csc 660: advanced operating systemsslide #1 csc 660: advanced os microkernels
TRANSCRIPT
CSC 660: Advanced Operating Systems Slide #2
Topics
1. What is a microkernel?
2. Mach and L4
3. Microkernel IPC
4. Microkernel Memory Management
5. Userspace Device Drivers
6. Nooks
7. Exokernels
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What is a Microkernel?
Kernel with minimal featuresAddress spaces
Interprocess communication (IPC)
Scheduling
Other OS features run as user-space servers.Device drivers
Filesystem
Pager
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Microkernel Philosophy
A concept is tolerated inside the microkernel only if moving it outside the kernel, i.e., permitting competing implementations would prevent the implementation of the systems' required functionality.
- Jochen Liedtke
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Why use Microkernels?
Flexibility: can implement competing versions of key OS features, like filesystem or paging, for best performance with applications.
Safety: server malfunction restricted to that server (even drivers), not affecting rest of OS.
Modularity: fewer interdepencies and a smaller trusted computing base (TCB).
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MachFirst generation microkernel.
Runs OS personality on top of microkernel.Core Abstractions
Tasks and Threads (kernel provides scheduling)Messages (instead of system calls)Memory Objects (allow userspace paging)
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Mach Abstractions
Task: unit of execution consisting of an address space, ports, and threads.
Thread: basic unit of execution, shares address space, ports with other threads in task.
Port: communication channel used to send messages between tasks. Tasks must have correct port rights to send message to a task.
Message: basic unit of communication consisting of a typed set of data objects.
Memory Object: source of memory tasks can map into their address space; includes files and pipes.
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Mach Threads and Messages• Threads have
multiple ports with different port rights.
• Send messages to ports instead of system calls.
• Task must have port rights to send message to port.
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Mach Innovations
Message passing instead of system calls.Provide uniform interface to kernel.Can extend messages w/o recompiling kernel.
Userspace pagingDifferent tasks can use different pagers.
Multiprocessor / distributed OS.Ports can reside on system across network.Message passing works identically across network as on local system with NetMsgServer forwarding messages across network.
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Mach Performance
System calls take 5-6X as long as UNIX.
Message PassingUses pointers, copy-on-write, and memory mapping to avoid unnecessary copies.
Port rights checks are expensive.
PagingPageout kernel thread determines system paging policy (which pages are paged out to disk.)
Pager servers handle actual writing.
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L4 Microkernel
• Second generation microkernel.
• Faster– IPC is about 10X faster than Mach.– IPC security checks moved to user space
processes if needed.
• Smaller– L4 is 12KB. Compare to Mach 3 (330KB)– Memory management policy moved entirely to
userspace.
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Microkernel IPC
Uniform way to handle kernel interactions.IPC Mechanisms
RegistersDirect copyMemory mapping
Most performance critical component.All interactions require 2 IPCs: request, response.Hand-off scheduling: CPU control may be transferred with message so recipient can respond without waiting to be rescheduled.
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Handle Interrupts as IPC
Microkernel captures interrupts.Doesn’t handle.
Forwards interrupts to process as IPC.
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Microkernel PagingMicrokernel forwards page fault to a pager server.
Kernel or server decides which pages need to be written to disk in low memory situations.
Pager server handles writing pages to disk.
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Recursive Address Spaces (L4)
• Initial address space controlled by first process.– Controls all available memory.
– Other address spaces empty at boot.
• Other processes obtain memory pages from first or from their other processes that got pages from first.
• Why is memory manager flexibility useful?– Different applications: real-time, multimedia, disk cache.
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Constructing Address Spaces
grant: remove page from your address space and give to another consenting process.
map: share page with another process.demap: remove page from all other processes that
received it directly or indirectly from demapper.
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User Space Device Driver
How do they work?Receive interrupts as IPC.
I/O ports mapped to user address space.
AdvantagesDevice drivers have 3-7X bugs as kernel code.
User space driver bugs don’t reduce reliability.
