NASA's Soil Moisture Active Passive satellite mission will use both an L-band radiometer and radar to produce global-scale measurements of soil moisture. L-band backscatter is also sensitive to the water content of vegetation. We found that a moderate dew increased the L-band backscatter of a soybean canopy by 1 dB. Dew thus has the potential to add error to satellite observations of soil moisture.

We investigate the Message Passing Interface Input/Output (MPI I/O) implementation issues for two overlapping access patterns: the overlaps among processes within a single I/O operation and the overlaps across a sequence of I/O operations. The former case considers whether I/O atomicity can be obtained in the overlapping regions. The latter focuses on the file consistency problem on parallel machines with client-side file caching enabled. Traditional solutions for both overlapping I/O problems use whole file or byte-range file locking to ensure exclusive access to the overlapping regions and bypass the file system cache. Unfortunately, not only can file locking serialize I/O, but it can also increase the aggregate communication overhead between clients and I/O servers. For atomicity, we first differentiate MPI's requirements from the Portable Operating System Interface (POSIX) standard and propose two scalable approaches, graph coloring and process-rank ordering, which can resolve access conflicts and maintain I/O parallelism. For solving the file consistency problem across multiple I/O operations, we propose a method called Persistent File Domains, which tackles cache coherency with additional information and coordination to guarantee safe cache access without using file locks.

Parallel file subsystems in today's high-performance computers adopt many I/O optimization strategies that were designed for distributed systems. These strategies, for instance client-side file caching, treat each I/O request process independently, due to the consideration that clients are unlikely related with each other in a distributed environment. However, it is inadequate to apply such strategies directly in the high-performance computers where most of the I/O requests come from the processes that work on the same parallel applications. We believe that client-side could perform more effectively if the subsystem is aware of the process scope of an application and regards all the application processes as a single client. In this paper, we propose the idea of caching which coordinates the application processes to manage cache data and achieve cache coherence without involving the I/O servers. To demonstrate this idea, we implemented a collective subsystem at user space as a library, which can be incorporated into any message passing interface implementation to increase its portability. The performance evaluation is presented with three I/O benchmarks on an IBM SP using its native parallel file system, GPFS. Our results show significant performance enhancement obtained by collective over the traditional approaches.

Summary form only given. In order for I/O systems to achieve high performance in a parallel environment, they must either sacrifice client-side file caching, or keep caching and deal with complex coherency issues. The most common technique for dealing with cache coherency in multiclient file caching environments uses file locks to bypass the client-side cache. Aside from effectively disabling cache usage, file locking is sometimes unavailable on larger systems. The high-level abstraction layer of MPI allows us to tackle cache coherency with additional information and coordination without using file locks. By approaching the cache coherency issue further up, the underlying I/O accesses can be modified in such a way as to ensure access to coherent data while satisfying the user's I/O request. We can effectively exploit the benefits of a file system's client-side cache while minimizing its management costs.

For concurrent I/O operations, atomicity defines the results in the overlapping file regions simultaneously read/written by requesting processes. Atomicity has been well studied at the file system level, such as POSIX standard. We investigate the problems arising from the implementation of MPI atomicity for concurrent overlapping write access and provide two programming solutions. Since the MPI definition of atomicity differs from the POSIX one, an implementation that simply relies on the POSIX file systems does not guarantee correct MPI semantics. To have a correct implementation of atomic I/O in MPI, we examine the efficiency of three approaches: I) file locking, 2) graph-coloring, and 3) process-rank ordering. Performance complexity for these methods are analyzed and their experimental results are presented for file systems including NFS, SGI's XFS, and IBM's GPFS.

This paper describes a unique visualization tool that has been used to analyze performance of the Cplant(tm) clusters at Sandia National Laboratories.As commodity cluster systems grow in size and complexity, understanding performance issues becomes more and more difficult.We have developed a tool that facilitates visual performance analysis within the context of the physical and runtime environment of a system. Combining an abstract system model with color-coding for both performance and job information enables quick fault isolation as well as insight into complex system behavior.

A great deal of effort has gone into developing open-ended inquiry activities for science education as well as complex computer tools for accessing scientific data to help students learn science.To be successful with these tools and activities, students need to learn a set of inquiry skills and to develop a new mode of classroom work reflective inquiry.In this paper we describe the design of the Progress Portfolio, a software environment to promote reflective inquiry, and we examine the influences of the unique practices and features of classroom contexts on our design process.K@ywOrdS