Throttling CPU usage in a Swift thread

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I want to traverse the file tree for a potentially large directory in a macOS app. It takes about 3 mins for my example case if I just do it, but the CPU spikes to 80% or so for those 3 minutes.

I can afford to do it more slowly on a background thread, but am not sure of what the best approach would be.

I thought of just inserting 1 millisecond sleep inside the loop, but I am not confident that won't have some negative impact on scheduling / disk IO etc. An alternative would be to do 1 second of work, then wait 2-3 seconds, but I am guessing there is something more elegant?

The core functionality I want is traversing a directory in a nested fashion checking file attributes:

let enumerator = FileManager.default.enumerator(atPath: filePath)
while let element = enumerator?.nextObject() as? String {
     // do something here
 }
swift macos
1 Answers

It's generally most energy efficient to spike the CPU for a short time than to run it at a low level for a longer time. As long as your process has a lower priority than other processes, running the CPU at even 100% for a short time isn't a problem (particularly if it doesn't turn the fans on). Modern CPUs would like to be run very hard for short periods of time, and then be completely idle. "Somewhat busy" for a longer time is much worse because the CPU can't power-off any subsystems.

Even so, users get very upset when they see high CPU usage. I used to work on system management software, and we spoke with Apple about throttling our CPU usage. They told us the above. We said "yes, but when users see us running at 100%, they complain to IT and try to uninstall our app." Apple's answer was to use sleep, like you're describing. If it makes your process take longer, then it will likely have a negative overall impact in total energy use. But I wouldn't expect it to cause any other trouble.

That said, if you are scanning the same directory tree more than once, you should look at File System Events and File Metadata Search which may perform this operations much more efficiently.

See also: Schedule Background Activity in the Energy Efficiency Guide for Mac Apps. I highly recommend this entire doc. There are many tools that have been added to macOS in recent years that may be useful for your problem. I also recommend Writing Energy Efficient Apps from WWDC 2017.

If you do need to scan everything directly with an enumerator, you can likely greatly improve things by using the URL-based API rather than the String-based API. It allows you to pre-fetch certain values (including attributeModificationDateKey, which may be of use here). Also, be aware of the fileAttributes property of DirectoryEnumerator, which caches the last-read file's attributes (so you don't need to query them again).

Three minutes is a long time; it's possible you're doing more work than needed. Run your operation using the find commandline tool and use that as a benchmark for how much time it should take.

user_973370
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