Concurrency

Spawning Concurrent Tasks

V's model of concurrency is similar to Go's.

go foo() runs foo() concurrently in a lightweight thread managed by the V runtime.

spawn foo() runs foo() concurrently in a different thread:

import math fn p(a f64, b f64) { // ordinary function without return value c := math.sqrt(a * a + b * b) println(c) } fn main() { spawn p(3, 4) // p will be run in parallel thread // It can also be written as follows // spawn fn (a f64, b f64) { // c := math.sqrt(a * a + b * b) // println(c) // }(3, 4) }

[!NOTE] Threads rely on the machine's CPU (number of cores/threads). Be aware that OS threads spawned with spawn have limitations in regard to concurrency, including resource overhead and scalability issues, and might affect performance in cases of high thread count.

Sometimes it is necessary to wait until a parallel thread has finished. This can be done by assigning a handle to the started thread and calling the wait() method to this handle later:

import math fn p(a f64, b f64) { // ordinary function without return value c := math.sqrt(a * a + b * b) println(c) // prints `5` } fn main() { h := spawn p(3, 4) // p() runs in parallel thread h.wait() // p() has definitely finished }

This approach can also be used to get a return value from a function that is run in a parallel thread. There is no need to modify the function itself to be able to call it concurrently.

import math { sqrt } fn get_hypot(a f64, b f64) f64 { // ordinary function returning a value c := sqrt(a * a + b * b) return c } fn main() { g := spawn get_hypot(54.06, 2.08) // spawn thread and get handle to it h1 := get_hypot(2.32, 16.74) // do some other calculation here h2 := g.wait() // get result from spawned thread println('Results: ${h1}, ${h2}') // prints `Results: 16.9, 54.1` }

If there is a large number of tasks, it might be easier to manage them using an array of threads.

import time fn task(id int, duration int) { println('task ${id} begin') time.sleep(duration * time.millisecond) println('task ${id} end') } fn main() { mut threads := []thread{} threads << spawn task(1, 500) threads << spawn task(2, 900) threads << spawn task(3, 100) threads.wait() println('done') } // Output: // task 1 begin // task 2 begin // task 3 begin // task 3 end // task 1 end // task 2 end // done

Additionally for threads that return the same type, calling wait() on the thread array will return all computed values.

fn expensive_computing(i int) int { return i * i } fn main() { mut threads := []thread int{} for i in 1 .. 10 { threads << spawn expensive_computing(i) } // Join all tasks r := threads.wait() println('All jobs finished: ${r}') } // Output: All jobs finished: [1, 4, 9, 16, 25, 36, 49, 64, 81]

Channels

Channels are the preferred way to communicate between threads. They allow threads to exchange data safely without requiring explicit locking. V's channels are similar to those in Go, enabling you to push objects into a channel on one end and pop objects from the other. Channels can be buffered or unbuffered, and you can use the select statement to monitor multiple channels simultaneously.

Syntax and Usage

Channels are declared with the type chan objtype. You can optionally specify a buffer length using the cap field:

ch := chan int{} // unbuffered - "synchronous" ch2 := chan f64{cap: 100} // buffered with a capacity of 100

Channels do not have to be declared as mut. The buffer length is not part of the type but a field of the individual channel object. Channels can be passed to threads like normal variables:

import time fn worker(ch chan int) { for i in 0 .. 5 { ch <- i // push values into the channel } } fn clock(ch chan int) { for i in 0 .. 5 { time.sleep(1 * time.second) println('Clock tick') ch <- (i + 1000) // push a value into the channel } ch.close() // close the channel when done } fn main() { ch := chan int{cap: 5} spawn worker(ch) spawn clock(ch) for { value := <-ch or { // receive/pop values from the channel println('Channel closed') break } println('Received: ${value}') } }

Buffered Channels

Buffered channels allow you to push multiple items without blocking, as long as the buffer is not full:

ch := chan string{cap: 2} ch <- 'hello' ch <- 'world' // ch <- '!' // This would block because the buffer is full println(<-ch) // "hello" println(<-ch) // "world"

Closing Channels

A channel can be closed to indicate that no further objects can be pushed. Any attempt to do so will then result in a runtime panic (with the exception of select and try_push() - see below). Attempts to pop will return immediately if the associated channel has been closed and the buffer is empty. This situation can be handled using an or {} block (see Handling options/results).

ch := chan int{} ch2 := chan f64{} // ... ch.close() // ... m := <-ch or { println('channel has been closed') } // propagate error y := <-ch2 ?

