golang并发三板斧系列之二:goroutine池用于并发

baixiaoustc · · 1692 次点击 · · 开始浏览    
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这是本系列文章的第二篇,第一篇在此[golang并发三板斧系列之一:channel用于通信和同步](http://baixiaoustc.com/2019/02/17/2019-02-17-golang-concurency-1-channel/)。 前文描述了手工作坊的时代,即老师傅带着小学徒并发地做一项工作,现在我们准备进入工业时代。 # Pipeline模型 Pipeline即流水线模型,这在现代工业是很常见的。模型分为数个阶段,每个阶段干不同的事情,但可以并行地去做。以造拖拉机为例来解释流水线的工作方式,假设装配一辆汽车需要四个步骤: * 第一步冲压:制作车身外壳和底盘等部件。 * 第二步焊接:将冲压成形后的各部件焊接成车身。 * 第三步涂装:将车身等主要部件清洗、化学处理、打磨、喷漆和烘干。 * 第四步总装:将各部件(包括发动机和向外采购的零部件)组装成车。 ![](http://image99.renyit.com/image/2019-05-22-1.jpeg) 比如要造一百辆拖拉机,如果每个阶段都等前一阶段的一百辆完成才开工,是对生产线的极大浪费。现代工业的流水线做法是每个阶段同时开工,在时间上并行起来。流水线的概念在计算机世界中也很普遍,拥有流水线的CPU可以在一个时钟周期内完成一条指令,而不是等待取指令、译码、取操作数、执行四个阶段才能完成一条指令: ![](http://image99.renyit.com/image/2019-05-22-2.png) golang的设计者们吸收了这一经典概念,使用channel把前后数个阶段串联起来,形成一个流水线: ```go func gen(nums ...int) <-chan int { out := make(chan int) go func() { for _, n := range nums { out <- n } close(out) }() return out } func sq(in <-chan int) <-chan int { out := make(chan int) go func() { for n := range in { out <- n * n } close(out) }() return out } func TestPipeline(t *testing.T) { c := gen(2, 3, 4) out := sq(c) for n := range out { log.Print(n) } } ``` 由于`sq`函数的入参和出参一样,故可以增加无数个阶段写为: ```go func TestPipelines(t *testing.T) { for n := range sq(sq(sq(gen(1, 2, 3, 4)))) { log.Print(n) } } ``` # FAN模型 可以说FAN模型是流水线的一种改进。可以观察到上述的流水线模型,每个阶段只起了一个goroutine,但是现实造拖拉机的时候,在组装轮子的时候可以4个工人一起上,这又是提高了并发。golang吸取了这种模型,在任务分发阶段,多个goroutine从同一个channel读取数据,直到关闭,称为FAN-OUT模型;在结果收集阶段,单个goroutine从多个channel读取数据,直到关闭,称为FAN-IN模型: [图片上传失败...(image-d2aaa-1558316671133)] ```go func merge(cs ...<-chan int) <-chan int { var wg sync.WaitGroup out := make(chan int) output := func(c <-chan int) { for n := range c { out <- n } wg.Done() } wg.Add(len(cs)) for _, c := range cs { go output(c) } go func() { wg.Wait() close(out) }() return out } func TestFan(t *testing.T) { c := gen(2, 3, 4) out1 := sq(c) out2 := sq(c) for n := range merge(out1, out2) { log.Print(n) } } ``` 并行起来之后,最终结果的顺序是不可控的: 2019/02/22 21:39:27 9 2019/02/22 21:39:27 16 2019/02/22 21:39:27 4 Process finished with exit code 0 # 以上模型能提升并行效率吗 ## CPU密集型 用浮点数的幂计算模拟CPU-BOUND,设计了如下模型用于比较: ```go //功能函数 func benchmarkList() []float64 { list := make([]float64, MAX) for n := 0; n < MAX; n++ { list[n] = float64(n) } return list } func cpubound(n float64) float64 { return math.Pow(3.1415926, n) } func genChan(nums []float64) <-chan float64 { out := make(chan float64) go func() { for _, n := range nums { out <- n } close(out) }() return out } func genChanBuffer(nums []float64) <-chan float64 { out := make(chan float64, BUFFERSIZE) go func() { for _, n := range nums { out <- n } close(out) }() return out } func cpuChan(in <-chan float64) <-chan float64 { out := make(chan float64) go func() { for n := range in { out <- cpubound(n) } close(out) }() return out } func cpuChanBuffer(in <-chan float64) <-chan float64 { out := make(chan float64, BUFFERSIZE) go func() { for n := range in { out <- cpubound(n) } close(out) }() return out } func mergeChan(cs ...