available_parallelism()

Async\available_parallelism() — returns the number of CPUs available to the process. Honours cgroup quotas, affinity, and container limits.

available_parallelism ​

(PHP 8.6+, True Async 1.0)

Async\available_parallelism() returns the number of CPUs available to the current process.

Description ​

php
namespace Async;

function available_parallelism(): int

Honours cgroup CPU quotas, sched_setaffinity, and similar constraints. This is the value libuv recommends for thread-pool / worker-pool sizing. Always >= 1.

In a container with cpu.max=2, the function returns 2, not the physical core count of the host. On bare metal — the number of logical cores minus affinity restrictions (if any).

Backend: uv_available_parallelism() with a fallback to uv_cpu_info.

Return value ​

int — number of CPUs, guaranteed >= 1.

Examples ​

Example #1 Pool size matching available CPUs ​

php
<?php
use Async\ThreadPool;
use function Async\available_parallelism;

// Idiomatic: the autodetect is already built into ThreadPool via workers=0,
// but the explicit call is useful when you scale something else.
$pool = new ThreadPool(workers: available_parallelism());

Example #2 HTTP server worker-pool size ​

php
<?php
use TrueAsync\HttpServer;
use TrueAsync\HttpServerConfig;
use function Async\available_parallelism;

$server = new HttpServer(
    (new HttpServerConfig())
        ->addListener('0.0.0.0', 8080)
        ->setWorkers(available_parallelism())
);

$server->start();

Example #3 Environment diagnostics ​

php
<?php
use function Async\available_parallelism;

echo "Process can use ", available_parallelism(), " CPU(s)\n";

// In Docker with `--cpus=2` → 2
// On a host with 16 cores and no limits → 16
// In a Kubernetes pod with requests/limits cpu=1 → 1

Notes ​

Tip: for ThreadPool and HttpServer::setWorkers() you do not have to call this function by hand at all — both components use available_parallelism() automatically when the pool size is set to 0.

On most IO-bound workloads it makes sense to overcommit by N + 1 or N + 2, because some workers will be blocked on I/O.

See also ​