Scalability and Symmetric Multiprocessing

By Danny Kalev, ITworld |  How-to

The Linux kernel offers a high scalability level. It's modular and
fast. Additionally, it has a small memory footprint and a clever
virtual memory system. This is why Linux is used on a variety of
platforms, ranging from the smallest embedded systems, through ordinary
PCs, to multi-user servers. One of the most important kernel features
is Symmetric Multiprocessing (SMP) support. Multiprocessing is hardly
new -- multiprocessor machines have been around for decades. However,
it used to be very expensive, asymmetric, and relied on special
hardware architectures. With the emergence of the SMP architecture
several years ago, multiprocessing has become more widespread. SMP is
one of the most challenging aspects in the design of a modern operating
system. In fact, many of the delays in the release of kernel 2.4 were
attributed to SMP enhancements. What is so special about SMP and why is
it so difficult to design a good SMP kernel?

First, let's have a quick overview of this architecture. SMP relies on
three basic principles:

* Multiprocessing: This means that the system consists of multiple
identical CPUs.

* Symmetry: Each CPU has an identical view of the system and has
the same capabilities.

* Singularity: Except for the CPU and their caches, everything else
is single: there's a single memory system, a single kernel, and a
single run queue.

In an SMP-enabled environment, threads and processes have no affinity
to a particular CPU. In one time-slice, the same thread may execute on
CPU #1 and on another time-slice, it may execute on CPU #2. Seemingly,
a two-processor machine can be twice as fast compared to a uniprocessor
machine. In practice, however, the scalability gain is even lower
because other resources such as memory, I/O buses, and the kernel
itself -- are still shared. Furthermore, most algorithms are fully or
partially sequential (single-threaded) by nature, and so are system
calls, memory access routines, and synchronized operations. Therefore,
there is an upper limit to the speedup increase that you can get from
multiprocessing - no matter how many processors your machine has. That
said, with every release, the Linux kernel scales better in terms of
the number of processors supported and the scheduling algorithm.
Release 2.4 will probably support 16 processors. Future releases may
reach 64 and even 256 processors.

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