<-chan float64) <-chan float64 { var wg sync.WaitGroup out := make(chan float64) output := func(c <-chan float64) { for n := range c { out <- n } wg.Done() } wg.Add(len(cs)) for _, c := range cs { go output(c) } go func() { wg.Wait() close(out) }() return out } func mergeChanBuffer(cs ...<-chan float64) <-chan float64 { var wg sync.WaitGroup out := make(chan float64, BUFFERSIZE) output := func(c <-chan float64) { for n := range c { out <- n } wg.Done() } wg.Add(len(cs)) for _, c := range cs { go output(c) } go func() { wg.Wait() close(out) }() return out } //测试函数 func BenchmarkCPUSequential(b *testing.B) { for i := 0; i < b.N; i++ { list := benchmarkList() var sum float64 for _, n := range list { sum += cpubound(n) } } } func BenchmarkCPUPipeline(b *testing.B) { for i := 0; i < b.N; i++ { list := benchmarkList() c := genChan(list) out := cpuChan(c) var sum float64 for n := range out { sum += n } } } func BenchmarkCPUPipelineBuffer(b *testing.B) { for i := 0; i < b.N; i++ { list := benchmarkList() c := genChanBuffer(list) out := cpuChanBuffer(c) var sum float64 for n := range out { sum += n } } } func BenchmarkCPUFan(b *testing.B) { for i := 0; i < b.N; i++ { list := benchmarkList() c := genChan(list) gonum := runtime.NumCPU() / 2 outs := make([]<-chan float64, gonum) for i := 0; i < gonum; i++ { outs[i] = cpuChan(c) } var sum float64 for n := range mergeChan(outs...) { sum += n } } } func BenchmarkCPUFanBuffer(b *testing.B) { for i := 0; i < b.N; i++ { list := benchmarkList() c := genChanBuffer(list) gonum := runtime.NumCPU() / 2 outs := make([]<-chan float64, gonum) for i := 0; i < gonum; i++ { outs[i] = cpuChanBuffer(c) } var sum float64 for n := range mergeChanBuffer(outs...) { sum += n } } } func BenchmarkCPUParallelize(b *testing.B) { for i := 0; i < b.N; i++ { list := benchmarkList() gonum := runtime.NumCPU() var sum float64 num := len(list) stride := num / gonum var wg sync.WaitGroup wg.Add(gonum) var mux sync.Mutex for g := 0; g < gonum; g++ { go func(g int) { start := g * stride end := start + stride if g == gonum-1 { end = num } var sumin float64 for _, n := range list[start:end] { sumin += cpubound(n) } mux.Lock() sum += sumin mux.Unlock() wg.Done() }(g) } wg.Wait() } } ``` 设MAX=1000000,BUFFERSIZE=1000。