Advanced Operating Systems   COMP9242 2002/S2

UNSW

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Popular Example: Mach Developed at CMU by Rashid and others[RTY$^+$88] from 1984 successor of Accent[FR86] and RIG[Ras88]. Goals: Tailorability: support different OS interfaces. Portability: almost all code H/W independent. Real-time capability. Multiprocessor and distribution support. Security. Coined term microkernel. Basic features of Mach -kernel Task and thread management; interprocess communication (asynchronous message-passing); memory object management; system call redirection; device support; multicomputer support. Mach tasks and threads Task consists of one or more threads. Task provides address space and other environment. Thread is active entity (basic unit of CPU utilisation) . Threads have own stacks, are kernel scheduled. Threads may run in parallel on multiprocessor. ``Privileged user-state program'' may be used to control scheduling. Task created from ``blueprint'' with empty or inherited address space. Activated by creating a thread in it. Mach IPC: Ports Addressing based on ports: port is a mailbox, allocated/destroyed via a system call; has a fixed-size message queue associated with it; is protected by (segregated) capabilities; has exactly one receiver, but possibly many senders; can have ``send-once'' capability to a port. Can pass the receive capability for a port to another process give up read access to the port. Kernel detects ports without senders or receiver. Processes may have many ports (UNIX server has 2000!) Ports can be grouped into port sets. Allows listening to many ports (like select()) Send blocks if queue is full except with send-once cap (used for server replies) Mach IPC: Messages Segregated capabilities: threads refer to them via local indices. kernel marshalls capabilities in messages. message format must identify caps Message contents: Send capability to destination port (mandatory) used by kernel to validate operation; optional send capability to reply port for use by receiver to send reply possibly other capabilities; ``in-line'' (by-value) data; ``out-of-line'' (by reference) data, using copy-on-write, may contain whole address spaces; Mach IPC Mach virtual memory management Address space constructed from memory regions initially empty populated by:  ${}$ explicit allocation  ${}$ explicitly mapping a memory object;  ${}$ inheriting from ``blueprint'' (as in Linux clone()), inheritance: not, shared or copied;  ${}$ allocated automatically by kernel during IPC when passing by-reference parameters; ==> sparse virtual memory use (unlike UNIX). 3 page states: unallocated, allocated & unreferenced, allocated & initialised Copy-on-write in Mach When data is copied (``blueprint'' or passed by-reference): source and destination share single copy, both virtual pages are mapped to the same frame. Marked as read-only. When one copy is modified, a fault occurs. Handling by kernel involves making a physical copy is made, VM mapping is changed to refer to the new copy. Advantage: efficient way of sharing/passing large amounts of data. Drawbacks: expensive for small amounts of data (page table manipulations) data must be properly aligned Mach address maps Address spaces represented as address maps: Any part of AS can be mapped to (part of) a memory object Compact representation of sparse address spaces Compare to multi-level page tables? Memory objects Kernel doesn't support file system Memory objects are an abstraction of secondary storage: can be mapped into virtual memory are cached by the kernel in physical memory pager invoked if uncached page is touched used by file system server to provide data Support data sharing by mapping objects into several address spaces Memory is only cache for memory objects User-level page fault handlers All actual I/O performed by pager; can be default pager (provided by kernel), or external pager, running at user-level. Intrinsic page fault cost: 2 IPCs Handling page faults Check protection & locate memory object uses address map Check cache, invoke pager if cache miss uses a hashed page table Check copy-on-write perform physical copy if write fault Enter new mapping into H/W page tables. Remote communication Client sends message to server on remote node. $A$ sends message to local proxy port for $B$'s receive port User-level network message server receives from proxy port NMS converts proxy port into (global) network port. NMS sends message to NMS on $B$'s node may need conversion (byte order...) Remote NMS converts network port into local port ($B$'s). Remote NMS sends message to that port. Mach Unix emulation emulation library in user address space handles IPC invoked by system call redirection (trampoline mechanism) supports binary compatibility Mach = Microkernel? Most OS services implemented at user level using memory objects and external pagers Provides mechanisms, not policies. Mostly hardware independent. 140 system calls. Size: 200k instructions. Performance??? tendency to move features into kernel Served as basis for OSF/1, MacOS X... Further information on Mach: [YTR$^+$87,CDK94,Sin97]