Self-Stabilizing Systems as a Base for Autonomic Computing Shlomi Dolev Yinnon Haviv, Reuven Yagel,...

Self-Stabilizing Systems as a Base for Autonomic Computing

Shlomi Dolev

Yinnon Haviv, Reuven Yagel,

Olga Brukman

Trustworthy Systems: Why Is It So Hard?

Corbató’91: "It almost goes without saying that ambitious systems never quite work as expected“http://larch-www.lcs.mit.edu:8001/~corbato/turing91/

"You must pay extreme attention to detail here. One wrong bit will make things fail… "http://my.execpc.com/~geezer/os/pm.htm

From Pentium’s manual:“… if the ESP or SP register is 1 when the PUSH instruction is executed, the processor shuts down due to a lack of stack space. No exception is generated to indicate this condition"

Mars Rover - Spirit…The Spirit rover has a radiation-hardened R6000 CPU from Lockheed-Martin Federal Systems…The operating system is Wind River Systems' Vx-Works.. …attempted to allocate more files than the RAM-based directory structure could accommodate. That caused an exception, which caused the task that had attempted the allocation to be suspended… …Spirit fell silent, alone on the emptiness of Mars, trying and trying to reboot

http://www.eetimes.com/sys/news/OEG20040220S0046

Linux and Windows do not Stabilize

Self-Stabilization

Self-healing, Self-managing, Self-*

Recovery Oriented Computing [Berkeley, Stanford]

Autonomic Computing [IBM]

Self-Stabilization Self-Stabilizing algorithm for mutual exclusion in a

ring topology [Dijkstra’74]

Well Established Theory !

Self-Stabilizing Systems

Elegant fault tolerant approach.Started at any state, the system convergences to a desired behavior.

Generally used in distributed systems. Routing, clock synchronization, leader election, etc.

Overcome transient faults in the system. Transient faults: soft-errors (“98% of RAM errors are soft

errors”), wrong CRC during communication etc.

Self-Stabilization

The combination and type of faults cannot be totally anticipated in on-going systemsAny on-going system must be self stabilizing (or manually monitored)

E L

First Self-Stabilizing Algorithm: Token Passing

Token Passing

1 P1: do forever

2 if x1=xn then

3 x1:=(x1+1)mod(n+1)

4 Pi(i ≠ 1):do forever

5 if xi≠xi-1 then

6 xi:=xi-1

Token Passing Cont.

Surely works when we start in

x1 = x2 = … = xn = 0.

One processor may change a state at a time.

{0; 0; 0; 0; 0};

{1; 0; 0; 0; 0};

{1; 1; 0; 0; 0};

{1; 1; 1; 0; 0};

{1; 1; 1; 1; 0};

{1; 1; 1; 1; 1};

{2; 1; 1; 1; 1};

{2; 2; 1; 1; 1};

{2; 2; 2; 1; 1};

{2; 2; 2; 2; 1};

{2; 2; 2; 2; 2}

…

Token Passing: Faults

Transient fault, soft errors, wrong CRC, unexpected temporal severe conditions, etc.

Assigns each processor with an arbitrary state (in the range of its state space).

For example {3; 4; 4; 1; 0}.

p2; p4; and p5 have tokens!

Will the system ever recover?

Token Passing: Automatic Recovery

p1 changes state infinitely often,

Otherwise, let s1 be the fixed state of p1,

p2 eventually copies s1 from p1, then

p3 eventually copies s1 from p2, then ...

pn eventually copies s1 from pn-1, then

p1 changes state.

p1 changes state in the order 4; 5; 0; 1; 2; 3; 4; 5; 0; ...

Token Passing: Automatic Recovery Cont.

In any initial state at least one state is missing, {4; 4; 1; 0; 2}, 3 and 5 are missing.

Once p1 reaches the missing state e.g., 5, all the processors must copy 5, before p1 reads 5 from pn and changes state to 0.

Will It Stabilize With mod (n - 2)?

Mod 3

{0,0,2,1,0} p1 {1,0,2,1,0} p5

{1,0,2,1,1} p4 {1,0,2,2,1} p3

{1,0,0,2,1} p2 {1,1,0,2,1}

+1 mod 3 !

Is Self-Stabilization a Toy?

