CS140 Operating Systems Final Review. Mar. 13 th, 2009 Derrick Isaacson

Transcription

1 CS140 Operating Systems Final Review Mar. 13 th, 2009 Derrick Isaacson 1

2 Final Exam Wed. Mar. 18 th 12:15 pm in Gates B01 Open book, open notes (closed laptop) Bring printouts You won t have time to learn the material but they will likely help as a quick reference Will cover all lectures 2

3 Outline 1. Virtual machines 2. Synchronization 3. Protection 4. Security 5. File systems 6. Network file systems 7. Networking 3

4 Virtual Machine Review Difference between VM and VMM Difference between Guest and Host OS Virtual Machine Guest OS Host OS Virtual Machine Monitor/ Hypervisor 4

5 More Virtual Machine Review Virtual Memory another layer of indirection Virtual Address address used by process inside of VM Physical Address virtualized hardware address of VM Machine Address true hardware address Page Tables Guest OS keeps its own page tables like always which now map from VA to PA VMM keeps a logical mapping from a VM s PA to MA VMM combines Guest OS page tables with it s own PA->MA mapping, into the shadow page table. While running VM, the actual cr3 register points to the shadow page directory. 5

6 Synchronization Is there a cycle? Is there deadlock? init(sema1, 1); init(sema2, 1); init(sema3, 2); init(sema4, 3); Yes to cycle. Yes to deadlock. fun1 { down(sema3); } down(sema1); fun2 { } down(sema3); down(sema1); down(sema2); fun3 { } down(sema2); down(sema3); 6

7 Synchronization 2 Is there a cycle? Is there deadlock? init(sema1, 2); init(sema2, 2); Yes to cycle. No to deadlock. fun1 { down(sema2); } down(sema1); fun2 { down(sema1); } fun3 { } down(sema1); down(sema2); fun4 { } down(sema2); 7

8 Synchronization - Deadlock Given limited resources A, B, C, D can I have deadlock in the following situations? Why/why not? I always acquire resources in alphabetical order No - no circularity in request graph An older thread can steal a resource from a younger one that holds it Yes, if 2 threads can have same timestamp No if timestamps unique we have preemption I request all resources at startup No no hold and wait 8

9 Protection ACLs versus Capabilities ACLs slice matrix by columns Capabilities slice along rows Security hole examples TOCTTOU Setuid ptrace Capability based systems never took off perceived complexity low performance because of more IPC 9

10 Security Bell-La Padula model Security level is (c, s) pair c = classification, s = category-set (c1, s1) dominates (c2, s2) iff c1 = c2 and Examples - (S, O, A) for subject S, object O, and access mode A L1 dominates L2 sometimes written L1? L2 or L2? L1 If current-level(s) = {top-secret, {Nuclear}} and level(o) = {secret, {Crypto}}, can A include observation? No level(s) must dominate level(o) If level(o 1 ) = {secret, {Nuclear}} and level(o 2 ) = {top-secret, {Nuclear}}, is it possible for S to observe O 1 and modify O 2? Yes level(o 2 ) must dominate level(o 1 ) s2? s1 10

11 Why was FFS fast? Larger block size File systems 1 - FFS Lessen internal fragmentation by placing pieces of files in fragments (for example ends of files) Clustering Place sequential blocks in adjacent sectors Keep inode in same cylinder as file data Keep inodes for a dir in same cylinder group Keep extra free space to help allocate near existing things Use bitmap for free blocks Keep in memory Easier to find contiguous blocks 11

12 File systems 2 Ordered Updates Performance vs. consistency Must follow three rules in ordering updates: 1. Never write pointer before initializing structure it points to 2. Never reuse a resource before nullifying all pointers to it 3. Never clear last pointer to live resource before setting new one Example - creating 1k file on FFS means you must: Problem: Solution: write filename in directory block write modification time in directory inode initialize file inode write file data block write free map multiple inodes are stored in same disk block and multiple directory entries are stored in same directory block develop cyclic dependencies soft updates maintain enough info in fs to rollback changes temporarily 12

13 File systems 3 Journaling Also improvement over basic FFS Keep a write-ahead log writes are to a consecutive portion of disk multiple updates are very fast record a checkpoint when all log entries up to a point have been processed allows you to truncate log XFS journaling file system for huge disks break disk into a few GB allocation groups use B+ trees for everything (directories, inodes, lookup tables) 13

14 Network File Systems Main points to remember Tradeoff between completely stateless systems (original NFS) and stateful ones (NFS 4, AFS) Stateless servers require clients to poll server to find out about changes Callbacks improve consistency and reduce server load, but complicate server requirements and failure modes Leases are a hybrid of polling and callbacks Server promises to let a client know about changes within a time period Failure modes simplified if lease time is less than crashreboot time 14

15 Key concepts: Networking IP provides host to host communication Unreliable (messages may get lost, reordered, or retransmitted) No control over transmission rate (congestion control) TCP/UDP provide process to process communication Layered on top of IP (IP gets message to the host, TCP/UDP gets it to the process) UDP simple layer on IP so it inherits IP s problems TCP a more complex layer on IP that provides reliability and congestion control Sockets abstraction for communication The several layers used in common networking stacks could cause a lot of copying as data is handed from one layer to next. mbuf libraries provide efficient, flexible data structures and utility functions to help avoid these problems 15

16 Questions? 16