Swift GCD Notify after concurrent execution Lock barrier

 

 

 

 

nslock

let queue = DispatchQueue(label: "Queue1", qos: DispatchQoS.default, attributes: DispatchQueue.Attributes.concurrent, autoreleaseFrequency: DispatchQueue.AutoreleaseFrequency.inherit, target: nil)
        let queue2 = DispatchQueue(label: "queue2", qos: DispatchQoS.default, attributes: DispatchQueue.Attributes.concurrent, autoreleaseFrequency: DispatchQueue.AutoreleaseFrequency.inherit, target: nil)
        let lock = NSLock()
        var array = Array(0...10000)
        func getLastItem() -> Int?{
            lock.lock()
            var temp: Int? = nil
            let count = array.count
            if count > 0{
                temp = array[count - 1]
            }
            lock.unlock()
            return temp
        }
        func removeLastItem(){
            lock.lock()
            array.removeLast()
            lock.unlock()
        }
        
        queue.async {
            for _ in 0 ..< 1000{
                removeLastItem()
            }
        }
        
        queue2.async {
            
            for _ in 0..<1000{
                print(getLastItem() ?? 0)
            }
            
        }

Lock wastes too much perfomance in the case of very few write opertions. Use Barrier instead.

Barrier will not execute until other threads finished executing

   let queue = DispatchQueue(label: "Queue1", qos: DispatchQoS.default, attributes: DispatchQueue.Attributes.concurrent, autoreleaseFrequency: DispatchQueue.AutoreleaseFrequency.inherit, target: nil)
        let queue2 = DispatchQueue(label: "queue2", qos: DispatchQoS.default, attributes: DispatchQueue.Attributes.concurrent, autoreleaseFrequency: DispatchQueue.AutoreleaseFrequency.inherit, target: nil)
        let arrayQueue = DispatchQueue(label: "arrayQueue", qos: DispatchQoS.default, attributes: DispatchQueue.Attributes.concurrent, autoreleaseFrequency: DispatchQueue.AutoreleaseFrequency.inherit, target: nil)
        
        var array = Array(0...1000)
        func getLastItem() -> Int?{
            return arrayQueue.sync {
                if array.count > 0 {
                    return array[array.count - 1]
                }
                return -1
                
            }
        }
        func removeLastItem(){
            let workItem = DispatchWorkItem(qos: DispatchQoS.default, flags: DispatchWorkItemFlags.barrier) {
                array.removeLast()
            }
            arrayQueue.async(execute: workItem)
        }
        
        queue.async {
            for _ in 0 ..< 1000{
                removeLastItem()
            }
        }
        
        queue2.async {
            
            for _ in 0..<1000{
                print(getLastItem() ?? 0)
            }
            
        }