1. CS 443 Advanced OS David R. Choffnes, Spring 2005 Application Performance and Flexibility on Exokernel Systems M. F. Kaashoek, D.R. Engler, G.R. Ganger,H.M. Briceno, R. Hunt, D. Mazieres, T. Pinckney, R. Grimm, J. Jannotti, K. Mackenzie MIT LCS Appears in SOSP 1997 Presented by: David R. Choffnes

2. 2 Why Exokernels? Application demands vary widely –Current OSs provide high-level interfaces and attempt to optimize for some “average-case” workload –Penalty for certain applications can be high –It would be better to give these untrusted applications direct access to hardware

3. 3 Exokernel Architecture Overview Goals –Limit OS (kernel) functionality to managing protection (and safe sharing) of hardware resources –Allow “applications” to manage resources Many of these “applications” will perform management typical of monolithic OSs – called library operating systems (libOSes) Traditional applications will then link to the libOSes instead of linking to a monolithic kernel Claim –Any programmer can specialize at libOS without affecting the rest of the system Too much responsibility in hands of app. programmer?

4. 4 Conventional OS user interface User process Kernel protects and manages all the system resources User process System calls

5. 5 Exokernels Exokernel protects but does not manage system resources User process

6. 6 LibOSes Exokernel User process libOS User process libOS

7. 7 Purpose of Paper Proof of concept: show that exokernels can introduce new interfaces that separate protection from management Show that exokernels do not limit performance of ordinary apps Show that apps can use exokernel to improve performance compared to traditional kernel

8. 8 Related Work (“Anything you can do I can do better”) Recast the debate over kernel architecture –Since exokernel operates at such a low level, it is “orthogonal” to the debate over monolithic/ukernel –Anything done to improve performance in a ukernel approach can and should be done in exokernel applications Virtual Machines –Solve the extensibility problem, but compartmentalize applications, which can lead to inefficiency Exokernels allow downloading of code

9. 9 Exokernels in a Nutshell (1) Exokernel principles –Separate protection from management Allocation, revocation, sharing and tracking of ownership –Expose allocation Allows apps to fully control what they allocate –Expose names Allows apps to use physical names, eliminating overhead of virtualization –Expose revocation Allow apps to recover from revocation instead of simply killing them –Expose information Allow app to know just about everything that the kernel knows

10. 10 Exokernels in a Nutshell (2) Kernel support for protected abstractions –Challenge: allow high-level access control without mandating an implementation or hindering application control of resources –Design techniques Same access control for all resources Binding of hardware resources as software abstractions –Buffer cache example Support downloading of code for abstractions –Allows extensibility to new forms of protection not represented by hardware –Abstractions reside in kernel (cheap shot at ukernels)

11. 11 Microkernels in a Nutshell (3) Protected sharing –LibOSes can trust applications not to muck with resources provided by exokernel, but cannot trust other libOSes that may have the same access. E.g., the fork problem –Exokernel provides four mechanisms to maintain invariants in shared abstractions Software regions (region can be read only through system call) Hierarchically-named capabilities (protect against buggy children) Wakeup predicates (protect against hanging) Robust critical sections (by disabling software interrupts) –Eliminates the need for locks –Can still lead to livelock?

12. 12 Microkernels in a Nutshell (3) –Optimize shared abstraction implementation according to level of trust Mutual trust –Similar to monolithic kernel programming Unidirectional –Protect shared resources from untrusted side Mutual distrust (defensive programming) –Worst case and rare –Must account for all kinds of attacks/problems

13. 13 Multiplexing Disk Storage (or: How I Learned to Stop Worrying and Love the Exokernel) Challenge –Support multiple concurrent file systems without partitioning –Give as much control as possible to libFSes as possible while protecting files from unauthorized access –Allow file systems to define arbitrary file formats Implementation: Stable Storage System –Simple/lightweight capability for new file formats –Allow safe sharing of disk blocks among libFSes –Efficient access –Allow cache sharing while supporting invariants

14. 14 XN (One Hack to Rule them All) Provides –Access to storage at level of disk blocks –Public-readable buffer cache registry –Free maps Purpose is to determine the access rights of a principal to a given disk block efficiently Challenge: multiple file formats Solution: UDF (untrusted deterministic functions) –Translate metadata from associated file format to a language that the kernel can understand –Stored in disk structures called templates

