You should read up on how a RWLock works. >
I am not going to answer to that ;-) About this code: https://play.golang.org/p/YOwuYFiqtlf 1. I wouldn't say that there are several goroutines writing, as there are several goroutines block in a mutex. Some might find this not important, although there are some people that have looked into this and think it is less efficient https://opensource.com/article/18/7/locks-versus-channels-concurrent-go ; I think it has to do on how the scheduler changes to another goroutine when the goroutine is lock on a channel. 2. *More importantly IMHO*, in your example the reader has to be aware that there is a lock. Hence, it is not "transparent" in the sense that the reader has to share something with the writer > > The Sleep(1) in the producer is only to add some delay to demonstrate it > gets terminated before the desired number of iterations. > > On Aug 29, 2019, at 12:13 AM, Leo Lara <l...@leopoldolara.com > <javascript:>> wrote: > > Hi Robert, > > To put you in context, it all started when I read > https://go101.org/article/channel-closing.html , that said that it is > impossible or at least you shouldn't close a channel that is being written > by several goroutines. Then I wrote this article with my solution > https://dev.to/leolara/closing-a-go-channel-written-by-several-goroutines-52j2 > also > in > https://medium.com/@leolara/closing-a-go-channel-written-by-several-goroutines-eba3a6c9404b > I > then created this issue https://github.com/go101/go101/issues/132 and > from there this topic was created by T L. > > Your example does not have several goruitnes writing so I think it is a > different problem. Perhaps that simple lock would work with several > goroutines, but I think there would be more contention with this lock. > > Anyway, I think I have already an optimisation to my code, I think using a > RW lock, if I put the "Add(1)" in a read lock and the wait in a Write lock > it might work better. The race condition that my lock prevents is only > related when an "Add" and a "Wait" run concurrently, several "Add" can run > concurrently. > > On Thursday, August 29, 2019 at 4:05:06 AM UTC+2, robert engels wrote: >> >> Here is a version using RWLock https://play.golang.org/p/YOwuYFiqtlf >> >> It won’t run correctly in the playground because it terminates when all >> routines are asleep - which happens during the test (not sure why it does >> this, as sleeping is different than a deadlock). >> >> It is probably less efficient, and less orderly than the other example >> using WaitGroup but you get the idea I hope. It forcibly terminates the >> writers before they complete by design. >> >> On Aug 28, 2019, at 4:09 PM, Michel Levieux <m.le...@capitaldata.fr> >> wrote: >> >> One should also be careful regarding the conceptual demands he or she is >> making. >> Having a shared resource (that is complex enough that it cannot be >> atomically accessed or modified) means essentially that "having multiple >> writers being transparent to the readers", fundamentally, is not possible. >> >> From the moment itself when such a resource is shared, there must be some >> sort of mecanism (that one using resources atomically usable) that ensures >> the integrity of it. >> Maybe what you're talking about is having it transparent in terms of >> code, in which case we both agree, but if you're looking for something >> transparent in essence, as in performance, logical construction and all the >> rest, I think there is a misunderstanding here: even if it was added in the >> language, there would be many many things going on under the hood, as it is >> already (and cannot really be otherwise) for channel use alone. >> >> As for the priority using selects, I think it's more of something to be >> dealt with on the "user-side". There are many kinds of priority in general, >> and trying to implement something in the language itself would IMO either >> be too specific compared to the nessecary time to do so or it would >> probably have a huge overhead on the "classical' use case of the select >> construct. >> + the fact that it is apparently already possible using RWMutexes. >> >> Le mer. 28 août 2019 à 22:37, Marcin Romaszewicz <mar...