| 59,7 → 59,53 |
|---|
| 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> |
| </section> |
| |
| <para></para> |
| <section> |
| <title>Memory management</title> |
| |
| <para>Memory management is another large subsystem in HelenOS. It serves |
| the kernel to satisfy its own memory allocation requests, provides |
| translation between virtual and physical memory addresses and manages |
| virtual address spaces of userspace tasks.</para> |
| |
| <para>Kernel allocates memory from the slab allocator, which itself |
| allocates memory from a buddy system based allocator of physical memory |
| frames.</para> |
| |
| <para>The virtual address translation layer currently supports two |
| mechanisms for mapping virtual memory pages to physical memory frames |
| (i.e. 4-level hierarchical page tables and global page hash table), and is |
| further extensible to other mechanisms.</para> |
| |
| <para>Userspace tasks depend on support of address spaces provided by the |
| kernel. Each address space is a set of mutually dijunctive address space |
| areas that group pages of common attributes. An address space area is |
| usually connected to, and backed by, an anonymous memory or an executable |
| image of some program. Anonymous memory address space areas can be easily |
| shared among address spaces.</para> |
| </section> |
| |
| <section> |
| <title>IPC</title> |
| |
| <para>Due to the fact that HelenOS is a microkernel, strong emphasis is |
| put on its IPC (Inter-Process Communication<footnote> |
| <para>The term Inter-Process Communication is slightly confusing |
| because in HelenOS terminology there are tasks instead of processes. |
| However, its abbreviation, IPC, is being publicly used as a standard |
| name for similar facilities. This book will therefore use the term IPC |
| to refer to communication among tasks.</para> |
| </footnote>). Tasks communicate by passing very short messages to one |
| another or by sending (i.e. sharing) address space areas when larger data |
| is to be transfered.</para> |
| |
| <para>The abstraction uses terms like phones, calls and answerboxes, but |
| is pretty similar to well-known abstraction of message queues. A task can |
| have multiple simultaneous simplex connections to several other tasks. A |
| connection leads from one of the source task's phones to the destination |
| task's answerbox. The phones are used as handles for making calls to other |
| tasks. Calls can be synchronous or asynchronous and can be forwarded from |
| one task to another.</para> |
| </section> |
| </chapter> |