结果让人大跌眼镜,无论是Pipeline模型还是Fan模型,都比不上普通的串行,只有普通的并发模型能有效提升: [baixiao@localhost go_concurrency]$ GOGC=off go test -cpu 1,8 -run none -bench CPU -benchtime 3s goos: linux goarch: amd64 BenchmarkCPUSequential 50 95148037 ns/op BenchmarkCPUSequential-8 30 101450384 ns/op BenchmarkCPUPipeline 10 512093124 ns/op BenchmarkCPUPipeline-8 5 864946495 ns/op BenchmarkCPUPipelineBuffer 20 219850707 ns/op BenchmarkCPUPipelineBuffer-8 10 370302165 ns/op BenchmarkCPUFan 5 715223945 ns/op BenchmarkCPUFan-8 5 913448396 ns/op BenchmarkCPUFanBuffer 10 320494600 ns/op BenchmarkCPUFanBuffer-8 10 427863250 ns/op BenchmarkCPUParallelize 100 95482003 ns/op BenchmarkCPUParallelize-8 200 19398520 ns/op PASS ok _/home/baixiao/go_concurrency 77.085s 可以得出以下结论: * Sequential完爆Pipeline和Fan * Pipeline和Fan在多核下均弱于单核,因为系统瓶颈根本不在并行上,而是channel造成的阻塞 * 给channel加了buffer之后,PipelineBuffer优于Pipeline、FanBuffer优于Fan,因为channel的阻塞减弱了 * 多核下的Parallelize:在座的各位都是垃圾 ## IO密集型 用随机sleep模拟IO-BOUND,设计了如下模型用于比较: ```go //功能函数 func iobound(n float64) float64 { time.Sleep(time.Duration(rand.Intn(10)) * time.Millisecond) return n } func ioChan(in <-chan float64) <-chan float64 { out := make(chan float64) go func() { for n := range in { out <- iobound(n) } close(out) }() return out } func ioChanBuffer(in <-chan float64) <-chan float64 { out := make(chan float64, BUFFERSIZE) go func() { for n := range in { out <- iobound(n) } close(out) }() return out } //测试函数 func BenchmarkIOSequential(b *testing.B) { for i := 0; i < b.N; i++ { list := benchmarkList() var sum float64 for _, n := range list { sum += iobound(n) } } } func BenchmarkIOPipeline(b *testing.B) { for i := 0; i < b.N; i++ { list := benchmarkList() c := genChan(list) out := ioChan(c) var sum float64 for n := range out { sum += n } } } func BenchmarkIOPipelineBuffer(b *testing.B) { for i := 0; i < b.N; i++ { list := benchmarkList() c := genChanBuffer(list) out := ioChanBuffer(c) var sum float64 for n := range out { sum += n } } } func BenchmarkIOFan(b *testing.B) { for i := 0; i < b.N; i++ { list := benchmarkList() c := genChan(list) gonum := runtime.NumCPU() / 2 outs := make([]<-chan float64, gonum) for i := 0; i < gonum; i++ { outs[i] = ioChan(c) } var sum float64 for n := range mergeChan(outs...) { sum += n } } } func BenchmarkIOFanBuffer(b *testing.B) { for i := 0; i < b.N; i++ { list := benchmarkList() c := genChanBuffer(list) gonum := runtime.NumCPU() / 2 outs := make([]<-chan float64, gonum) for i := 0; i < gonum; i++ { outs[i] = ioChanBuffer(c) } var sum float64 for n := range mergeChanBuffer(outs...) { sum += n } } } func BenchmarkIOParallelize(b *testing.B) { for i := 0; i < b.N; i++ { list := benchmarkList() gonum := runtime.NumCPU() var sum float64 num := len(list) stride := num / gonum var wg sync.WaitGroup wg.Add(gonum) var mux sync.Mutex for g := 0; g < gonum; g++ { go func(g int) { start := g * stride end := start + stride if g == gonum-1 { end = num } var sumin float64 for _, n := range list[start:end] { sumin += iobound(n) } mux.Lock() sum += sumin mux.Unlock() wg.Done() }(g) } wg.Wait() } } ``` 设MAX=100,BUFFERSIZE=1000。 [baixiao@localhost go_concurrency]$ GOGC=off go test -cpu 1,8 -run none -bench IO -benchtime 3s goos: linux goarch: amd64 BenchmarkIOSequential 10 441609349 ns/op BenchmarkIOSequential-8 10 457200927 ns/op BenchmarkIOPipeline 10 454147034 ns/op BenchmarkIOPipeline-8 10 467264740 ns/op BenchmarkIOPipelineBuffer 10 456459492 ns/op BenchmarkIOPipelineBuffer-8 10 452286832 ns/op BenchmarkIOFan 100 36995490 ns/op BenchmarkIOFan-8 100 36950275 ns/op BenchmarkIOFanBuffer 100 37555702 ns/op BenchmarkIOFanBuffer-8 100 36851459 ns/op BenchmarkIOParallelize 50 88653231 ns/op BenchmarkIOParallelize-8 50 87061568 ns/op PASS ok _/home/baixiao/go_concurrency 54.007s 可以得出以下结论: * 在IO密集的情况下,Fan模型吊打所有 * channel带buffer没有效率提升 * 所有的模型,在多核下都没有提升 ## 结论是? 1. 不带buffer的channel由于「强同步」特性,无法提高并行,甚至拖累效率 2. CPU密集型的场景,多核并行能提升效率 3. IO密集型的场景,多核并行不能提升效率 4. Pipeling模型有何用途?我没看出来 5. Waitgroup模型在CPU密集型场景有优势 6. Fan模型在IO密集型场景有优势 **坑:runtime.NumCPU()不会随着runtime.GOMAXPROCS()改变,前者代表的是系统全部的核数,后者代表的是可同时使用的核数** # goroutine池 ## pool模型 设计一个pool,需要考虑几个方面:输入是什么,做什么事情,多少worker一起执行? 现抽象出一个goroutine pool的模型代码,可以自定义输入类型,执行函数,worker数量。 ```go func genPoolChanBuffer(nums []float64) <-chan interface{} { out := make(chan interface{}, BUFFERSIZE) go func() { for _, n := range nums { out <- n } close(out) }() return out } type Handler func(*Work) type Work struct { input interface{} } type Pool struct { channum int workernum int wg *sync.WaitGroup ch chan Work Func Handler } func (p Pool) RunWorker() { for i := 0; i < p.workernum; i++ { p.wg.Add(1) go func() { defer p.wg.Done() for work := range p.ch { p.Func(&work) } }() } } func (p Pool) FeedWorker(in <-chan interface{}) { go func() { for n := range in { work := Work{ input: n, } p.ch <- work } close(p.ch) }() } func (p Pool) Wait() { p.wg.Wait() } func InitPool(channum, workernum int, f Handler) *Pool { return &Pool{ channum: channum, workernum: workernum, wg: &sync.WaitGroup{}, ch: make(chan Work, channum), Func: f, } } ``` ## 对比测试 设计了四个对比测试,观察在CPU密集型和IO密集型的情况下该pool的表现,另外还旨在探索什么情况下worker的数量会越多越好?带Min的测试中设置`gonum`为系统核数的一半,带Max的测试中设置`gonum`为系统核数的十倍,`gonum`即为worker数量。 ```go //测试函数 func BenchmarkCPUPool(b *testing.B) { channum := 100 gonum := runtime.NumCPU() for i := 0; i < b.N; i++ { f := func(w *Work) { if v, ok := w.input.(float64); ok { cpubound(v) } } list := benchmarkList() c := genPoolChanBuffer(list) p := InitPool(channum, gonum, f) p.RunWorker() p.FeedWorker(c) p.Wait() } } func BenchmarkCPUPoolMin(b *testing.B) { channum := 100 gonum := runtime.NumCPU() / 2 for i := 0; i < b.N; i++ { f := func(w *Work) { if v, ok := w.input.(float64); ok { cpubound(v) } } list := benchmarkList() c := genPoolChanBuffer(list) p := InitPool(channum, gonum, f) p.RunWorker() p.FeedWorker(c) p.Wait() } } func BenchmarkCPUPoolMax(b *testing.B) { channum := 100 gonum := 10 * runtime.NumCPU() for i := 0; i < b.N; i++ { f := func(w *Work) { if v, ok := w.input.