It is also not uniform for powers of two. If there a 8 cases, the sixth one will only get a 80% chance of the first one.
On Friday, October 27, 2017 at 4:33:47 PM UTC-4, Axel Wagner wrote: > > The code suggests, that select is indeed not fair: > > https://github.com/golang/go/blob/a4d03a9bf7604b727abd0a1ebfb118ff6366ee50/src/runtime/stubs.go#L109 > Which seems fine to me. A fair, uniform PRNG for ranges that are not > powers of two gets a fair bit of overhead and no program should depend on > uniformity of select for correctness anyway. > > On Fri, Oct 27, 2017 at 10:28 PM, T L <tapi...@gmail.com <javascript:>> > wrote: > >> It looks the distribution of the RNG results depends on the number of >> cases. >> The third case only gets the lowest possibility of there are 6 cases, >> >> >> On Friday, October 27, 2017 at 4:21:12 PM UTC-4, T L wrote: >>> >>> A better example to show the inhomogeneity of the selected possibilities >>> of each case. >>> >>> package main >>> >>> import "fmt" >>> >>> func main() { >>> foo := make(chan struct{}) >>> close(foo) >>> var a, b, c, d, e, f, g float64 >>> fa := func(){a++} >>> fb := func(){b++} >>> fc := func(){c++} >>> fd := func(){d++} >>> fe := func(){e++} >>> ff := func(){f++} >>> fg := func(){g++} >>> for i := 0; i < 1000000; i++ { >>> select { >>> case <-foo: fa() >>> case <-foo: fb() >>> case <-foo: fc() >>> case <-foo: fd() >>> case <-foo: fe() >>> case <-foo: ff() >>> case <-foo: fg() >>> } >>> } >>> fmt.Println(a/a) >>> fmt.Println(b/a) >>> fmt.Println(c/a) >>> fmt.Println(d/a) >>> fmt.Println(e/a) >>> fmt.Println(f/a) >>> fmt.Println(g/a) >>> } >>> >>> result: >>> >>> 1 >>> 0.9999052991869258 >>> 0.9015652691532394 >>> 0.9691884140319548 >>> 0.966320332264567 >>> 0.9669020658305938 >>> 0.9604624105415533 >>> >>> On Friday, October 27, 2017 at 4:08:39 PM UTC-4, T L wrote: >>>> >>>> The following program constantly prints a value larger than 1.02 >>>> >>>> package main >>>> >>>> import "fmt" >>>> >>>> func main() { >>>> foo, bar := make(chan struct{}), make(chan struct{}) >>>> close(foo) >>>> close(bar) >>>> x, y := 0.0, 0.0 >>>> f := func(){x++} >>>> g := func(){y++} >>>> for i := 0; i < 1000000; i++ { >>>> select { >>>> case <-bar: g() >>>> case <-foo: f() >>>> case <-bar: g() >>>> case <-foo: f() >>>> case <-bar: g() >>>> case <-foo: f() >>>> } >>>> } >>>> fmt.Println(x/y) >>>> } >>>> >>>> if the last two cases are removed, then it becomes quit fair. >>>> >>> -- >> 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 <javascript:>. >> 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.
