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authorThomas Schwinge <thomas@codesourcery.com>2018-05-25 15:37:30 +0200
committerThomas Schwinge <thomas@codesourcery.com>2018-05-25 15:37:30 +0200
commitab97868f7e5097ce6d141c7783c1e07eaa9c4ba1 (patch)
treea6b6959705f1b2dcb36fe920675cecbcf8a3dc94 /community/gsoc
parent371a782cd09a6f0c7f9f99cf336351635c6fa7c9 (diff)
Revert page removal of "Drop physical memory management project"
..., to avoid breaking existing links. Instead, mark as obsolete. This reverts parts of commit 371a782cd09a6f0c7f9f99cf336351635c6fa7c9.
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+[[!meta copyright="Copyright © 2015, 2016, 2018 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"]]
+
+[[!template id=highlight text="""/!\ Obsolete /!\
+
+---
+
+This is no longer valid as a Google Summer of Code project; it was basically
+done by Richard."""]]
+
+
+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>