This paper presents a study of I/O scheduling techniques applied to the I/O forwarding layer. In high-performance computing environments, applications rely on parallel file systems (PFS) to obtain good I/O performance even when handling large amounts of data. To alleviate the concurrency caused by thousands of nodes accessing a significantly smaller number of PFS servers, intermediate I/O nodes are typically applied between processing nodes and the file system. Each intermediate node forwards requests from multiple clients to the system, a setup which gives this component the opportunity to perform optimizations like I/O scheduling. We evaluate scheduling techniques that improve spatiality and request size of the access patterns. We show they are only partially effective because the access pattern is not the main factor for read performance in the I/O forwarding layer. A new scheduling algorithm, TWINS, is presented to coordinate the access of intermediate I/O nodes to the data servers. Our proposal decreases concurrency at the data servers, a factor previously proven to negatively affect performance. The proposed algorithm is able to improve read performance from shared files by up to 28% over other scheduling algorithms and by up to 50% over not forwarding I/O.

International audienceThis article presents our approach to provide input/output (I/O) scheduling with double adaptivity: to applications and devices. In high-performance computing environments, parallel file systems provide a shared storage infrastructure to applications. In the situation where multiple applications access this shared infrastructure concurrently, their performance can be impaired because of interference. Our work focuses on I/O scheduling as a tool to improve performance by alleviating interference effects. The role of the I/O scheduler is to decide the order in which applications' requests must be processed by the parallel file system's servers, applying optimizations to adjust the resulting access pattern for improved performance. Our approach to improve I/O scheduling results is based on using information from applications' access patterns and storage devices' sensitivity to access sequentiality. We have applied machine learning to provide the ability to automatically select the best scheduling algorithm for each situation. Our approach improves performance by up to 75% over an approach that uses the same scheduling algorithm to all situations, without adaptability. Our results evidence that both aspects – applications and storage devices – are essential to make good scheduling decisions

We present a comprehensive survey on parallel I/O in the high performance computing (HPC) context. This is an important field for HPC because of the historic gap between processing power and storage latencies, which causes applications performance to be impaired when accessing or generating large amounts of data. As the available processing power and amount of data increase, I/O remains a central issue for the scientific community. In this survey, we focus on a traditional I/O stack, with a POSIX parallel file system. We present background concepts everyone could benefit from. Moreover, through the comprehensive study of publications from the most important conferences and journals in a five-year time window, we discuss the state of the art of I/O optimization approaches, access pattern extraction techniques, and performance modeling, in addition to general aspects of parallel I/O research. Through this approach, we aim at identifying the general characteristics of the field and the main current and future research topics.

International audiencePower consumption is one of the main challenges to achieve Exascale performance. Current research trends aim atovercoming power consumption constraints using low-power processors. Although new processors feature sensors that enableprecise power measurements, they provide different interfaces to collect data, making it difficult to correlate performance withenergy consumption. To overcome this issue, the authors developed a platform-independent tool that collects power andenergy data from homogeneous and heterogeneous systems. Using this tool, they provide a detailed comparison between alow-power processor (ARM big.LITTLE) and a high performance processor (Intel Sandy Bridge-EP) using all applicationsfrom the NAS parallel benchmarks and a real-world soil irrigation simulator. The results show that the average power demandof Intel Sandy Bridge-EP is within 12.6× to 152.4× higher than ARM big.LITTLE, whereas its average energy consumptionis within 1.6× to 7.1× superior. Overall, ARM big.LITTLE presented a better performance/energy trade-off when it takes<9.2× the execution time of Intel Sandy Bridge-EP to solve the same problem

I/O optimization techniques such as request scheduling can improve performance mainly for the access patterns they target, or they depend on the precise tune of parameters. In this paper, we propose an approach to adapt the I/O forwarding layer of HPC systems to the application access patterns by tuning a request scheduler. Our case study is the TWINS scheduling algorithm, where performance improvements depend on the time window parameter, which depends on the current workload. Our approach uses a reinforcement learning technique-contextual bandits-to make the system capable of learning the best parameter value to each access pattern during its execution, without a previous training phase. We evaluate our proposal and demonstrate it can achieve a precision of 88% on the parameter selection in the first hundreds of observations of an access pattern. After having observed an access pattern for a few minutes (not necessarily contiguously), we demonstrate that the system will be able to optimize its performance for the rest of the life of the system (years).

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In this paper, we improve the performance of server-side I/O scheduling on parallel file systems by transparently including information about the applications' access patterns. Server-side I/O scheduling is a valuable tool on multiapplication scenarios, where the applications' spatial locality suffers from interference caused by concurrent accesses to the file system. We present AGIOS, an I/O scheduling library for parallel file systems. We guide scheduler's decisions by including information about the applications' future requests. This information is obtained from traces generated by the scheduler itself, without changes in application or file system. Our approach shows performance improvements under different workloads of 46.3% on average when compared to a scenario without an I/O scheduler, and of 25.1% when compared to a scheduler which does not use information about future accesses.

The historical gap between processing and data access speeds causes many applications to spend a large portion of their execution on I/O operations. From the point of view of a large-scale, expensive, supercomputer, it is important to ensure applications achieve the best I/O performance to promote an efficient usage of the machine. In this paper, we evaluate the I/O infrastructure of the Santos Dumont supercomputer, the largest one from Latin America. More specifically, we investigate the performance of collective I/O operations. By conducting an analysis of a scientific application that uses the machine, we identify large performance differences between the available MPI implementations. We then further study the observed phenomenon using the BT-IO and IOR benchmarks, in addition to a custom microbenchmark. We conclude that the customized MPI implementation by Bull (used by more than 20% of the jobs) presents the worst performance for small collective write operations. Our results are being used to help the Santos Dumont users to achieve the best performance for their applications. Additionally, by investigating the observed phenomenon, we provide information to help improve future MPI-IO collective write implementations.