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docs/background.rst

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+Background and Motivation
+=========================
+
+With the growing complexity of high-end supercomputers, the current
+system software stack faces significant challenges as we move forward to
+exascale and beyond. The necessity to deal with extreme degree of
+parallelism, heterogeneous architectures, multiple levels of memory
+hierarchy, power constraints, etc., advocates operating systems that can
+rapidly adapt to new hardware requirements, and that can support novel
+programming paradigms and runtime systems. On the other hand, a new
+class of more dynamic and complex applications are also on the horizon,
+with an increasing demand for application constructs such as in-situ
+analysis, workflows, elaborate monitoring and performance tools. This
+complexity relies not only on the rich features of POSIX, but also on
+the Linux APIs (such as the */proc*, */sys* filesystems, etc.) in
+particular.
+
+Two Traditional HPC OS Approaches
+---------------------------------
+
+Traditionally, light-weight operating systems specialized for HPC
+followed two approaches to tackle scalable execution of large-scale
+applications. In the full weight kernel (FWK) approach, a full Linux
+environment is taken as the basis, and features that inhibit attaining
+HPC scalability are removed, i.e., making it light-weight. The pure
+light-weight kernel (LWK) approach, on the other hand, starts from
+scratch and effort is undertaken to add sufficient functionality so that
+it provides a familiar API, typically something close to that of a
+general purpose OS, while at the same time it retains the desired
+scalability and reliability attributes. Neither of these approaches
+yields a fully Linux compatible environment.
+
+The Multi-kernel Approach
+-------------------------
+
+A hybrid approach recognized recently by the system software community
+is to run Linux simultaneously with a lightweight kernel on compute
+nodes and multiple research projects are now pursuing this direction.
+The basic idea is that simulations run on an HPC tailored lightweight
+kernel, ensuring the necessary isolation for noiseless execution of
+parallel applications, but Linux is leveraged so that the full POSIX API
+is supported. Additionally, the small code base of the LWK can also
+facilitate rapid prototyping for new, exotic hardware features.
+Nevertheless, the questions of how to share node resources between the
+two types of kernels, where do device drivers execute, how exactly do
+the two kernels interact with each other and to what extent are they
+integrated, remain subjects of ongoing debate.