Talk Outline

Self Stabilizing Microprocessor [DH04]

Self Stabilizing Operating System [DY04]

Self-Stabilization Preserving Compiler[DH05]

Self-Stabilizing Automatic Recoverer For

Eventual Byzantine Software [BDK03]

Recovery Oriented Programming[BD05]

Self-Stabilizing MicroprocessorOvercoming Soft-Errors

Shlomi Dolev and Yinnon A. Haviv

17th International Conference on Architecture of Computing Systems (ARCS)

Motivation

Soft-Errors: Single Event Upsets (SEU)

Caused by cosmic ray / other disruptions.

Cause a logical gate to flip its content.

Currently handled only in memories.

Significant impact on the microprocessors.

Soft-Errors - Current Solutions

Obtaining masking using probabilistic approaches: Information redundancy (ECC / Parity) Space redundancy Time redundancy Failure detection / recovery.

Known solutions: IBM S-390 Compaq NonStop Himalaya IROC

Self-Stabilizing Microprocessor

Self-stabilizing algorithms assume that the microprocessor executes them. Soft-errors may cause the microprocessor to be stuck

in a faulty state.

A microprocessor is self-stabilizes if: Started in any internal state, converges in a finite

number of steps into the set of safe states. Microprocessor’s safe state – in which it performs

“fetch-decode-execute” cycle

Proving Convergence Proving that there exists no “bad” cycle in the transition graph

of the microprocessor. Too large ! (we must explore the entire graph) Using an abstraction:~ Group together states in which the

micro-code program counter is the same.

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Self-Stabilizing Microprocessor: Summary

Soft-errors are here to stay, we should: Design our systems to mask them. Self-stabilize following a non-masked error.

We provide methodology for validating self-stabilization property of microprocessors.

Talk Outline

Self Stabilizing Microprocessor [DH04]

Self Stabilizing Operating System [DY04]

Self-Stabilization Preserving Compiler[DH05]

Self-Stabilizing Automatic Recoverer For

Eventual Byzantine Software [BDK03]

Recovery Oriented Programming[BD05]

Toward Self-Stabilizing Operating System (SOS)

Shlomi Dolev and Reuven Yagel,SAACS’04 Workshop, Zaragoza

Talk Outline

The first self-stabilizing algorithm (of Dijkstra)

Self Stabilizing Microprocessor [DH04]

Self Stabilizing Operating System [DY04]

Self-Stabilization Preserving Compiler[DH05]

Self-Stabilizing Automatic Recoverer For Eventual Byzantine Software [BDK03]

Recover Oriented Programming[BD05]

Basic Directions

Black-box Take existing OS (Unix, Windows, RTOS) Add stabilization layer

Carefully tailoring a tiny kernel Processor scheduling Memory management Device allocation

Assumptions

Every configuration (processor/memory) is possibleAt least some program code is hardwired (in ROM) and is correct – Harvard Model

Processor: Instruction manual (e.g. x86\IA-32) defines a

transition function. Self-stabilizing [DH04]

Black Box

Requirements: Defining a legal execution is usually impractical At least - restore original state (variables + code), infinitely often

Periodic Reset Re-install and Execute Watchdog timer (self-stabilizing) Periodic processor reset During bootstraps OS reinstall from ROM

Weak self-stabilization E = (ci, ai, ci+1, …., RRE, c1, a1, c2, a2, …., ci, ai, ci+1, …., RRE, c1, a1,

c2, a2, …. Is it always acceptable?

Alternative: Periodic re-install code only, add consistency check and enforcement

Tailored Kernel

Tiny Scheduler Tiny Memory Manager

Requirements: Self-stabilizing Fair Process stabilization preserving (e.g. validity of

P.C. value)

Tiny SOS Scheduler

; increase task10 mov word ax, [currentProc]11 and ax, PROC_MASK...

; load task state...;restore ip52 mov ax, [bx+4];validate ip53 and ax, IP_MASK54 mov word [ss:STACK TOP], ax;restore general registers55 mov cx, word [bx+12] 56 mov dx, word [bx+14] 57 mov si, word [bx+16] 58 mov di, word [bx+18]

~70 lines of a real machine assembly code16bit Real mode & 32bit Protected mode.Standard build and emulation tools (Nasm, ld, Bochs)Detailed proof of requirement preservation

Tiny SOS Memory Manager

Requirements: Consistency of memory hierarchy Self-stabilization preservation

Any State

Process(ing)Next ProcessValidated & Ready

Clock tick / execute next

Some Error

Some Error

Some Error

Establish SchedulerConsistency

Tiny SOS Scheduler

Any State

Process(ing)Next ProcessValidated & Ready

Clock tick / execute next

NMI / load PC with scheduler handler

Establish SchedulerConsistency

Tiny SOS Scheduler

Sketch of Proof

In every execution E, the code of the scheduler is started to be executed and is executed from the first instruction to the last instruction infinitely often

In every execution E of the scheduler each process is executed infinitely often

The self-stabilizing scheduler preservers stabilization of processes.

