Hi Lee, Thanks for pointing out my misuse of words. I've actually borne in mind that parallelism and concurrency are different things, but I made a slip. I'll try to be more careful next time.
On Tuesday, 17 April 2018 04:09:06 UTC+8, Lee Painton wrote: > > Also, as Rob Pike has stressed in the past, concurrency is not > parallelism. Concurrency is a design principle that enables parallelism, > but goroutines are concurrency constructs and do not automatically run in > parallel. > > On Monday, April 16, 2018 at 12:47:18 PM UTC-4, andrey mirtchovski wrote: >> >> 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 <dotsl...@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...@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.
