On Friday, May 11, 2018 at 11:54:16 AM UTC-4, Jake Montgomery wrote:
>
> As others on this thread have pointed out, this use case does not actually
> require select where the cases are checked in order.
>
> However, for completeness, if you need to check two channels, and they
> must be checked in order, then use two select statements with defaults:
> for {
> select {
> case wi := <-work_ch:
> self.do_the_work(wi)
> default:
> }
> select {
> case <-self.idle_request:
> self.idle_response <- true
> default:
> }
> }
>
> Again, this should rarely be necessary, or even useful. But there it is.
>
Good point, but it is too cpu consuming.
I think the following one is better.
for {
select {
case <-self.idle_request:
self.idle_response <- true
default:
}
select {
case wi := <-work_ch:
self.do_the_work(wi)
case <-self.idle_request:
self.idle_response <- true
}
}
But I think if it would be even better if Go support non-random select
blocks.
>
>
> On Friday, May 4, 2018 at 10:24:56 AM UTC-4, Andriy Pylypenko wrote:
>>
>> Hi,
>>
>> I have a scenario which I haven't found a way to implement with the Go
>> channels and select statement while it's no problem to do it with the
>> select()/poll() functions. The scenario is as follows.
>>
>> Let me use a Go-like pseudo-code to demonstrate the idea. There is a
>> source of a work items which are processed by several goroutines. Here is
>> the source procedure:
>>
>> work_ch := make(chan *WorkItem)
>>
>> for {
>> work_ch <- NewWorkItem()
>> }
>>
>> And here is the worker thread:
>>
>> for {
>> wi := <-work_ch
>> self.do_the_work(wi)
>> }
>>
>> Now I need to suspend the work. I stop supplying the work items and want
>> to make sure the worker goroutines are done with the work. The source
>> procedure becomes something like this:
>>
>> for {
>> if suspend_requested {
>> for _, worker := range workers {
>> worker.idle_request <- true
>> }
>> for _, worker := range workers {
>> <-worker.idle_response
>> }
>> report_suspend_is_success()
>> wait_until_unsuspended()
>> }
>> work_ch <- NewWorkItem()
>> }
>>
>> And I want to modify the worker thread like this:
>>
>> for {
>> select {
>> case wi := <- work_ch:
>> self.do_the_work(wi)
>> case <-self.idle_request:
>> self.idle_response <- true
>> }
>> }
>>
>> However this last snippet of code does not work because the cases within
>> the select statement are randomly chosen when both channels are ready so I
>> have no guarantee that the first case is executed first.
>>
>> Talking about the select() or poll() functions from the C library which
>> evidently were an inspiration for the Go select statement, there is no
>> problem with the described approach. It's because the select() function
>> returns a complete information about the state of all the monitored
>> descriptors and allows the programmer to decide what he wants to read or
>> write and in which order, based on readiness of every descriptor.
>>
>> I think it would be a great idea to have some function similar to
>> select()/poll() but working with the Go channels.
>>
>
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