How to make a select case non-blocking in for loop without default

Question

I have this piece of code.

func Start() bool {
    for {
        if checkSomthingIsTrue() {
            if err := doSomthing(); err != nil {
                continue
            }
        }
        select {
        case <-ctx.Done():
            return true
        }
    }
}

How to make the above function non blocking without using default: case. reason to not use default case is because its eating up 100% CPU always.

Answer: I have used time.Ticker to throttle Thanks

`select` blocks if none of the communication ops are ready to proceed. You can only make a `select` without `default` non-blocking if you ensure at least one of its comm ops can proceed. — icza, Jun 17 '21 at 14:58
If you don't want a `default` because it "eats up" your CPU, then it looks you don't want `checkSomthingIsTrue()` to be called continuously. Use a `time.Timer` or `time.Ticker` to achieve some "sleep" between iterations, and still monitor the context while sleeping. — icza, Jun 17 '21 at 15:00
If you don't want a loop to use 100% CPU you want it to block. Can you explain how you want a non-blocking statement to block? — JimB, Jun 17 '21 at 15:00
There is a literal answer, but it's useless: just make sure one of the channels you select on is always ready. For instance, add a closed channel to the list of channels. But then you'll use 100% CPU on the closed channel. — torek, Jun 17 '21 at 16:03

Dietrich Epp · Accepted Answer · 2021-06-17T15:09:36.747

There is a fundamental misunderstanding here. A thread can do only two things:

A thread can block, waiting for something.
A thread can run, using CPU.

If a thread never blocks, then it uses 100% of the available CPU. You cannot make non-blocking code use less than 100% of the available CPU.

You have three options:

Use non-blocking code, and accept the 100% CPU usage.

Redesign checkSomthingIsTrue() so it uses a channel, and can be put inside the select block.

for {
    select {
    case <-ctx.Done():
        return true
    case <-whenSomethingIsTrue():
        if err := doSomthing(); err != nil {
            continue
        }
    }
}

Use a timeout to throttle the loop, for example:

// Poll every 100ms.
const pollInterval = 100 * time.Millisecond
for {
    select {
    case <-ctx.Done():
        return true
    case <-time.After(pollInterval):
        if checkSomthingIsTrue() {
            if err := doSomthing(); err != nil {
                continue
            }
        }
    }
}

Also note that continue makes no sense, but that is a different issue.

we can use [time.Tikcer](https://gobyexample.com/tickers) instead of `time.After` right? — nipuna, Jun 17 '21 at 15:09
@nipuna: Sure, if you prefer, you can use `Ticker`. The timing will be different. — Dietrich Epp, Jun 17 '21 at 15:10
You can use a `time.Ticker` but you may need to call Stop() on it when you are done otherwise you may leak tickers.`time.After()` cleans itself up after the duration. A `time.Timer` will be the same as `time.After()` for timing but you control when it is cleaned up. — Bracken, Jun 17 '21 at 19:31

How to make a select case non-blocking in for loop without default

1 Answers1