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[[!meta copyright="Copyright © 2008, 2009, 2010 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]]."]]"""]]
[[!tag open_issue_gnumach open_issue_hurd open_issue_viengoos]]
[[microkernel/Mach]] interfaces do not allow for proper resource accounting,
when a server allocates resources on behalf of a client.
Mach can't do a good job at resource management, as it doesn't have enough
information how resources are used: which data is important and which is
discardable, for example.
These issues are what Neal Walfield is working on with his new kernel
[[microkernel/viengoos]].
# Kernel
Inside the [[kernel]], there is commonly a need to allocate resources according
to externally induced demand, dynamically. For example, for memory-management
data structures (page tables), process table entries, thread control blocks,
[[capability]] tables, incoming network packages, blocks that are read in from
disk, the keyboard type-ahead buffer for a in-kernel keyboard driver. Some of
these are due to actions driven by user-space requests, others are due to
actions internal to the the kernel itself. Some of these buffers can be sized
statically (keyboard type-ahead buffer), and are thus unproblematic. Others
are not, and should thus be attributed to their user space entities. In the
latter (ideal) case, all resources -- that is, including those needed inside
the kernel -- that a user space task needs for execution are provided by itself
(and, in turn, provided by its parent / principal), and the kernel itself does
not need to allocate any resources dynamically out of an its own memory pool.
This avoids issues like [[microkernel/Mach]]'s [[zalloc_panics]] upon user
space processes allocating too many [[microkernel/mach/port]]s, for example.
[[!toggleable id=fof_plos09 text="""[[!template id=note
text="*[[fof\_plos09|microkernel/barrelfish]]*:
{{$microkernel/barrelfish#fof_plos09}}"]]"""]]
[[!toggleable id=sel4 text="""[[!template id=note
text="[[*sel4*|microkernel/l4]]: {{$microkernel/l4#sel4}}"]]"""]]
In [[!toggle id=fof_plos09 text="[fof\_plos09]"]], the authors describe in
section 3 how they model their [[capability]] system according to [[!toggle
id=sel4 text="[sel4]"]] using a *retype* operation that *takes an existing
capability and produces one or more derived capabilities [...] used to create
new kernel-level memory objects (such as page tables or execution contexts)
from capabilities to raw regions of RAM*.
This is, of course, non-trivial to implement, and also requires changing the
[[RPC]] interfaces, for example, but it is a valid approach, a research topic.
([[!taglink open_issue_documentation]]: compare this to Linux [`vmsplice`'s
SPLICE_F_GIFT
flag](http://www.kernel.org/doc/man-pages/online/pages/man2/vmsplice.2.html#DESCRIPTION).)
# Further Examples
* [[configure max command line length]]
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