num_cpus

The `num_cpus` topic refers to a Rust crate (library) that provides a simple and cross-platform way to determine the number of logical CPUs (or cores) available to the current process. This information is crucial for applications that need to optimize their performance by leveraging parallel processing, such as multi-threaded servers, data processing tools, or scientific simulations. By knowing the number of available CPUs, an application can intelligently size thread pools, distribute workloads, or configure concurrent tasks to make the most efficient use of system resources.

Key aspects:

1. Purpose: The primary goal of `num_cpus` is to answer the question: "How many CPU cores can my program effectively use?" It helps in avoiding both under-utilization (not enough parallelism) and over-utilization (too many threads contending for a limited number of cores, leading to context switching overhead).
2. Functionality: The crate exposes a single main function, `num_cpus::get()`, which returns an `usize` representing the count of logical CPUs. It abstracts away the complexities of querying this information on different operating systems (Windows, Linux, macOS, etc.).
3. Logical vs. Physical CPUs: It typically reports the number of *logical* CPUs, which includes hyper-threading (HT) cores. For example, a physical core with hyper-threading enabled might appear as two logical CPUs to the operating system. For most parallel computing scenarios, the logical CPU count is the more relevant metric.
4. Container/Virtual Machine Environments: When running inside a container (e.g., Docker) or a virtual machine, `num_cpus` will report the number of CPUs allocated to that specific container or VM, not necessarily the total number of CPUs on the host machine. This is generally the desired behavior, as the application should only concern itself with the resources it has access to.
5. Usage: To use `num_cpus`, you add it as a dependency in your `Cargo.toml` and then call `num_cpus::get()` in your Rust code. The function is generally safe and reliable.

In summary, `num_cpus` is a small but powerful utility that enables Rust applications to be more resource-aware and performant by providing a reliable way to query system CPU capabilities.

```rust
// First, add num_cpus to your Cargo.toml:
// [dependencies]
// num_cpus = "1.17"

fn main() {
    // Get the number of logical CPUs available to the process.
    let num_cpus = num_cpus::get();

    println!("Number of logical CPUs available: {}", num_cpus);

    // Example of using this information for a thread pool (conceptual):
    // In a real application, you might use a library like 'rayon' or 'crossbeam'
    // to manage actual threads, but this shows the principle.
    let num_worker_threads = num_cpus.saturating_sub(1).max(1); // Use all but one, or at least one.
    println!("Suggested number of worker threads: {}", num_worker_threads);

    // More practical use case: Spawning tasks for each CPU
    println!("\nSpawning a task for each CPU (conceptual):");
    for i in 0..num_cpus {
        // In a real scenario, you'd spawn actual threads or use a thread pool
        // like tokio's runtime or rayon's parallel iterators.
        println!("  - Initializing task for CPU {}", i);
    }
}
```