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Understanding the Linux Kernel, 3rd Edition by Marco Cesati, Daniel P. Bovet

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Runqueue Balancing in Multiprocessor Systems

We have seen in Chapter 4 that Linux sticks to the Symmetric Multiprocessing model (SMP ); this means, essentially, that the kernel should not have any bias toward one CPU with respect to the others. However, multiprocessor machines come in many different flavors, and the scheduler behaves differently depending on the hardware characteristics. In particular, we will consider the following three types of multiprocessor machines:

Classic multiprocessor architecture

Until recently, this was the most common architecture for multiprocessor machines. These machines have a common set of RAM chips shared by all CPUs.

Hyper-threading

A hyper-threaded chip is a microprocessor that executes several threads of execution at once; it includes several copies of the internal registers and quickly switches between them. This technology, which was invented by Intel, allows the processor to exploit the machine cycles to execute another thread while the current thread is stalled for a memory access. A hyper-threaded physical CPU is seen by Linux as several different logical CPUs.

NUMA

CPUs and RAM chips are grouped in local "nodes" (usually a node includes one CPU and a few RAM chips). The memory arbiter (a special circuit that serializes the accesses to RAM performed by the CPUs in the system, see the section "Memory Addresses" in Chapter 2) is a bottleneck for the performance of the classic multiprocessor systems. In a NUMA architecture, when a CPU accesses ...

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