Systems Seminar Schedule Monday, 18 Februrary, 4pm: – “New Wine in Old Bottles” - Douglas Thain 4 March: – No seminar: Paradyn/Condor Week Tuesday, 19 March, 3pm: – “The Microsoft.NET System” - Mike Litzkow Tuesday, 2 April, 3pm: – “Condor and the Grid” - Miron Livny Monday, 15 April, 4pm: – “Exploiting Gray-Box Knowledge of Buffer-Cache Management” - Nathan Burnett Monday, 29 April, 4pm: – “Bridging the Information gap in Storage Protocol Stacks” - Tim Denehy

New Wine in Old Bottles: Java on Condor Douglas Thain University of Wisconsin 18 February 2002

Abstract We have added Java support to Condor. I’ll 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!)

Outline Why Java and Condor? Architecture Initial Experience A Little Error Theory Changes for the Better Conclusions

Java for Scientific Computing Java 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 Conference – The Java Grande Forum A. 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.

Limitations Java 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 Computing Condor 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 INFN Central Manager condor schedd Job condor startd RAMcpu condor startd RAMcpu condor startd RAMcpu condor startd RAMcpu condor startd RAMcpu condor startd RAMcpu Job condor startd RAMcpu condor startd RAMcpu condor startd RAMcpu condor startd RAMcpu condor startd RAMcpu condor startd RAMcpu Job condor startd RAMcpu condor startd RAMcpu condor startd RAMcpu condor startd RAMcpu condor startd RAMcpu condor startd RAMcpu Job UWCS Central Manager

226 Condor Pools 5576 Condor Hosts Top 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 Reality Coupling 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.

Outline Why Java and Condor? Architecture Initial Experience A Little Error Theory Changes for the Better Conclusions

scheddstartd Match Maker Machine Policies Job Policies Home File System Claiming Protocol Activation Protocol Matchmaking Protocol Execution Protocol shadowstarter Fork The Job Fork Creates the execution environment. Exports the details, policy, and I/O services.

JVM Fork startershadow Fork Home File System Wrapper I/O Library The Job I/O ServerI/O Proxy Secure Remote I/O Local System Calls Local I/O (Chirp)

User Interface condor_status -java Name JavaVendor Ver State Activity LoadAv Mem aish.cs.wisc. Sun Microsy Owner Idle anfrom.cs.wis Sun Microsy Owner Idle babe.cs.wisc. Sun Microsy Claimed Busy Machines Owner Claimed Unclaimed Matched Preempting INTEL/LINUX Total

User Interface universe = java executable = Main.class jar_files = MyLibrary.jar input = infile output = outfile arguments = Main queue condor_submit

I/O Interface Input, 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 -> ChirpInputStream – FileOutputStream -> ChirpOutputStream

Application Java Standard Libraries Java Virtual Machine Operating System C Standard Library Chirp I/O Library Added a new library on existing interfaces. User must call new constructors. JNI Java 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 JVM’s own operations.

Outline Why Java and Condor? Architecture Initial Experience A Little Error Theory Changes for the Better Conclusions

Initial Experience Bad 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 can’t initialize. – Home file system offline.

Initial Experience Although 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?

Outline Why Java and Condor? Architecture Initial Experience A Little Error Theory Changes for the Better Conclusions

A Little Error Theory Build 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 Terminology Failure – An externally-visible deviation from specifications. Error – An internal data state that leads to a failure. Fault – An external event that creates an error. A. Avizienis and J.C. Laprie. Dependable computing: From concepts to design diversity. IEEE 74(5) May 1986.

Example ClientServer What is sqrt(4)? Hmm, sqrt(4) is... Hmm, sqrt(9) is... Answer: 3 ERROR FAILURE FAULT

Implicit errors – The system claims to have reached a valid result, but an auditor claims it is invalid. Example: sqrt(3)==2 Explicit errors – The system tells us it cannot complete the desired action. Example: file not found. Escaping errors – The 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 K. Ekandham and A. Bernstein. Some new Transitions in hierarchical level structures. Operating Systems Review 12(4), 1978.

Program Virtual Memory System Physical Memory Backing Store load data Could 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. Parent Process Escaping error: Tell the parent that the program could not complete. Normal Exit Abnormal Exit

Interface Contracts int 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: , October B. Meyer. Object-Oriented Software Construction. Prentice Hall, 1997.

Exceptions int 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.

Program Virtual File System Memory Disk open Success, FileNotFound, AccessDenied Parent Process Normal Exit Abnormal Exit MemoryCorrupt, DiskOffline, PigeonLost INTERFACE IMPLEMENTATION

Error Scope In 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 Scope The 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. ErrorScopeHandler FileNotFoundFileCalling Function RPC DisconnectProcessParent Process Cache Coherency Problem MachineHypervisor or Operator PVM Node CrashPVM ClusterParent Process

Error Detail The 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!

Lessons Principle 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 error’s scope.

Outline Why Java and Condor? Architecture Initial Experience A Little Error Theory Changes for the Better Conclusions

Java and Condor Revisited What did we do wrong? We focussed on error detail without considering error scope.

Java and Condor Revisited To fix the system, we revisited the notion of error scope throughout. Two examples: – JVM exit code – I/O errors

JVM Exit Code DetailScopeExit Code Program exited by completing mainProgram0 Program exited through System.exit(x)Programx Exception: Null pointer.Program1 Exception: Out of memory.Virtual Machine 1 Exception: Java Misconfigured.Remote Resource 1 Exception: Home file system offline.Local Resource 1 Exception: Program image corrupt.Job1

JVM startershadow Fork Home File System Wrapper I/O Library The Job Result File JVM Result Result of Execution Attempt + Result of Program, If any. Starter Result + Program Result

I/O Error Scope All 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 starter Wrapper I/O Library The Job Result File JVM Result Result of Execution Attempt + Result of Program, If any. To I/O Proxy Error Outside Program Scope Error Inside Program Scope

Outline Why Java and Condor? Architecture Initial Experience A Little Error Theory Changes for the Better Conclusions

Conclusion We 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 Thought There isn’t 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 Thought The 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 Thought An 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?

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

c2 cluster tux lab istat skywalker.cs.wisc.edu

Acknowledgements Although 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: – These slides: – Douglas Thain – Questions now?