我是如何一步步的在并行编程中将lock锁次数降到最低实现无锁编程

在并行编程中,经常会遇到多线程间操作共享集合的问题,很多时候大家都很难逃避这个问题做到一种无锁编程状态,你也知道一旦给共享集合套上lock之后,并发和伸缩能力往往会造成很大影响,这篇就来谈谈如何尽可能的减少lock锁次数甚至没有。

一:缘由

1. 业务背景

昨天在review代码的时候,看到以前自己写的这么一段代码,精简后如下:

        private static List<long> ExecuteFilterList(int shopID, List<MemoryCacheTrade> trades, List<FilterConditon> filterItemList, MatrixSearchContext searchContext)        {            var customerIDList = new List<long>();            var index = 0;            Parallel.ForEach(filterItemList, new ParallelOptions() { MaxDegreeOfParallelism = 4 },                            (filterItem) =>            {                var context = new FilterItemContext()                {                    StartTime = searchContext.StartTime,                    EndTime = searchContext.EndTime,                    ShopID = shopID,                    Field = filterItem.Field,                    FilterType = filterItem.FilterType,                    ItemList = filterItem.FilterValue,                    SearchList = trades.ToList()                };                var smallCustomerIDList = context.Execute();                lock (filterItemList)                {                    if (index == 0)                    {                        customerIDList.AddRange(smallCustomerIDList);                        index++;                    }                    else                    {                        customerIDList = customerIDList.Intersect(smallCustomerIDList).ToList();                    }                }            });            return customerIDList;        }

这段代码实现的功能是这样的,filterItemList承载着所有原子化的筛选条件,然后用多线程的形式并发执行里面的item,最后将每个item获取的客户人数集合在高层进行整体求交,画个简图就是下面这样。

2. 问题分析

其实这代码存在着一个很大的问题,在Parallel中直接使用lock锁的话,filterItemList有多少个,我的lock就会锁多少次,这对并发和伸缩性是有一定影响的,现在就来想想怎么优化吧!

3. 测试案例

为了方便演示,我模拟了一个小案例,方便大家看到实时结果,修改后的代码如下:

        public static void Main(string[] args)        {            var filterItemList = new List<string>() { "conditon1", "conditon2", "conditon3", "conditon4", "conditon5", "conditon6" };            ParallelTest1(filterItemList);        }        public static void ParallelTest1(List<string> filterItemList)        {            var totalCustomerIDList = new List<int>();            bool isfirst = true;            Parallel.ForEach(filterItemList, new ParallelOptions() { MaxDegreeOfParallelism = 2 }, (query) =>            {                var smallCustomerIDList = GetCustomerIDList(query);                lock (filterItemList)                {                    if (isfirst)                    {                        totalCustomerIDList.AddRange(smallCustomerIDList);                        isfirst = false;                    }                    else                    {                        totalCustomerIDList = totalCustomerIDList.Intersect(smallCustomerIDList).ToList();                    }                    Console.WriteLine($"{DateTime.Now} 被锁了");                }            });            Console.WriteLine($"最后交集客户ID:{string.Join(",", totalCustomerIDList)}");        }        public static List<int> GetCustomerIDList(string query)        {            var dict = new Dictionary<string, List<int>>()            {                ["conditon1"] = new List<int>() { 1, 2, 4, 7 },                ["conditon2"] = new List<int>() { 1, 4, 6, 7 },                ["conditon3"] = new List<int>() { 1, 4, 5, 7 },                ["conditon4"] = new List<int>() { 1, 2, 3, 7 },                ["conditon5"] = new List<int>() { 1, 2, 4, 5, 7 },                ["conditon6"] = new List<int>() { 1, 3, 4, 7, 9 },            };            return dict[query];        }------ output ------2020/04/21 15:53:34 被锁了2020/04/21 15:53:34 被锁了2020/04/21 15:53:34 被锁了2020/04/21 15:53:34 被锁了2020/04/21 15:53:34 被锁了2020/04/21 15:53:34 被锁了最后交集客户ID:1,7

二:第一次优化

从结果中可以看到,filterItemList有6个,锁次数也是6次,那如何降低呢? 其实实现Parallel代码的FCL大神也考虑到了这个问题,从底层给了一个很好的重载,如下所示:

public static ParallelLoopResult ForEach<TSource, TLocal>(OrderablePartitioner<TSource> source, ParallelOptions parallelOptions, Func<TLocal> localInit, Func<TSource, ParallelLoopState, long, TLocal, TLocal> body, Action<TLocal> localFinally);

这个重载很特别,多了两个参数localInit和localFinally,过会说一下什么意思,先看修改后的代码体会一下

        public static void ParallelTest2(List<string> filterItemList)        {            var totalCustomerIDList = new List<int>();            var isfirst = true;            Parallel.ForEach<string, List<int>>(filterItemList,              new ParallelOptions() { MaxDegreeOfParallelism = 2 },              () => { return null; },             (query, loop, index, smalllist) =>             {                 var smallCustomerIDList = GetCustomerIDList(query);                 if (smalllist == null) return smallCustomerIDList;                 return smalllist.Intersect(smallCustomerIDList).ToList();             },            (finalllist) =>            {                lock (filterItemList)                {                    if (isfirst)                    {                        totalCustomerIDList.AddRange(finalllist);                        isfirst = false;                    }                    else                    {                        totalCustomerIDList = totalCustomerIDList.Intersect(finalllist).ToList();                    }                    Console.WriteLine($"{DateTime.Now} 被锁了");                }            });            Console.WriteLine($"最后交集客户ID:{string.Join(",", totalCustomerIDList)}");        }------- output ------2020/04/21 16:11:46 被锁了2020/04/21 16:11:46 被锁了最后交集客户ID:1,7Press any key to continue . . .

很好,这次优化将lock次数从6次降到了2次,这里我用了 new ParallelOptions() { MaxDegreeOfParallelism = 2 } 设置了并发度为最多2个CPU核,程序跑起来后会开两个线程,将一个大集合划分为2个小集合,相当于1个集合3个条件,第一个线程在执行3个条件的起始处会执行你的localInit函数,在3个条件迭代完之后再执行你的localFinally,第二个线程也是按照同样方式执行自己的3个条件,说的有点晦涩,画一张图说明吧。

三: 第二次优化

如果你了解Task<T>这种带有返回值的Task,这就好办了,多少个filterItemList就可以开多少个Task,反正Task底层是使用线程池承载的,所以不用怕,这样就完美的实现无锁编程。

        public static void ParallelTest3(List<string> filterItemList)        {            var totalCustomerIDList = new List<int>();            var tasks = new Task<List<int>>[filterItemList.Count];            for (int i = 0; i < filterItemList.Count; i++)            {                tasks[i] = Task.Factory.StartNew((query) =>                {                    return GetCustomerIDList(query.ToString());                }, filterItemList[i]);            }            Task.WaitAll(tasks);            for (int i = 0; i < tasks.Length; i++)            {                var smallCustomerIDList = tasks[i].Result;                if (i == 0)                {                    totalCustomerIDList.AddRange(smallCustomerIDList);                }                else                {                    totalCustomerIDList = totalCustomerIDList.Intersect(smallCustomerIDList).ToList();                }            }            Console.WriteLine($"最后交集客户ID:{string.Join(",", totalCustomerIDList)}");        }------ output -------最后交集客户ID:1,7Press any key to continue . . .

四:总结

我们将原来的6个lock优化到了无锁编程,但并不说明无锁编程就一定比带有lock的效率高,大家要结合自己的使用场景合理的使用和混合搭配。

好了,本篇就说到这里,希望对您有帮助。


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