15. 15 XN (continued) UDF allows libFS to define each of its types once per file system –Better than separating capabilities from (meta)data blocks or using self-descriptive blocks Requirements for XN –Every operation on disk data must be guarded Implemented at bind time (when page associated with disk block is loaded into page table) –Determine access rights unambiguously (use libFS’s metadata) –Prevent crashes from corrupting FS (no writing pointers to uninitialized data, no freeing block until there are no pointers to it)

16. 16 XN (continued) For protected sharing among libFS’s: –Coherent caching of disk blocks (in-kernel, systemwide protected cache registry) –Atomic metadata updates (lock cache registry entries) –Well-formed updates (use libFS’s UDF)

17. 17 XN “Disk Ordering” Preventing FS corruption –Use reference counting for deallocation –Use tainted blocks to indicate pointers to uninitialized data Tainted blocks cannot be written to disk If FS is marked “temporary” or is not connected to a FS root, operations on tainted pages are allowed

18. 18 XN Buffer Cache Registry Tracks mapping and state BCR is mapped into every app’s memory space XN allows libFS to determine its own caching/backing store policies XN allows any process to flush dirty pages to disk (even w/o write access to such pages)

19. 19 XN Usage –Lots of details you don’t need to know right now C-FFS –FFS for Xok –Uses kernel downloading, but should use UDFs –Demonstrates that traditional FS abstractions and requirements can be mapped to exokernel capabilities without “much” effort Future work –Wait, you haven’t tested this with more than one FS!? More XN (Because 5 slides aren’t enough)

20. 20 Xok/ExOS (aka, worst OS name since The Hurd) Xok safely multiplexes physical resources –CPU: Round-robin scheduling –Network: Dynamic packet filters –RAM: Apps must use system call to modify page table, but Xok allows user-level pagers –Wakeup predicates: evaluated when an app is about to be executed  low overhead –Access control: hierarchically-named capabilities

21. 21 ExOS 1.0 ExOS: libOS that supports most of the abstractions in 4.4BSD Primary goals: simplicity and flexibility Implemenation –UNIX abstractions Most shared state is protected using Xok’s protection mechanisms, but some simply uses shared memory Bogus claim of fault isolation Processes –Memory mapping tricks to improve efficiency –Fork trickiness for lazy page copying IPC: software regions, “directed yield”, timers, upcalls… File descriptors use shared memory, allow apps to install own file functions Files: (already discussed) mounting done via shared memory

22. 22 ExOS 1.0 Implemenation (cont) Shared libraries –ExOS maps shared libraries into address space at load time –Significantly reduces memory consumption for exokernel; similar in size to normal UNIX –No dynamic linking cost

23. 23 Application Performance (Finally!) Base performance of unaltered UNIX apps linked against ExOS is comparable to OpenBSD and FreeBSD Some apps perform faster on ExOS due to file system (no detailed description why) Estimated protection cost appears to be low

24. 24 Exploiting Extensibility (1) Fast binary emulation allows large improvements in simple operations (getpid) XCP improves copy performance by a factor of two –Take advantage of access to hardware to perform copy using DMA without CPU touching data

25. 25 Exploiting Extensibility (2) Cheetah HTTP/1.0 Server – integration with Disk, TCP/IP stack allow significant performance improvement

26. 26 Exploiting Extensibility (3) Global performance –Weakest point, uses ridiculously unrealistic workloads, then claims that performance is OK –Does not seem to support RT apps –Lots of hand-waving about deriving info for global tuning

27. 27 Experience “Clear” Advantages –Exposing kernel data structures Mapping into user space eliminates system call overhead Allows apps to come up with new ways of using info –CPU interface Allows explicit control over synchronization –Libraries Greatly simplifies development process by removing the “reboot” step Costs –Not easy to design exokernel interfaces –Information can be lost between interfaces –Self paging is difficult

28. 28 Lessons If I have time for this, which I doubt, I’ll wing it

29. 29 Conclusion Exokernels are feasible! Exokernels can be used to provide services to unmodified apps meant for conventional OSes! Exokernels can actually boost performance compared to existing popular/conventional OSes! Easier to develop Open questions, but still viable.