Talk Outline

Self Stabilizing Microprocessor [DH04]

Self Stabilizing Operating System [DY04]

Self-Stabilization Preserving Compiler[DH05]

Self-Stabilizing Automatic Recoverer For

Eventual Byzantine Software [BDK03]

Recover Oriented Programming[BD05]

Self-Stabilization Preserving Compiler

Shlomi Dolev, Yinnon A. Haviv,Department of Computer Science

Ben-Gurion University, Israel

Mooly Sagiv,Department of Computer Science

Tel Aviv University, Israel

Motivation

Transient malfunctions.

Single processor: Hardware glitches. Soft-Errors.

Distributed environment: Processor crashes / recoveries. Link errors.

Resulting in an unpredictable system state.

Coping with Transient Errors

Masking (safety factor) achieved by: Information redundancy (e.g., ECC). Time/Space redundancy. (e.g., TMR)

Self-Stabilization [Dijkstra74]: Assuming any system state (caused by errors). Recovering by converging into legal behavior. Existing algorithms for distributed tasks:

Routing, leader election, mutual exclusion, etc.

Self-Stabilizing Algorithms – a Solution to Soft-Errors?

Self-Stabilizing algorithm assumes that the microprocessor executes it. Soft-Errors may cause the microprocessor to be

stuck in a faulty state.

Composition of self-stabilizing algorithms creates a self-stabilizing system. Make the microprocessor eventually fetch-decode-

execute machine code.

The Gap.

Need a transformation between: Input program P written in a high abstraction

language, e.g., (D)ASM. Output program Q in a machine language, say, JVM.

Existing compilers? P and Q behaves the same when started in the

initial state. What if Q reaches an unexpected state due to

soft-error experienced by microprocessor?

Trivial Example

A statement of the form:For each i in {0..9} do f(i)

May be compiled to Start with cx=12 inside the loop…

Moreover: Any runtime mechanism can get stuck / inconsistent.

mov ax, 10 mov cx, 0loop1: push cx call f inc cx cmp cx,ax jne loop

Stabilization Preserving Compiler – a closer look

State space of P

Ensuring that Q eventually behaves as P:

State space of Q

Self-Stabilization Preserving Compiler: Summary

Front end of compiler established. Typed version of ASM. JavaCC as a parser generator.

Interpreter (used as a model).

Fast stabilization vs. optimizations.

Self Stabilization preserving compiler. Language with clear semantics from any state. Innovative demands from compiler.

Talk Outline

Self Stabilizing Microprocessor [DH04]

Self Stabilizing Operating System [DY04]

Self-Stabilization Preserving Compiler[DH05]

Self-Stabilizing Automatic Recoverer For Eventual Byzantine Software [BDK03]

Recover Oriented Programming[BD05]

Self-Stabilization and Evolving Systems

Real world systems cannot be verified exhaustively…

We enforce safety and live-ness specifications

Contract between the client, project manager and programmers, that is checked on line!

Make sure that the additional (thin) monitoring and recovering layer is self-stabilizing

A change can be made to the

implementation/specification

to support evolving environments

Self-Stabilizing Recoverer for Eventual Byzantine Software

Olga Brukman, Shlomi DolevDepartment of Computer Science

Ben-Gurion University, Israel

Hillel Kolodner,Haifa Research Labs

IBM, Israel

Software Contains Bugs

Heisenbugs, corrupt states, leaked resources are common… Correct and faultless SW is hard Long-lived running programs, e.g., OS

Usually software is tested when starting from initial state and considering limited time scenarios.

Fault Model Reflecting Reality

Software packages can be trusted to work as required after restart.Eventual Byzantine software.System administrators and users use reboot to deal with faults.

Middleware Architecture

OS

Kern

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OMR

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<Preds,RActs>2

…<Preds,RActs>n

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Acts

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<Preds,RActs>

<Preds,RActs>

<Pred

s,RActs>

Monitor-Restarter for Process and Subsystem

<Pred,RActs>1

<Pred,RActs>2

…

Restart Actions – Mature Approach

Subsystem waits for completion of a restart of its components.Restart action may vary, depending on component internal state. Reschedule Roll-back Kill & Restart

Few restart attempts with more drastic restart actions.