@gmail.com> a >> écrit : >> >>> Think of a channel as existing for the lifetime of a particular data >>> stream, and not have it be associated with either producer or consumer. >>> Here's an example: >>> >>> https://play.golang.org/p/aEAXXtz2X1g >>> >>> The channel here is closed after all producers have exited, and all >>> consumers continue to run until the channel is drained of data. >>> >>> The producers are managed by something somewhere in your code - and that >>> is the scope at which it makes sense to create channel ownership. I've used >>> a waitgroup to ensure that the channel is closed after all producers exit, >>> but you can use whatever barrier construct you want. >>> >>> Even if you must have a channel per producer, you can safely close the >>> producer side, without notifying the downstream about this. The example >>> early in the thread uses multiple channels, with one channel being used to >>> signal that the producers should exit. Channels aren't really the right >>> model for this, you want a thread safe flag of some sort. For example: >>> >>> var exitFlag uint64 >>> func producer(chan data int, wg *sync.WaitGroup) { >>> defer wg.Done() >>> for { >>> shouldExit := atomic.LoadUint64(&exitFlag) >>> if shouldExit == 1 { >>> return >>> } >>> chan <- rand.Intn(100) >>> } >>> } >>> >>> Here's 10 producers and 3 consumers sharing a channel and closing it >>> safely upon receiving an exit flag: >>> https://play.golang.org/p/RiKi1PGVSvF >>> >>> -- Marcin >>> >>> On Wed, Aug 28, 2019 at 11:29 AM Leo Lara <l...@leopoldolara.com> wrote: >>> >>>> I do not think priority select is *necessary*, it could be a nice >>>> addition if the performance does not change. >>>> >>>> On Wednesday, August 28, 2019 at 8:27:36 PM UTC+2, Leo Lara wrote: >>>>> >>>>> Hi Robert, >>>>> >>>>> From the article: """To bound more the problem, in my case, you >>>>> control the writers but not the readers""" >>>>> >>>>> So what I was trying to do was to be able to close, with mutiple >>>>> writers, while being transparent for the readers. The readers only need >>>>> to >>>>> read as usual form the channel. >>>>> >>>>> For example, if you want to write a library where the user just reads >>>>> from a channel, this is an approach I found where the user of the lirbary >>>>> deos nto have to do anything special. Of course, there might be another >>>>> solution, but if you need to modify the reader we are talking about a >>>>> different problem. >>>>> >>>>> Cheers!! >>>>> >>>>> On Wednesday, August 28, 2019 at 7:17:24 PM UTC+2, Robert Engels wrote: >>>>>> >>>>>> A better solution is to wrap the writes using a RWLock, grab the read >>>>>> lock for writing, and the Write lock for closing. Pretty simple. >>>>>> >>>>>> Just encapsulate it all in a MultiWriterChannel struct - generics >>>>>> would help here :) >>>>>> >>>>>> -----Original Message----- >>>>>> From: Leo Lara >>>>>> Sent: Aug 28, 2019 11:24 AM >>>>>> To: golang-nuts >>>>>> Subject: [go-nuts] Re: An old problem: lack of priority select cases >>>>>> >>>>>> This is connected with my article: >>>>>> https://dev.to/leolara/closing-a-go-channel-written-by-several-goroutines-52j2 >>>>>> >>>>>> I think there I show it is possible to workaround that limitation >>>>>> using standard Go tools. Of course, the code would be simple with >>>>>> priority >>>>>> select, but also perhaps select would become less efficient. >>>>>> >>>>>> On Wednesday, August 28, 2019 at 6:06:33 PM UTC+2, T L wrote: >>>>>>> >>>>>>> The old thread: >>>>>>> https://groups.google.com/forum/#!topic/golang-nuts/ZrVIhHCrR9o >>>>>>> >>>>>>> Go channels are flexible, but in practice, I often encountered some >>>>>>> situations in which channel are hard to use. >>>>>>> Given an example: >>>>>>> >>>>>>> import "math/rand" >>>>>>> >>>>>>> type Producer struct { >>>>>>> data chan int >>>>>>> closed chan struct{} >>>>>>> } >>>>>>> >>>>>>> func NewProducer() *Producer { >>>>>>> p := &Producer { >>>>>>> data: make(chan int), >>>>>>> closed: make(chan struct{}), >>>>>>> } >>>>>>> >>>>>>> go p.run() >>>>>>> >>>>>>> return p >>>>>>> } >>>>>>> >>>>>>> func (p *Produce) Stream() chan int { >>>>>>> return p.data >>>>>>> } >>>>>>> >>>>>>> func (p *Producer) run() { >>>>>>> for { >>>>>>> // If non-blocking cases are selected by their appearance >>>>>>> order, >>>>>>> // then the following slect block is a perfect use. >>>>>>> select { >>>>>>> case(0) <-p.closed: return >>>>>>> case p.data <- rand.Int(): >>>>>>> } >>>>>>> } >>>>>>> } >>>>>>> >>>>>>> func (p *Produce) Clsoe() { >>>>>>> close(p.closed) >>>>>>> close(p.data) >>>>>>> } >>>>>>> >>>>>>> func main() { >>>>>>> p := NewProducer() >>>>>>> for n := p.Stream() { >>>>>>> // use n ... >>>>>>> } >>>>>>> } >>>>>>> >>>>>>> >>>>>>> If the first case in the select block in the above example has a >>>>>>> higher priority than the second one, >>>>>>> then coding will be much happier for the use cases like the above >>>>>>> one. >>>>>>> >>>>>>> In short, the above use case requires: >>>>>>> * for receivers, data streaming end is notified by the close of a >>>>>>> channel. >>>>>>> * for senders, data will never be sent to closed channel. >>>>>>> >>>>>>> But, as Go 1 doesn't support priority select cases, it is much >>>>>>> tedious to implement the code >>>>>>> satisfying the above listed requirements. The final implementation >>>>>>> is often very ugly and inefficient. >>>>>>> >>>>>>> Does anyone else also experience the pain? >>>>>>> >>>>>> >>>>>> -- >>>>>> 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 golan...@googlegroups.com. >>>>>> To view this discussion on the web visit >>>>>> https://groups.google.com/d/msgid/golang-nuts/b284f880-034a-4721-8686-ef48d3e2c14c%40googlegroups.com >>>>>> >>>>>> <https://groups.google.com/d/msgid/golang-nuts/b284f880-034a-4721-8686-ef48d3e2c14c%40googlegroups.com?utm_medium=email&utm_source=footer> >>>>>> . >>>>>> >>>>>> >>>>>> >>>>>> >>>>>> >>>> -- >>>> 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 golan...@googlegroups.com. >>>> To view this discussion on the web visit >>>> https://groups.google.com/d/msgid/golang-nuts/aeb38a0a-8268-42d7-a8eb-ce5ef01c5380%40googlegroups.com >>>> >>>> <https://groups.google.com/d/msgid/golang-nuts/aeb38a0a-8268-42d7-a8eb-ce5ef01c5380%40googlegroups.com?utm_medium=email&utm_source=footer> >>>> . >>>> >>> >>> -- >>> 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 golan...@googlegroups.com. >>> To view this discussion on the web visit >>> https://groups.google.com/d/msgid/golang-nuts/CA%2Bv29LvcUhUvrZb_8AKYWj0A%2Bqd5LKBPmbz-RVBb%3DJn_gNZE6w%40mail.gmail.com >>> >>> <https://groups.google.com/d/msgid/golang-nuts/CA%2Bv29LvcUhUvrZb_8AKYWj0A%2Bqd5LKBPmbz-RVBb%3DJn_gNZE6w%40mail.gmail.com?utm_medium=email&utm_source=footer> >>> . >>> >> >> -- >> 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 golan...@googlegroups.com. >> To view this discussion on the web visit >> https://groups.google.com/d/msgid/golang-nuts/CANgi337s1Low95QvqJUAOTsqcVji7uMQ_jr%3DFftpt2uMz5_XSQ%40mail.gmail.com >> >> <https://groups.google.com/d/msgid/golang-nuts/CANgi337s1Low95QvqJUAOTsqcVji7uMQ_jr%3DFftpt2uMz5_XSQ%40mail.gmail.com?utm_medium=email&utm_source=footer> >> . >> >> >> -- > 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 golan...@googlegroups.com <javascript:>. > To view this discussion on the web visit > https://groups.google.com/d/msgid/golang-nuts/e5f37679-bdfb-4da3-854e-fea35cf82cb7%40googlegroups.com > > <https://groups.google.com/d/msgid/golang-nuts/e5f37679-bdfb-4da3-854e-fea35cf82cb7%40googlegroups.com?utm_medium=email&utm_source=footer> > . > > -- You received this message because you are subscribed to the Google Groups "golang-nuts" group. 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