User space driver bugs don’t reduce security.
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User Space Device Driver
driver thread:
wait for (msg, sender)
if sender = my hw interrupt
read/write i/o ports
reset hw interrupt
else
pass
end
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Nooks
Problem: Most kernel bugs in device drivers.Drivers written by less experienced programmers.
Drivers are tested less than core kernel code.
Solution: Lightweight protection domains.Kernel-mode env w/ restricted mem write access.
Isolate drivers from kernel code.
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Nooks Goals
1. Isolation: Isolate kernel from extension failures.
2. Recovery: Automatic recovery after extension failure so applications can continue execution.
3. Backwards compatibility: Extensions should not have to be rewritten to use Nooks.
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Exokernels
Problem with traditional OSMost resource management decisions made once in a global fashion.
Exokernel solution• Let programmers make resource management
decisions when they write their applications.• Allows experimentation.• Allows for high performance for applications
that don’t fit OS assumptions, e.g. RDBMS.
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What makes Exokernels Different?
• Separate security from abstraction.– ex: Protect disk blocks not files.
• Exokernel securely multiplexes hardware.• Move abstractions into userspace libraries
called library operating systems (libOSes.)• Exokernels vs Microkernels
– Microkernel concerned with implementing kernel in user space rather than kernel space.
– Exokernel concerned with separating security from abstraction to give applications control.
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Exokernel Tasks
1. Tracking ownership of resources.
2. Performing access control by guarding all usage or binding points.
3. Revoking access to resources.
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Resource Revocation
Invisible revocation– Most OSes deallocate memory, CPU without
informating application.
Visible revocation– Exokernels visibly request that a resource be returned to
the kernel.
– Ex: Exokernel informs app that CPU is revoked at end of time slice, and app responds by saving required processor state.
– If application does not return resource, exokernel will take it from the application.
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Exokernel Performance
Aegis/ExOS vs Ultrix performanceSystem calls 10X faster.
IPC 10-20+X faster.
Virtual memory1-5X faster.
OS syscall matrix pipe lrpc
Aegis 2.9 5.2s 22.6 10.4
Ultrix 33.7 5.2s 231 457
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Cheetah Web Server
Exokernel web server performance features:– Transmits data directly from page cache w/o copying.
– Colocates hyperlinked files within filesystem.
– Network stack tuned to reduce packets by 20%.
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Exokernel Portability
Apps that directly use exokernel aren’t portable to different architectures.
Exokernel tied closely to hardware.
Library operating systems can provide portability for other applications.
LibOSes can provide POSIX interface.
Can run multiple LibOSes on exokernel.
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Microkernels in Use
MachUnderlying microkernel for UNIX systems.Examples: Mac OS X, MkLinux, NeXTStep
QNXPOSIX-compliant real-time OS for embedded sys.Fits on a single floppy.Underlying microkernel for Cisco IOS XR.
SymbianMicrokernel OS for cell phones.
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Key Points1. Microkernel provides minimal features
1. Address spaces2. IPC3. Scheduling
2. Microkernel advantages1. Flexibility2. Safety3. Modularity
3. Early microkernels were slow, but flexible memory/disk policies can allow for superior application performance.
4. Exokernels focus on separation of protection from abstraction instead of focusing on user/kernel divide.
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4. Hermann Härtig, Michael Hohmuth, Jochen Liedtke, Sebastian Schönberg, “The performance of μ-kernel-based systems”. Proc. 16th ACM symposium on Operating Systems Principles (SOSP), 1997.
5. Jochen Liedtke. “On µ-Kernel Construction,” Proc. 15th ACM Symposium on Operating System Principles (SOSP), December 1995
6. Jochen Liedtke, “Towards Real Microkernels,” Communications of the ACM, 39(9):70-77, September 1996.
7. Avi Silberchatz et. al., Operating System Concepts, 7th edition, http://codex.cs.yale.edu/avi/os-book/os7/online-dir/Mach.pdf, 2004.
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