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<h1 class="main">Concurrency</h1>
<h2 id="spawning-concurrent-tasks">Spawning Concurrent Tasks <a href="concurrency.html#spawning-concurrent-tasks" class="header-anchor" aria-hidden="true">#</a></h2>
<p>V's model of concurrency is going to be very similar to Go's.
For now, <code>spawn foo()</code> runs <code>foo()</code> concurrently in a different thread:</p>
<div class="language-v">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)
}
</div>
<blockquote class="tip">
<p>[!NOTE]
Threads rely on the machine's CPU (number of cores/threads).
Be aware that OS threads spawned with <code>spawn</code>
have limitations in regard to concurrency,
including resource overhead and scalability issues,
and might affect performance in cases of high thread count.</p>
</blockquote>
<p>There's also a <code>go</code> keyword. Right now <code>go foo()</code> will be automatically renamed via vfmt
to <code>spawn foo()</code>, and there will be a way to launch a coroutine with <code>go</code> (a lightweight
thread managed by the runtime).</p>
<p>Sometimes it is necessary to wait until a parallel thread has finished. This can
be done by assigning a <em>handle</em> to the started thread and calling the <code>wait()</code> method
to this handle later:</p>
<div class="language-v">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
}
</div>
<p>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.</p>
<div class="language-v">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`
}
</div>
<p>If there is a large number of tasks, it might be easier to manage them
using an array of threads.</p>
<div class="language-v">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
</div>
<p>Additionally for threads that return the same type, calling <code>wait()</code>
on the thread array will return all computed values.</p>
<div class="language-v">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]
</div>
<h2 id="channels">Channels <a href="concurrency.html#channels" class="header-anchor" aria-hidden="true">#</a></h2>
<p>Channels are the preferred way to communicate between threads. V's channels work basically like
those in Go. You can push objects into a channel on one end and pop objects from the other end.
Channels can be buffered or unbuffered and it is possible to <code>select</code> from multiple channels.</p>
<h3 id="syntax-and-usage">Syntax and Usage <a href="concurrency.html#syntax-and-usage" class="header-anchor" aria-hidden="true">#</a></h3>
<p>Channels have the type <code>chan objtype</code>. An optional buffer length can be specified as the <code>cap</code> field
in the declaration:</p>
<div class="language-v">ch := chan int{} // unbuffered - "synchronous"
ch2 := chan f64{cap: 100} // buffer length 100
</div>
<p>Channels do not have to be declared as <code>mut</code>. 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:</p>
<div class="language-v">fn f(ch chan int) {
// ...
}
fn main() {
ch := chan int{}
spawn f(ch)
// ...
}
</div>
<p>Objects can be pushed to channels using the arrow operator. The same operator can be used to
pop objects from the other end:</p>
<div class="language-v">// make buffered channels so pushing does not block (if there is room in the buffer)
ch := chan int{cap: 1}
ch2 := chan f64{cap: 1}
n := 5
// push
ch <- n
ch2 <- 7.3
mut y := f64(0.0)
m := <-ch // pop creating new variable
y = <-ch2 // pop into existing variable
</div>
<p>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 <code>select</code> and
<code>try_push()</code> - 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 <code>or {}</code> block (see <a href="type-declarations.html#handling-optionsresults">Handling options/results</a>).</p>
<div class="language-v">ch := chan int{}
ch2 := chan f64{}
// ...
ch.close()
// ...
m := <-ch or {
println('channel has been closed')
}
// propagate error
y := <-ch2 ?
</div>
<h3 id="channel-select">Channel Select <a href="concurrency.html#channel-select" class="header-anchor" aria-hidden="true">#</a></h3>
<p>The <code>select</code> 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 <a href="statements-&-expressions.html#match">match</a> command:</p>
<div class="language-v">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')
}
</div>
<p>The timeout branch is optional. If it is absent <code>select</code> waits for an unlimited amount of time.
It is also possible to proceed immediately if no channel is ready in the moment <code>select</code> is called
by adding an <code>else { ... }</code> branch. <code>else</code> and <code><timeout></code> are mutually exclusive.</p>
<p>The <code>select</code> command can be used as an <em>expression</em> of type <code>bool</code>
that becomes <code>false</code> if all channels are closed:</p>
<div class="language-v">if select {
ch <- a {
// ...
}
} {
// channel was open
} else {
// channel is closed
}
</div>
<h3 id="special-channel-features">Special Channel Features <a href="concurrency.html#special-channel-features" class="header-anchor" aria-hidden="true">#</a></h3>
<p>For special purposes there are some builtin fields and methods:</p>
<div class="language-v">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`
</div>
<p>The <code>try_push/pop()</code> methods will return immediately with one of the results
<code>.success</code>, <code>.not_ready</code> or <code>.closed</code> - 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 <code>.len</code> and
<code>.closed</code> should not be used to make decisions.
Use <code>or</code> branches, error propagation or <code>select</code> instead (see <a href="concurrency.html#syntax-and-usage">Syntax and Usage</a>
and <a href="concurrency.html#channel-select">Channel Select</a> above).</p>
<h2 id="shared-objects">Shared Objects <a href="concurrency.html#shared-objects" class="header-anchor" aria-hidden="true">#</a></h2>
<p>Data can be exchanged between a thread and the calling thread via a shared variable.
Such variables should be created as <code>shared</code> and passed to the thread as such, too.
The underlying <code>struct</code> contains a hidden <em>mutex</em> that allows locking concurrent access
using <code>rlock</code> for read-only and <code>lock</code> for read/write access.</p>
<div class="language-v">struct St {
mut:
x int // data to be shared
}
fn (shared b St) g() {
lock b {
// read/modify/write b.x
}
}
fn main() {
shared a := St{
x: 10
}
spawn a.g()
// ...
rlock a {
// read a.x
}
}
</div>
<p>Shared variables must be structs, arrays or maps.</p>
</main>
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