An official website of the United States government
Posit is a recently proposed alternative to the floating point representation (FP). It provides tapered accuracy. Given a fixed number of bits, the posit representation can provide better precision for some numbers compared to FP, which has generated significant interest in numerous domains. Being a representation with tapered accuracy, it can introduce high rounding errors for numbers outside the above golden zone. Programmers currently lack tools to detect and debug errors while programming with posits. This paper presents PositDebug, a compile-time instrumentation that performs shadow execution with high pre- cision values to detect various errors in computation using posits. To assist the programmer in debugging the reported error, PositDebug also provides directed acyclic graphs of instructions, which are likely responsible for the error. A contribution of this paper is the design of the metadata per memory location for shadow execution that enables productive debugging of errors with long-running programs. We have used PositDebug to detect and debug errors in various numerical applications written using posits. To demonstrate that these ideas are applicable even for FP programs, we have built a shadow execution framework for FP programs that is an order of magnitude faster than Herbgrind.
more »
« less
Fast shadow execution for debugging numerical errors using error free transformations
This paper proposes, EFTSanitizer, a fast shadow execution framework for detecting and debugging numerical errors during late stages of testing especially for long-running applications. Any shadow execution framework needs an oracle to compare against the floating point (FP) execution. This paper makes a case for using error free transformations, which is a sequence of operations to compute the error of a primitive operation with existing hardware supported FP operations, as an oracle for shadow execution. Although the error of a single correctly rounded FP operation is bounded, the accumulation of errors across operations can result in exceptions, slow convergences, and even crashes. To ease the job of debugging such errors, EFTSanitizer provides a directed acyclic graph (DAG) that highlights the propagation of errors, which results in exceptions or crashes. Unlike prior work, DAGs produced by EFTSanitizer include operations that span various function calls while keeping the memory usage bounded. To enable the use of such shadow execution tools with long-running applications, EFTSanitizer also supports starting the shadow execution at an arbitrary point in the dynamic execution, which we call selective shadow execution. EFTSanitizer is an order of magnitude faster than prior state-of-art shadow execution tools such as FPSanitizer and Herbgrind. We have discovered new numerical errors and debugged them using EFTSanitizer.
more »
« less
- PAR ID:
- 10603115
- Publisher / Repository:
- Association for Computing Machinery (ACM)
- Date Published:
- Journal Name:
- Proceedings of the ACM on Programming Languages
- Volume:
- 6
- Issue:
- OOPSLA2
- ISSN:
- 2475-1421
- Format(s):
- Medium: X Size: p. 1845-1872
- Size(s):
- p. 1845-1872
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
null (Ed.)This paper proposes a new approach for debugging errors in floating point computation by performing shadow execution with higher precision in parallel. The programmer specifies parts of the program that need to be debugged for errors. Our compiler creates shadow execution tasks, which execute on different cores and perform the computation with higher precision. We propose a novel method to execute a shadow execution task from an arbitrary memory state, which is necessary because we are creating a parallel shadow execution from a sequential program. Our approach also ensures that the shadow execution follows the same control flow path as the original program. Our runtime automatically distributes the shadow execution tasks to balance the load on the cores. Our prototype for parallel shadow execution, PFPSanitizer, provides comprehensive detection of errors while having lower performance overheads than prior approaches.more » « less
-
Developing Big Data Analytics often involves trial and error debugging, due to the unclean nature of datasets or wrong assumptions made about data. When errors (e.g. program crash, outlier results, etc.) arise, developers are often interested in pinpointing the root cause of errors. To address this problem, BigSift takes an Apache Spark program, a user-defined test oracle function, and a dataset as input and outputs a minimum set of input records that reproduces the same test failure by combining the insights from delta debugging with data provenance. The technical contribution of BigSift is the design of systems optimizations that bring automated debugging closer to a reality for data intensive scalable computing. BigSift exposes an interactive web interface where a user can monitor a big data analytics job running remotely on the cloud, write a user-defined test oracle function, and then trigger the automated debugging process. BigSift also provides a set of predefined test oracle functions, which can be used for explaining common types of anomalies in big data analytics--for example, finding the origin of the output value that is more than k standard deviations away from the median. The demonstration video is available at https://youtu.be/jdBsCd61a1Q.more » « less
-
Melquiond, Guillaume; Tang, Ping_Tak_Peter (Ed.)Recent advances have made numeric debugging tools much faster by using double-double oracles, and numeric analysis tools much more accurate by using condition numbers. But these techniques have downsides: double-double oracles have correlated error so miss floating-point errors while condition numbers cannot cleanly handle over- and underflow. We combine both techniques to avoid these downsides. Our combination, EXPLANIFLOAT, computes condition numbers using double- double arithmetic, which avoids correlated errors. To handle over- and underflow, it introduces a separate logarithmic oracle. As a result, EXPLANIFLOAT achieves a precision of 80.0% and a recall of 96.1% on a collection of 546 difficult numeric benchmarks: more accurate than double-double oracles yet dramatically faster than arbitrary-precision condition number computations.more » « less
-
Cybercrime scene reconstruction that aims to reconstruct a previous execution of the cyber attack delivery process is an important capability for cyber forensics (e.g., post mortem analysis of the cyber attack executions). Unfortunately, existing techniques such as log-based forensics or record-and-replay techniques are not suitable to handle complex and long-running modern applications for cybercrime scene reconstruction and post mortem forensic analysis. Specifically, log-based cyber forensics techniques often suffer from a lack of inspection capability and do not provide details of how the attack unfolded. Record-and-replay techniques impose significant runtime overhead, often require significant modifications on end-user systems, and demand to replay the entire recorded execution from the beginning. In this paper, we propose C2SR, a novel technique that can reconstruct an attack delivery chain (i.e., cybercrime scene) for post-mortem forensic analysis. It provides a highly desired capability: interactable partial execution reconstruction. In particular, it reproduces a partial execution of interest from a large execution trace of a long-running program. The reconstructed execution is also interactable, allowing forensic analysts to leverage debugging and analysis tools that did not exist on the recorded machine. The key intuition behind C2SR is partitioning an execution trace by resources and reproducing resource accesses that are consistent with the original execution. It tolerates user interactions required for inspections that do not cause inconsistent resource accesses. Our evaluation results on 26 real-world programs show that C2SR has low runtime overhead (less than 5.47%) and acceptable space overhead. We also demonstrate with four realistic attack scenarios that C2SR successfully reconstructs partial executions of long-running applications such as web browsers, and it can remarkably reduce the user’s efforts to understand the incident.more » « less
