Compiler-Directed Collective-I/O

• Authors: Mahmut Kandemir
• Venue: IEEE Transactions on Parallel and Distributed Systems
• Year: 2001

Current approaches to parallel I/O demand extensive user effort to obtain acceptable performance. This is in part due to difficulties in understanding the characteristics of a wide variety of I/O devices and in part due to inherent complexity of I/O software. While parallel I/O systems provide users with environments where persistent data sets can be shared between parallel processors, the ultimate performance of I/O-intensive codes depends largely on the relation between data access patterns exhibited by parallel processors and storage patterns of data in files and on disks. In cases where access patterns and storage patterns match, we can exploit parallel I/O hardware by allowing each processor to perform independent parallel I/O. In order to keep performance decent under circumstances in which data access patterns and storage patterns do not match, several I/O optimization techniques have been developed in recent years. Collective I/O is such an optimization technique that enables each processor to do I/O on behalf of other processors if doing so improves the overall performance. While it is generally accepted that collective I/O and its variants can bring impressive improvements as far as the I/O performance is concerned, it is difficult for the programmer to use collective I/O in an optimal manner. In this paper, we propose and evaluate a compiler-directed collective I/O approach which detects the opportunities for collective I/O and inserts the necessary I/O calls in the code automatically. An important characteristic of the approach is that instead of applying collective I/O indiscriminately, it uses collective I/O selectively only in cases where independent parallel I/O would not be possible or would lead to an excessive number of I/O calls. The approach involves compiler-directed access pattern and storage pattern detection schemes that work on a multiple application environment. We have implemented the necessary algorithms in a source-to-source translator and within a stand-alone tool. Our experimental results on an SGI/Cray Origin 2000 multiprocessor machine demonstrate that our compiler-directed collective I/O scheme performs very well on different setups built using nine applications from several scientific benchmarks. We have also observed that the I/O performance of our approach is only 5.23 percent worse than an optimal scheme.