(float64); ok { cpubound(v) } } list := benchmarkList() c := genPoolChanBuffer(list) p := InitPool(channum, gonum, f) p.RunWorker() p.FeedWorker(c) p.Wait() } } func BenchmarkIOPool(b *testing.B) { channum := 100 gonum := runtime.NumCPU() for i := 0; i < b.N; i++ { f := func(w *Work) { if v, ok := w.input.(float64); ok { iobound(v) } } list := benchmarkList() c := genPoolChanBuffer(list) p := InitPool(channum, gonum, f) p.RunWorker() p.FeedWorker(c) p.Wait() } } func BenchmarkIOPoolMin(b *testing.B) { channum := 100 gonum := runtime.NumCPU() / 2 for i := 0; i < b.N; i++ { f := func(w *Work) { if v, ok := w.input.(float64); ok { iobound(v) } } list := benchmarkList() c := genPoolChanBuffer(list) p := InitPool(channum, gonum, f) p.RunWorker() p.FeedWorker(c) p.Wait() } } func BenchmarkIOPoolMax(b *testing.B) { channum := 100 gonum := 10 * runtime.NumCPU() for i := 0; i < b.N; i++ { f := func(w *Work) { if v, ok := w.input.(float64); ok { iobound(v) } } list := benchmarkList() c := genPoolChanBuffer(list) p := InitPool(channum, gonum, f) p.RunWorker() p.FeedWorker(c) p.Wait() } } ``` 设MAX=100,BUFFERSIZE=1000。 [baixiao@localhost go_concurrency]$ GOGC=off go test -cpu 1,8 -run none -bench Pool -benchtime 3s goos: linux goarch: amd64 BenchmarkCPUPool 100000 38070 ns/op BenchmarkCPUPool-8 50000 77872 ns/op BenchmarkCPUPoolMin 100000 33286 ns/op BenchmarkCPUPoolMin-8 50000 71690 ns/op BenchmarkCPUPoolMax 30000 143250 ns/op BenchmarkCPUPoolMax-8 20000 281619 ns/op BenchmarkIOPool 200 21382667 ns/op BenchmarkIOPool-8 200 21467617 ns/op BenchmarkIOPoolMin 100 37068480 ns/op BenchmarkIOPoolMin-8 100 37350682 ns/op BenchmarkIOPoolMax 500 9438800 ns/op BenchmarkIOPoolMax-8 500 9519574 ns/op PASS ok _/home/baixiao/go_concurrency 64.149s 可以得出以下结论: * CPU密集型场景中多核下均弱于单核 * CPU密集型场景中worker数量太多只能起反作用 * IO密集型场景中多核并行不能提升效率 * IO密集型场景中worker数量在一定范围内能有效提升效率 * Pool模型由于用到了channel,多核都不能提升效率 # channel是个好东西? 在第一篇里,我们讲到channel是goroutine之间通信和同步的重要工具,也是golang中重要的关键字之一,说明golang的设计者们很看重这个特性。 但是实际上channel的性能较一般,分析源码可知,channel中的数据无论读写都会加mutex锁,造成高并发时的较大瓶颈,这个从我们的对比测试中也都可以看出来。 ![image](http://upload-images.jianshu.io/upload_images/17052744-5671925d03f9f0f5.png?imageMogr2/auto-orient/strip%7CimageView2/2/w/1240) 上图来自[文章](http://lessisbetter.site/2019/03/03/golang-channel-design-and-source/)。 另外,channel目前都还有整个社群都无法调优的问题,比如[runtime: select on a shared channel is slow with many Ps](https://github.com/golang/go/issues/20351)。 --- 本篇的模型可以类比为你现在是个企业主,开工厂进行工业生产了。 --- 所有代码都在[https://github.com/baixiaoustc/go_concurrency/blob/master/second_post_test.go](https://github.com/baixiaoustc/go_concurrency/blob/master/second_post_test.go)中能找到。

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