Systems Seminar ScheduleMonday, 18 Februrary, 4pm:New Wine in Old Bottles - Douglas Thain4 March:No seminar: Paradyn/Condor WeekTuesday, 19 March, 3pm:The Microsoft .NET System - Mike LitzkowTuesday, 2 April, 3pm:Condor and the Grid - Miron LivnyMonday, 15 April, 4pm:Exploiting Gray-Box Knowledge of Buffer-Cache Management - Nathan BurnettMonday, 29 April, 4pm:Bridging the Information gap in Storage Protocol Stacks - Tim Denehy

New Winein Old Bottles:Java on CondorDouglas ThainUniversity of Wisconsin18 February 2002

AbstractWe have added Java support to Condor. Ill tell you how it works and how to use it. There are some nifty features for end users. Adding this code forced us to think about the fundamental problem of coupling systems and representing errors.A lesson: One must consider the scope of an error as well as its detail.

Disclaimer:

This is still rough around the edges.(Someone had to go first!)

OutlineWhy Java and Condor?ArchitectureInitial ExperienceA Little Error TheoryChanges for the BetterConclusions

Java for Scientific ComputingJava is emerging as a tool for large scale (Grande) scientific computing.More accessible to domain scientists.Simplified porting.Faster development, debugging.User communities are forming:ACM Java Grande ConferenceThe Java Grande ForumA. Globus, E. Langhirt, M. Livny, R. Ramamurthy, M. Solomon, and S. Traugott. JavaGenes and Condor: Cycle-Scavenging genetic algorithms. ACM Conf on Java Grande, 2000.

LimitationsJava floating point and complex arithmetic do not yet satisfy all of the scientific community.Arguments continue between industry and academia.Java is yet slower than comparable programs in C/C++/Fortran.WAT compilers and JIT compilers are catching up.You choose: 2x slowdown vs 5x machines.Can we really harness 5x machines while still maintaining platform independence?

Condor for Scientific ComputingCondor creates a high-throughput computing system on a community of computers.A high-throughput computing system seeks to maximize the amount of work done over a long period of time.A community of computers may be any collection of machines that agree to work together.

Condor Enables Ordinary Users

226 Condor Pools5576 Condor HostsTop 10 Condor Pools:

The Hype:Java:Write once, run anywhere!Condor:Submit once, run everywhere!The Grid:Uniform, dependable, consistent, pervasive, and inexpensive computing.

The RealityCoupling systems is not trivial!The easy part:Putting java in front of the program name.The tricky parts:Java installation messes.Unavailable file systems.Distinguishing program errors from environmental errors.

OutlineWhy Java and Condor?ArchitectureInitial ExperienceA Little Error TheoryChanges for the BetterConclusions

scheddstartdMatchMakerMachinePoliciesJobPoliciesHomeFileSystemCreates the execution environment.Exports the details, policy, and I/O services.

HomeFileSystem

User Interfacecondor_status -java

Name JavaVendor Ver State Activity LoadAv Mem

aish.cs.wisc. Sun Microsy 1.2.2 Owner Idle 0.000 249anfrom.cs.wis Sun Microsy 1.2.2 Owner Idle 0.030 249 babe.cs.wisc. Sun Microsy 1.2.2 Claimed Busy 1.120 123...

Machines Owner Claimed Unclaimed Matched PreemptingINTEL/LINUX 514 101 408 5 0 0 Total 514 101 408 5 0 0

User Interfaceuniverse = javaexecutable = Main.classjar_files = MyLibrary.jarinput = infileoutput = outfilearguments = Main 1 2 3queuecondor_submit

I/O InterfaceInput, output, and error files are automatically transferred to/from the execution site.Any other named files may be transferred as well.To do online I/O without transferring whole files, you must make small changes to the code:FileInputStream -> ChirpInputStreamFileOutputStream -> ChirpOutputStream

ApplicationJava Standard Libraries Java Virtual MachineOperating SystemC Standard LibraryChirp I/O Library Added a new library on existing interfaces. User must call new constructors.JNIJava symbols are fully qualified, so transparent replacedment of classes is not possible.Could replace native methods in the JVM, but this ties us to open-source JVMs.Could trap real system calls, but these are complex (asynchronous, nonblocking, threaded) and may be difficult to distringuish from the JVMs own operations.

OutlineWhy Java and Condor?ArchitectureInitial ExperienceA Little Error TheoryChanges for the BetterConclusions

Initial ExperienceBad news: Nearly any unexpected failure would cause the job to be returned to the user:Out of memory at execution site.Java misconfigured at execution site.I/O proxy cant initialize.Home file system offline.

Initial ExperienceAlthough this was correct in some sense -- the information was true -- it was very frustrating.Users want to know when their program fails by design (NullPointerException,) but not if it fails due to the environment.What did we do wrong?

OutlineWhy Java and Condor?ArchitectureInitial ExperienceA Little Error TheoryChanges for the BetterConclusions

A Little Error TheoryBuild on standard definitions from fault-tolerance and programming languages.Some brief examples to get the idea.Return to Condor and use the theory to understand our design mistakes.

Fault Tolerance TerminologyFailureAn externally-visible deviation from specifications. ErrorAn internal data state that leads to a failure.FaultAn external event that creates an error.A. Avizienis and J.C. Laprie. Dependable computing: From concepts to design diversity. IEEE 74(5) May 1986.

