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 viengoos.
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 Mach's zalloc panics upon user space processes allocating too many ports, for example.
fof plos09: Pierre-Evariste Dagand, Andrew Baumann, Timothy Roscoe. Filet-o-Fish: practical and dependable domain-specific languages for OS development. PLOS '09, October 11, 2009, Big Sky, Montana, USA.
In [fof_plos09], the authors describe in section 3 how they model their capability system according to [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.
IRC, freenode, #hurd, 2011-07-31
< braunr> one of the biggest problems on the hurd is that, when a client makes a call, kernel (and other) resources are allocated on behalf of the server performaing the requested action < braunr> performing* < braunr> this makes implementing scheduling and limits difficult < CTKArcher> And could changing the kernel change anything to that ? < braunr> yes but you'd probably need to change its interface as well < braunr> iirc, the critique describes resource containers < braunr> but no work has been done on the current hurd (hence the hurdng attempts)