Computational Model: rsf-executionAn execution E is rsf (restart supporting fair)-execution iff E is a fair execution in which every subsystem subi that is initialised during E respects its specification function ssi.

Requirement: Every rsf-execution E has a suffix in which the system respects its specification function ss.

Tools for Implementation – Black Box Approach

Software package is a black box.

Package is monitored by recording it’s IO (e.g., strace in Linux).

Monitors are independent of specific implementation

Tools for Implementation – Transparent Box Approach

Software package implementation tool is known.

Run-Time Reflection tools are used to monitor and restart the package.

Possible in Java, C++, CORBA, COM.

Practical Experience: Printers Problem

Corrupted pdf, doc or ps file sent to printing server.

Printer can’t print the file.

Cause retries by printing server Printer is “stuck” on one job.

Predicate for printing server: Restrict number of retries, try format conversions,

send error message to user.

Self-Stabilizing Automatic Recoverer: Summary

Theory foundations of self-stabilization and restart techniques could serve as a basis for the new paradigms.General framework for design and correctness proof for autonomic recoverer.

Printers experience coordinated with IBM.

Recovery Oriented Programming

Olga Brukman and Shlomi DolevDepartment of Computer Science

Ben-Gurion University, Israel

Towards Robust Software

Programming Structural programming, OOD, Design Patterns…

Testing and debugging Unit testing [JUnit, CppUnit]… Design By Contract (Eiffel) …

Formal specification languages ASM, IO Automata, NURPL

Model checkingOnline recovery ROC [PBB02]. Self-Stabilizing Autonomic Recoverer for Eventual

Byzantine Software [BDK03] - black box software packages.

Our Contribution

Program invariants derived from design specifications. Checked every time invariant variables are updated.

Automatic code generation for invariant verification and recovery upon invariant violation.Invariants are verified during runtime. Change of invariant variable is pre-checked in sand-

box. Violation is prevented and replaced with a recovery action.

Our Contribution Cont.

Recovery action is chosen depending on the current state and history.

Roll back & resume.Wait.Reschedule.Kill & restart.

External Monitoring

Monitoring the whole task to avoid transient faults occurrence after which

invariant variables are not changed ( and no invariant checks are done)

liveness problem – monitor over time

Producer Consumer Example

Producer, Consumer – threads

Queue – a circular bounded length queue int queue[size] int start – position of

current first element in the queue

int end – position of the first empty place in the queue

boolean empty

Producer

Consumer

Queue

Invariants for Queue

start, end are in some values range

({0,..,size-1})

Queue is not empty iff start != end

A possible invariant is:

(start in {0,..,size-1}, end in{0,.., size-1}) &&

(start != end => empty == false)

Given Implementation Codeclass Queue {

private int[] queue; private int start, end, size; private boolean empty; public Queue(int size){

queue = new int[size];start = 0; end =0; empty= true;

} public synchronized int dequeue() {

int result; while (empty) wait(); result = queue[start]; start= (start + 1) % size; empty = (start == end) ? true; notifyAll(); return result; } public synchronized void enqueue(int value) { while (start == end && !empty) //while full wait(); queue[end] = value; end = (end + 1) % size; empty=false; notifyAll(); }}

Code Transformationclass Queue {invariant: (start in {0,..,size-1}, end in {0,.., size-1}) &&

(start != end => empty == false) recovery actions: {this = new Queue(); empty = false;}

private int[] queue; private int start, end, size; private boolean empty; public Queue(int size){

queue = new int[size];atomic{ start = 0; end =0; empty= true;}

} public synchronized int get() {

int result; while (empty) wait(); result = queue[start]; atomic{ start= (start + 1) % size; empty = (start == end) ? true;} notifyAll(); return result; }

Code Transformation Cont. public synchronized void put(int value) {

while (start == end && !empty) //while full wait(); queue[end] = value; atomic{ end = (end + 1) % size; empty=false;} notifyAll(); }

}

External Monitoring

The possible invariant is:

(start in {0,..,size-1}, end in {0,.., size-1}) &&

(start != end => empty = false) &&

(start = end && empty = false => producer.wait()) && (empty = true =>consumer.wait()) &&

[(producer.wait() && !consumer.wait()) || (!producer.wait() && consumer.wait())]

New recovery actions: interrupt producer/consumer and initialize it.

Recovery Oriented Programming: Summary

Programming with self-stabilization enforcing.Eventually safe execution.

Talk Conclusions

Self-Stabilization as an effective paradigm for creating robust systems.

Rigorous approach for designing basic system components Microprocessor Operating system Compiler Evolving and Recovery Oriented