoreDuet, a sub-system concerned with monitoring environmental and process load conditions. The disadvantage of that was that, like cron, launchd isn’t flexible on timing: the moment that it decides to run a large backup could coincide with intense user activity, resulting in slowing of response to the user and prolonged backup duration.Īt some stage over the following years, before macOS 10.12 Sierra in 2016, Apple introduced a whole new system designed to schedule background activities more flexibly, Duet Activity Scheduler (DAS).
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Just over two years later, when Apple released Time Machine in Mac OS X 10.5, it was scheduled to make backups every hour using launchd.
The first couple of years with launchd had their moments, as it took a while before it became reliable. In replacing cron, launchd took on the responsibility of running scheduled background activities too. In this new model, one of the early tasks of the kernel after it has been booted is to load launchd, which then becomes the master launcher and launches everything else, in conjunction with launchctl and property lists in LaunchDaemons and LaunchAgents folders in Library folders. But Dave Zarzycki was busy designing and coding a replacement service for cron and a lot more besides: launchd, which arrived in Mac OS X 10.4 Tiger, in 2005. The traditional service for running scheduled background activities in Unix is cron, and that’s what the first versions of Mac OS X shipped with. This article explains how those activities are scheduled. The most prominent example of the latter are Time Machine backups performed by backupd, which should occur every hour. Some like logd run in the background all the time, while others are scheduled to run every now and then.
MacOS runs dozens, maybe even hundreds, of services.
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