Note: buffered channels can be closed while they have unread values in them. The buffered values can be retrieved, even after the closing:

ich := chan int{cap: 5} for i in 0 .. 5 { ich <- i } for _ in 0 .. 2 { x := <-ich or { break } eprintln('>> loop 0..2 | x: ${x} | ich.closed: ${ich.closed}') } ich.close() for { x := <-ich or { break } eprintln('>> final loop | x: ${x} | ich.closed: ${ich.closed}') }

... will produce:

>>  loop 0..2 | x: 0 | ich.closed: false
>>  loop 0..2 | x: 1 | ich.closed: false
>> final loop | x: 2 | ich.closed: true
>> final loop | x: 3 | ich.closed: true
>> final loop | x: 4 | ich.closed: true

Note: reading from the .closed field of the channel in the example, is done just for clarity of illustration. The recommended way to pop values from the channel is with: x := <-ich or { break } in a for loop, which will cleanly break out of the loop, when the channel is closed and empty. Avoid manually checking, whether the channel was closed or not, because that can introduce data races, if you are not careful.

Channel Select

The select command allows monitoring several channels at the same time without noticeable CPU load. It consists of a list of possible transfers and associated branches of statements - similar to the match command:

import time fn main() { ch := chan f64{} ch2 := chan f64{} ch3 := chan f64{} mut b := 0.0 c := 1.0 // ... setup spawn threads that will send on ch/ch2 spawn fn (the_channel chan f64) { time.sleep(5 * time.millisecond) the_channel <- 1.0 }(ch) spawn fn (the_channel chan f64) { time.sleep(1 * time.millisecond) the_channel <- 1.0 }(ch2) spawn fn (the_channel chan f64) { _ := <-the_channel }(ch3) select { a := <-ch { // do something with `a` eprintln('> a: ${a}') } b = <-ch2 { // do something with predeclared variable `b` eprintln('> b: ${b}') } ch3 <- c { // do something if `c` was sent time.sleep(5 * time.millisecond) eprintln('> c: ${c} was send on channel ch3') } 500 * time.millisecond { // do something if no channel has become ready within 0.5s eprintln('> more than 0.5s passed without a channel being ready') } } eprintln('> done') }

The timeout branch is optional. If it is absent select waits for an unlimited amount of time. It is also possible to proceed immediately if no channel is ready in the moment select is called by adding an else { ... } branch. else and <timeout> are mutually exclusive.

The select command can be used as an expression of type bool that becomes false if all channels are closed:

if select { ch <- a { // ... } } { // channel was open } else { // channel is closed }

Special Channel Features

For special purposes there are some builtin fields and methods:

ch := chan int{cap: 2} println(ch.try_push(42)) // `.success` if pushed, `.not_ready` if full, `.closed` if closed println(ch.len) // Number of items in the buffer println(ch.cap) // Buffer capacity println(ch.closed) // Whether the channel is closed
struct Abc { x int } a := 2.13 ch := chan f64{} res := ch.try_push(a) // try to perform `ch <- a` println(res) l := ch.len // number of elements in queue c := ch.cap // maximum queue length is_closed := ch.closed // bool flag - has `ch` been closed println(l) println(c) mut b := Abc{} ch2 := chan Abc{} res2 := ch2.try_pop(mut b) // try to perform `b = <-ch2`

The try_push/pop() methods will return immediately with one of the results .success, .not_ready or .closed - dependent on whether the object has been transferred or the reason why not. Usage of these methods and fields in production is not recommended - algorithms based on them are often subject to race conditions. Especially .len and .closed should not be used to make decisions. Use or branches, error propagation or select instead (see Syntax and Usage and Channel Select above).

Shared Objects

Data can be exchanged between a thread and the calling thread via a shared variable. Such variables should be created as shared and passed to the thread as such, too. The underlying struct contains a hidden mutex that allows locking concurrent access using rlock for read-only and lock for read/write access.

Note: Shared variables must be structs, arrays or maps.

Example of Shared Objects

struct Counter { mut: value int } fn (shared counter Counter) increment() { lock counter { counter.value += 1 println('Incremented to: ${counter.value}') } } fn main() { shared counter := Counter{} spawn counter.increment() spawn counter.increment() rlock counter { println('Final value: ${counter.value}') } }

Difference Between Channels and Shared Objects

Purpose:

  • Channels: Used for message passing between threads, ensuring safe communication.
  • Shared objects: Used for direct data sharing and modification between threads.

Synchronization:

  • Channels: Implicit (via channel operations)
  • Shared objects: Explicit (via rlock/lock blocks)
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