[SR-15785] TaskGroup addTask is blocking with poor performance

[jshier]

Previous ID	SR-15785
Radar	rdar://problem/88219931
Original Reporter	@jshier
Type	Bug
Status	Resolved
Resolution	Done

Environment

macOS 12.2, Xcode 13.2.1 and 13.3, 2020 i9 iMac, M1 Max MacBook Pro.

Additional Detail from JIRA

Votes	0
Component/s	swift
Labels	Bug, Concurrency, Performance
Assignee	@Catfish-Man
Priority	Medium

md5: cd628c72dea7e813640f5edee1597d34

Issue Description:

Attempting to enqueue many children, while quite easy with TaskGroup, is more than linear when you get into the tens of thousands. As a fundamental concurrency primitive, it would be beneficial if this performance could be improved.

Additionally, addTask seems to block the execution of children, letting memory balloon as the children are enqueued but aren't be executed.

Here's a simple example:

@main
struct ConcurrencyTest {
    static func main() async {
        let start = CFAbsoluteTimeGetCurrent()
        @Sendable
        func doNothing() {}
        
        await withTaskGroup(of: Void.self) { group in
            for _ in 0..<100_000 {
                group.addTask(operation: doNothing)
            }
            
            await group.waitForAll()
        }
        print(CFAbsoluteTimeGetCurrent() - start)
    }
}

This code takes 35.6s to execute on a 2020 i9 iMac and, shockingly, 89s on an M1 Max MacBook Pro.

[jshier]

Simply batching the execution allows 100_000 child tasks to complete in 0.125s on both Intel and M1.

@main
struct ConcurrencyTest {
    static func main() async {
        let start = CFAbsoluteTimeGetCurrent()
        await withTaskGroup(of: Void.self) { group in
            @Sendable
            func doNothing() {}

            var tasksToPerform = 99_990
            for _ in 0..<10 {
                group.addTask(operation: doNothing)
            }

            for await _ in group {
                if tasksToPerform > 0 {
                    group.addTask(operation: doNothing)
                    tasksToPerform -= 1
                }
            }
        }
        print(CFAbsoluteTimeGetCurrent() - start)
    }
}

[weissi]

Thanks for filing this! I had seen this too pre-release (can't find the radar now...) and hoped this was fixed 🙁.

[weissi]

@swift-ci create

[weissi]

CC @rjmccall/@ktoso

[ktoso]

We looked into this with David and Rokhini, PR here: #41165