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Working duties in parallel – The.Swift.Dev.


Having the ability to run duties in parallel is sweet, it could possibly velocity up issues for certain when you’ll be able to make the most of a number of CPU cores, however how can we truly implement these sort of operations in Swift? 🤔

There are a number of methods of working parallel operations, I had an extended article concerning the Grand Central Dispatch (GCD) framework, there I defined the variations between parallelism and concurrency. I additionally demonstrated the way to arrange serial and concurrent dispatch queues, however this time I would prefer to focus a bit extra on duties, staff and jobs.

Think about that you’ve got an image which is 50000 pixel vast and 20000 pixel lengthy, that is precisely one billion pixels. How would you alter the colour of every pixel? Properly, we might do that by iterating by way of every pixel and let one core do the job, or we might run duties in parallel.

The Dispatch framework gives a number of methods to resolve this challenge. The primary answer is to make use of the concurrentPerform operate and specify some variety of staff. For the sake of simplicity, I’ll add up the numbers from zero to 1 billion utilizing 8 staff. 💪

import Dispatch

let staff: Int = 8
let numbers: [Int] = Array(repeating: 1, depend: 1_000_000_000)

var sum = 0
DispatchQueue.concurrentPerform(iterations: staff) { index in
    let begin = index * numbers.depend / staff
    let finish = (index + 1) * numbers.depend / staff
    print("Employee #(index), gadgets: (numbers[start..<end].depend)")

    sum += numbers[start..<end].cut back(0, +)
}

print("Sum: (sum)")

Cool, however nonetheless every employee has to work on various numbers, possibly we should not begin all the employees directly, however use a pool and run solely a subset of them at a time. That is fairly a simple process with operation queues, let me present you a primary instance. 😎

import Basis

let staff: Int = 8
let numbers: [Int] = Array(repeating: 1, depend: 1_000_000_000)

let operationQueue = OperationQueue()
operationQueue.maxConcurrentOperationCount = 4

var sum = 0
for index in 0..<staff {
    let operation = BlockOperation {
        let begin = index * numbers.depend / staff
        let finish = (index + 1) * numbers.depend / staff
        print("Employee #(index), gadgets: (numbers[start..<end].depend)")
        
        sum += numbers[start..<end].cut back(0, +)
    }
    operationQueue.addOperation(operation)
}

operationQueue.waitUntilAllOperationsAreFinished()

print("Sum: (sum)")

Each of the examples are above are extra ore much less good to go (if we glance by way of at doable information race & synchronization), however they depend upon further frameworks. In different phrases they’re non-native Swift options. What if we might do one thing higher utilizing structured concurrency?

let staff: Int = 8
let numbers: [Int] = Array(repeating: 1, depend: 1_000_000_000)

let sum = await withTaskGroup(of: Int.self) { group in
    for i in 0..<staff {
        group.addTask {
            let begin = i * numbers.depend / staff
            let finish = (i + 1) * numbers.depend / staff
            return numbers[start..<end].cut back(0, +)
        }
    }

    var abstract = 0
    for await consequence in group {
        abstract += consequence
    }
    return abstract
}

print("Sum: (sum)")

Through the use of process teams you’ll be able to simply setup the employees and run them in parallel by including a process to the group. Then you’ll be able to look ahead to the partial sum outcomes to reach and sum every thing up utilizing a thread-safe answer. This method is nice, however is it doable to restrict the utmost variety of concurrent operations, similar to we did with operation queues? 🤷‍♂️

func parallelTasks<T>(
    iterations: Int,
    concurrency: Int,
    block: @escaping ((Int) async throws -> T)
) async throws -> [T] {
    strive await withThrowingTaskGroup(of: T.self) { group in
        var consequence: [T] = []

        for i in 0..<iterations {
            if i >= concurrency {
                if let res = strive await group.subsequent() {
                    consequence.append(res)
                }
            }
            group.addTask {
                strive await block(i)
            }
        }

        for strive await res in group {
            consequence.append(res)
        }
        return consequence
    }
}


let staff: Int = 8
let numbers: [Int] = Array(repeating: 1, depend: 1_000_000_000)

let res = strive await parallelTasks(
    iterations: staff,
    concurrency: 4
) { i in
    print(i)
    let begin = i * numbers.depend / staff
    let finish = (i + 1) * numbers.depend / staff
    return numbers[start..<end].cut back(0, +)
}

print("Sum: (res.cut back(0, +))")

It’s doable, I made a little bit helper operate much like the concurrentPerform methodology, this manner you’ll be able to execute various duties and restrict the extent of concurrency. The principle concept is to run various iterations and when the index reaches the utmost variety of concurrent gadgets you wait till a piece merchandise finishes and you then add a brand new process to the group. Earlier than you end the duty you additionally need to await all of the remaining outcomes and append these outcomes to the grouped consequence array. 😊

That is it for now, I hope this little article will assist you to to handle concurrent operations a bit higher.

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