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docs/archtecture.rst

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+Architectural Overview
+======================
+
+At the heart of the stack is a low-level software infrastructure called
+Interface for Heterogeneous Kernels (IHK). IHK is a general framework
+that provides capabilities for partitioning resources in a many-core
+environment (e.g.,CPU cores and physical memory) and it enables
+management of lightweight kernels. IHK can allocate and release host
+resources dynamically and no reboot of the host machine is required when
+altering configuration. IHK also provides a low-level inter-kernel
+messaging infrastructure, called the Inter-Kernel Communication (IKC)
+layer. An architectural overview of the main system components is shown
+below.
+
+.. figure:: mckernel.png
+   :alt: arch
+
+McKernel is a lightweight kernel written from scratch. It is designed
+for HPC and is booted from IHK. McKernel retains a binary compatible ABI
+with Linux, however, it implements only a small set of performance
+sensitive system calls and the rest are offloaded to Linux.
+Specifically, McKernel has its own memory management, it supports
+processes and multi-threading with a simple round-robin cooperative
+(tick-less) scheduler, and it implements signaling. It also allows
+inter-process memory mappings and it provides interfaces to hardware
+performance counters.
+
+Functionality
+-------------
+
+An overview of some of the principal functionalities of the IHK/McKernel
+stack is provided below.
+
+System Call Offloading
+~~~~~~~~~~~~~~~~~~~~~~
+
+System call forwarding in McKernel is implemented as follows. When an
+offloaded system call occurs, McKernel marshals the system call number
+along with its arguments and sends a message to Linux via a dedicated
+IKC channel. The corresponding proxy process running on Linux is by
+default waiting for system call requests through an ioctl() call into
+IHK’s system call delegator kernel module. The delegator kernel module’s
+IKC interrupt handler wakes up the proxy process, which returns to
+userspace and simply invokes the requested system call. Once it obtains
+the return value, it instructs the delegator module to send the result
+back to McKernel, which subsequently passes the value to user-space.
+
+Unified Address Space
+~~~~~~~~~~~~~~~~~~~~~
+
+The unified address space model in IHK/McKernel ensures that offloaded
+system calls can seamlessly resolve arguments even in case of pointers.
+This mechanism is depicted below and is implemented as follows.
+
+.. figure:: unified_address_space_en.png
+   :alt: unified_ap
+
+First, the proxy process is compiled as a position independent binary,
+which enables us to map the code and data segments specific to the proxy
+process to an address range which is explicitly excluded from McKernel’s
+user space. The grey box on the right side of the figure demonstrates
+the excluded region. Second, the entire valid virtual address range of
+McKernel’s application user-space is covered by a special mapping in the
+proxy process for which we use a pseudo file mapping in Linux. This
+mapping is indicated by the blue box on the left side of the figure.