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<title>Architecture overview</title>
<para>The HelenOS operating system is designed as a relatively small
microkernel assisted with a set of userspace drivers and server tasks.
HelenOS is not very radical in what subsystems should or should not be
implemented in the kernel - in some cases, both kernel and userspace drivers
exist. The reason for creating the system as a microkernel is prosaic. Even
though it is initially more difficult to get the same level of functionality
from a microkernel than it is in the case of a simple monolithic kernel, a
microkernel is much easier to maintain once the pieces have been put to work
together. Therefore, the kernel of HelenOS, as well as the essential
userspace libraries thereof can be maintained by only a few developers who
understand them completely. In addition, a microkernel based operating
system reaches completion sooner than monolithic kernels as the system can
be used even without some traditional subsystems (e.g. block devices,
filesystems and networking).</para>
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<caption>HelenOS architecture overview</caption>
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<para>HelenOS is comprised of the kernel and userspace server tasks. The
kernel provides scheduling, memory management and IPC. It also contains
essential device drivers that control the system clock and other devices
necessary to guarantee a safe environment. Userspace communicates with the
kernel through a small set of syscalls. The userspace layer consists of
tasks with different roles, capabilities and privileges. Some of the tasks
serve as device drivers, naming servers, managers of various kinds and some
are just ordinary user programs. All of them communicate with other threads
via kernel-provided IPC.</para>
<section>
<title>Scheduling</title>
<para>Kernel's unit of execution flow is a thread. A thread is an entity
that executes code and has a stack that takes up some space in memory. The
relation between kernel and userspace threads is 1:1:n, meaning that there
can be several pseudo threads running within one userspace thread that
maps to one kernel thread. Threads are grouped into tasks by functionality
they provide (i.e. several threads implement functionality of one task).
Tasks serve as containers of threads, they provide linkage to address
space and are communication endpoints for IPC.</para>
<para>The scheduler deploys several run queues on each processor. A thread
ready for execution is put into one of the run queues, depending on its
priority and its current processor, from where it is eventually picked up
by the scheduler. Special purpose kernel threads strive to keep processors
balanced by thread migration. Threads are scheduled by the round robing
scheduling policy with respect to multiple priority run queues.</para>
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