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[[!meta copyright="Copyright © 2015, 2016 Free Software Foundation, Inc."]]
[[!meta license="""[[!toggle id="license" text="GFDL 1.2+"]][[!toggleable
id="license" text="Permission is granted to copy, distribute and/or modify this
document under the terms of the GNU Free Documentation License, Version 1.2 or
any later version published by the Free Software Foundation; with no Invariant
Sections, no Front-Cover Texts, and no Back-Cover Texts. A copy of the license
is included in the section entitled
[[GNU Free Documentation License|/fdl]]."]]"""]]
[[!meta title="Physical memory management"]]
GNU Mach is currently suffering from severe limitations caused by the way
it manages physical memory. For example, since it requires pages to be mapped
in kernel space in order to be used, the maximum amount of usable physical
memory is currently around 800MB (or 1.8GB if a 2/2 split is set). And
because the page allocator is unable to easily return blocks of contiguous
pages, the kernel has to use virtual memory to provide contiguity.
But the kernel virtual space is separate from the direct mapping of
physical memory, so the larger it is, the less physical pages available.
The size of the kernel space is currently around 200MB, with around 100MB
for kernel objects. This small size prevents the system from achieving
scalability, since a panic occurs when the kernel is unable to allocate
a kernel object such as a port. In addition, the kernel uses mainly tables
to store IPC rights. When a table is full, it is enlarged through a kernel
specific version of realloc(). When a file system starts managing many
files (e.g. because some of their content is cached in physical memory),
these tables can get big enough to make realloc() fail because of
fragmentation.
The goal of this project is to make as much physical memory available as
possible for both the kernel and applications, by rewriting the page
allocator into a buddy allocator to support contiguous block allocations,
using it directly instead of virtual memory as the backend of the slab
allocator for kernel objects, and, if time allows it, transform IPC right
tables (e.g. into radix trees) and get rid of realloc().
This project requires a good understanding of virtual memory (both physical
mappings at the MMU level and virtual mappings at the VM level), and strong
skills in C programming. Note that some work has already been done in the
X15 project about this, and can be reused as a reference.
Useful links :
* <https://www.sceen.net/mapping-physical-memory-directly/>
* <http://git.sceen.net/rbraun/x15.git/>
* <https://git.sceen.net/rbraun/librbraun.git/plain/rdxtree.h>
Possible mentors: Richard Braun
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