A CPU thread is a portion of a computer's CPU instructions that represents a task running within an individual CPU core — which together with other cores form the hardware component of the overall CPU. These threads help pass these sequences of instructions to the processor cores where work is performed.
How do CPU threads work?
Modern machines — from personal computers to robust datacenter servers — are expected to multitask reliably. A processor with more cores can help boost efficiency by synchronizing multiple threads running simultaneously on each core. This process, called multithreading, can provide a more responsive user experience. Gone are the days when just one task could be performed per core at any given time.
Multithreading helps reduce CPU stalling from task switching while targeting individual components comprising the core. For example, one thread might instruct the arithmetic logic unit (ALU) to do something, while another thread concurrently targets the floating-point unit (FPU). This depends on the workload in question since individual tasks can vary so widely. Multithreading enables parallel processing, to extract more CPU performance without worrying about blocking. Some chipmakers such as AMD have implemented a variant of multithreading called simultaneous multithreading (also called hyperthreading on Intel processors) — though this includes the concepts surrounding virtual cores.
Simultaneous multithreading (SMT) divides one physical CPU core into multiple (typically two) virtual cores. Each of these cores functions as a separate unit to allow more threads to execute at the same time. SMT is implemented at the hardware level to unlock more efficient overall performance from the CPU without requiring complex programming to support it. SMT trades some processing power for additional concurrency, though at the expense of having slightly less deterministic performance, as threads share the same resources. Conversely, multithreading without SMT offers greater performance at the cost of some concurrency. Each approach shines depending on the program's or application's processing requirements.
The number of available cores varies widely per CPU. While laptops and workstation desktops might offer less overall processing power since workloads are generally lighter, an enterprise-grade mainframe CPU might contain tens or even hundreds of cores to tackle complex computations.
While close to it, thread switching within a CPU isn't 100% seamless. There's a small, almost imperceptible delay between closing out one instruction and processing another — which can introduce some measurable latency. However, modern CPUs mask this small inefficiency well without meaningfully impacting overall system performance, unless at massive scale.
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How does HAProxy handle CPU thread management?
HAProxy is built for optimized thread management and multithreading, intelligently organizing threads to prevent data transmission between CPU cores located far apart from one another. This capability is bolstered by features such as automatic CPU binding and intelligently creating thread groups based on CPU mapping to reduce data sharing between threads, zero-copy forwarding (preventing L3 cache evictions by reducing unneeded buffering), load-balancing algorithms that scale with many threads, and more to boost application delivery performance.
To learn more about CPU thread management enhancements in HAProxy, check out our How HAProxy takes advantage of multi-core CPUs blog.