ExampleClientServerHmm, sqrt(4) is...Hmm, sqrt(9) is...ERRORFAILUREFAULT

Implicit errorsThe system claims to have reached a valid result, but an auditor claims it is invalid. Example: sqrt(3)==2Explicit errorsThe system tells us it cannot complete the desired action. Example: file not found.Escaping errorsThe system detects an error, but has no method of reporting it, so it escapes by an alternate route. Example: core dump, kernel panic.John B. Goodenough, Exception Handling: issues and a proposed notation. CACM 18(120, December 1975.K. Ekandham and A. Bernstein. Some new Transitions in hierarchical level structures. Operating Systems Review 12(4), 1978.

ProgramVirtual Memory SystemPhysicalMemoryBackingStoreloaddataCould return a default value, but that creates an implicit error.Would like to return an explicit error, but a load insn has no exit code.Escaping error: Tell the parent that the program could not complete.

Interface Contractsint load( int address );

The implementor must either compute a result that conforms to the contract, or is obliged to cause an escaping error.C. Hoare. An axiomatic basis for computer programming. CACM 12(10:576-580, October 1969.B. Meyer. Object-Oriented Software Construction. Prentice Hall, 1997.

Exceptionsint open( String filename ) throws FileNotFound, AccessDenied;

A language with exceptions provides more structure to the contract. A declared exception is an explicit error. Yet, escaping errors are still possible.

ProgramVirtual File SystemMemoryDiskopenSuccess,FileNotFound,AccessDeniedMemoryCorrupt,DiskOffline,PigeonLostINTERFACEIMPLEMENTATION

Error ScopeIn order to be accepted by end users, a distributed system must be able to distinguish between errors computed by the program and errors forced upon it by the environment.

We use the term scope to draw the distinction.

Error ScopeThe scope of an error is the portion of the system that it invalidates.An error must be delivered to the process responsible for managing that scope.

ErrorScopeHandlerFileNotFoundFileCalling FunctionRPC DisconnectProcessParent ProcessCache Coherency ProblemMachineHypervisor or OperatorPVM Node CrashPVM ClusterParent Process

Error DetailThe detail of an error describes in phenomenological terms the cause of the error.In the right hands, the detail is useful. In the wrong hands, the detail can be misleading.Suppose open returns AccessDenied...File is not accessible - Ok.Library containing open is not accessible - Problem!

LessonsPrinciple 1:A routine must not generate an implicit error as a result of receiving an explicit error.Principle 2:An escaping error converts a potential implicit error into an explicit error at a higher level.Principle 3:An escaping error must be propagated to the program that manages the errors scope.

OutlineWhy Java and Condor?ArchitectureInitial ExperienceA Little Error TheoryChanges for the BetterConclusions

Java and Condor Revisited

What did we do wrong?

We focussed on error detail without considering error scope.

Java and Condor RevisitedTo fix the system, we revisited the notion of error scope throughout. Two examples:JVM exit codeI/O errors

JVM Exit Code

DetailScopeExit CodeProgram exited by completing mainProgram0Program exited through System.exit(x)ProgramxException: Null pointer.Program1Exception: Out of memory.Virtual Machine1Exception: Java Misconfigured.Remote Resource1Exception: Home file system offline.Local Resource1Exception: Program image corrupt.Job1

JVM

startershadowHomeFileSystemResultFileJVM ResultResult of Execution Attempt + Result of Program, If any.Starter Result +Program Result

I/O Error ScopeAll Java I/O operations throw a single exception type -- IOException.Our mistake: convert all detected errors into IOExceptions and pass them to the program.Makes sense for FileNotFound, but not for ProxyUnavailable or CredentialsExpired.

JVM

starterResultFileJVM ResultResult of Execution Attempt + Result of Program, If any.To I/O ProxyError OutsideProgram ScopeError InsideProgram Scope

OutlineWhy Java and Condor?ArchitectureInitial ExperienceA Little Error TheoryChanges for the BetterConclusions

ConclusionWe started building the Java Universe with some naive assumptions about errors.On encountering practical difficulties, we thought more abstractly about errors and developed the notion of scope and detail.By routing errors according to their scope, we made the system more robust and usable.

Food for ThoughtThere isnt always an easy way to propagate an error to the scope handler.Escaping error to parent process:Raise a POSIX signal.Escaping error to the starter:Throw a Java Error, trapped by the Wrapper, placed in file, read after process exits.

Food for ThoughtThe mere use of exceptions in a program does not imply a disciplined error management.For example, throws IOException is a very vague statement about an interface.What is an implementor allowed to throw?Can open() return FileNotFound?(Probably.)Can read() throws FileNotFound?(Asking for trouble.)What about ConnectionRefused?

Food for ThoughtAn contract can govern more than simply the interface specification.Consider this self-cleaning program:fd = open(file);unlink(file);close(fd);Works on UNIX, fails on WinNT.Can an interface (code+docs) really state all the necessary semantic information?Should it?

DeploymentAs of February 14th, the Java Universe is running on 515 RedHat 7.2 machines.Will be rolled out as part of Condor 6.3.2 on all platforms in the regular release schedule.Sun JDK 1.2.2 on UNIX machines.Sun JDK 1.3.2 on WinNT machines.Is the Java Universe available on my machine?condor_status -java

c2 clustertux labistatskywalker.cs.wisc.edu

AcknowledgementsAlthough we me take credit (or blame) for the most recent changes, the Condor architecture has dealt with errors for many years. Much credit goes to the core designers, esp. Mike Litzkow, Todd Tannenbaum, and Derek Wright.

More Info:The Condor Project:http://www.cs.wisc.edu/condorThese slides:http://www.cs.wisc.edu/~thainDouglas Thainthain@cs.wisc.eduQuestions now?