On Friday, October 27, 2017 at 4:51:15 PM UTC-4, Axel Wagner wrote: > > Yes. The random number generator employed (right above from what I linked > to; the fastrand function), apparently, is unsurprisingly imperfect. >
Why not to maintain a global start index for each goroutine? The value of the start index is the number of executed select blocks in a goroutine. So that we can change the fastrandn implementation to "return uint32(uint64(fastrand() + current_g.numExecutedSelects) * uint64(n) >> 32)" > > On Fri, Oct 27, 2017 at 10:47 PM, T L <tapi...@gmail.com <javascript:>> > wrote: > >> 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> 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. >>>> 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...@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.
