In short, your concurrency is too fine-grained. Adding concurrency primitives requires locking which is expensive, and creating a lot of goroutines does consume resources, even if we consider it relatively cheap.
If you slice the problem slightly differently it can be made faster: one goroutine per number, but each goroutine calculates its number the "normal" way, without using concurrency: https://play.golang.org/p/lKYSbuK79sB $ time ./tt 20 > /dev/null # new code real 0m3.374s user 0m5.129s sys 0m0.016s $ time ./t 20 > /dev/null # your original non-concurrent program real 0m4.593s user 0m4.538s sys 0m0.019s On Mon, Apr 16, 2018 at 9:09 AM, Tashi Lu <dotslash...@gmail.com> wrote: > Hi all, > > As a newbie I tried to implement a simple program calculating the Catalan > numbers, which are the numbers satisfying the recursion equation c(1) = 1; > c(n) = c(n - 1) * c(1) + c(n - 2) * c(2) + ... c(1) * c(n). > > At first, I implemented it without channels: > package main > > import ( > "fmt" > "os" > "strconv" > ) > > func catalan(n int) int { > if n == 1 { > return 1 > } > res := 0 > for i := 1; i < n; i++ { > res += catalan(n - i) * catalan(i) > } > return res > } > > func main() { > n, _ := strconv.Atoi(os.Args[1]) > for i := 1; i <= n; i++ { > fmt.Println(catalan(i)) > } > } > > > Then I thought the calculation can be easily made concurrent, so I wrote the > version below: > package main > > import ( > "fmt" > "os" > "strconv" > ) > > func catalan(n int, ch chan int) { > if n == 1 { > ch <- 1 > return > } > res := 0 > for i := 1; i < n; i++ { > ch1 := make(chan int) > ch2 := make(chan int) > go catalan(n - i, ch1) > go catalan(i, ch2) > res += <-ch1 * <-ch2 > close(ch1) > close(ch2) > } > ch <- res > } > > func main() { > n, _ := strconv.Atoi(os.Args[1]) > for i := 1; i <= n; i++ { > q := make(chan int) > go catalan(i, q) > fmt.Println(<-q) > close(q) > } > } > > > But I found the second version was unexpectly slower than the first: > go run catalan.go 15 0.07s user 0.66s system 257% cpu 0.281 total > vs > go run catalan-parallel.go 15 3.80s user 0.97s system 130% cpu 3.662 total > > What's the reason behind this? How can I improve this concurrent version to > make it faster? > > -- > You received this message because you are subscribed to the Google Groups > "golang-nuts" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to golang-nuts+unsubscr...@googlegroups.com. > For more options, visit https://groups.google.com/d/optout. -- You received this message because you are subscribed to the Google Groups "golang-nuts" group. To unsubscribe from this group and stop receiving emails from it, send an email to golang-nuts+unsubscr...@googlegroups.com. For more options, visit https://groups.google.com/d/optout.
