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The I/O access patterns of parallel programs often consist of accesses to a large number of small, noncontiguous pieces of data. If an application's I/O needs are met by making many small, distinct I/O requests, however, the I/O performance degrades drastically. T o a void this problem, MPI-IO allows users to access a noncontiguous data set with a single I/O function call. This feature provides MPI-IO implementations an opportunity to optimize data access.We describe how our MPI-IO implementation, ROMIO, delivers high performance in the presence of noncontiguous requests. We explain in detail the two k ey optimizations ROMIO performs: data sieving for noncontiguous requests from one process and collective I/O for noncontiguous requests from multiple processes. We describe how one can implement these optimizations portably on multiple machines and le systems, control their memory requirements, and also achieve high performance. We demonstrate the performance and portability with performance results for three applications|an astrophysics-application template (DIST3D), the NAS BTIO benchmark, and an unstructured code (UNSTRUC)|on ve di erent parallel machines: HP Exemplar, IBM SP, I n tel Paragon, NEC SX-4, and SGI Origin2000.

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An ab initio calculation of the 12 C elastic form factor, and sum rules of longitudinal and transverse response functions measured in inclusive (e, e ) scattering, is reported, based on realistic nuclear potentials and electromagnetic currents. The longitudinal elastic form factor and sum rule are found to be in satisfactory agreement with available experimental data. A direct comparison between theory and experiment is difficult for the transverse sum rule. However, it is shown that the calculated one has large contributions from two-body currents, indicating that these mechanisms lead to a significant enhancement of the quasi-elastic transverse response. This fact may have implications for the anomaly observed in recent neutrino quasi-elastic charge-changing scattering data off 12 C. The current picture of the nucleus as a system of protons and neutrons interacting among themselves via two-and three-body forces and with external electroweak probes via one-and two-body currents-a dynamical framework we will refer to below as the standard nuclear physics approach (SNPA)-has been shown to reproduce satisfactorily a variety of empirical properties of light nuclei with mass number A ≤ 12, including energy spectra [1][2][3][4][5][6][7], static properties [1,3,4,8,9] of low-lying states, such as charge radii, and magnetic and quadrupole moments, and longitudinal electron scattering [10,11]. However, it has yet to be established conclusively whether such a picture quantitatively and successfully accounts for the observed electroweak structure and response of these systems, at least those with A > 4, in a wide range of energy and momentum transfers. This issue has acquired new and pressing relevance in view of the anomaly seen in recent neutrino quasielastic charge-changing scattering data on 12 C [12], i.e., the excess, at relatively low energy, of measured cross section relative to theoretical calculations. Analyses based on these calculations have led to speculations that our present understanding of the nuclear response to chargechanging weak probes may be incomplete [13], and, in particular, that the momentum-transfer dependence of the axial form factor of the nucleon may be quite different from that obtained from analyses of pion electroproduction data [14] and measurements of neutrino and anti-neutrino reactions on protons and deuterons [15][16][17][18]. However, it should be emphasized that the calculations on which these analyses are based use rather crude models of nuclear structure-Fermi gas or local density approximations of the nuclear matter spectral function-as well as simplistic treatments of the reaction mechanism, and do not fit the picture outlined above. Conclusions based on them should therefore be viewed with caution.The present work provides the first step towards a comprehensive study, within the SNPA, of the quasi-elastic electroweak response functions of light nuclei. We report an exact quantum Monte Carlo (QMC) calculation of the elastic form factor and sum rules associated with the longitudi...

We discuss the issues involved in implementing MPI-IO portably on multiple machines and le systems and also achieving high performance. One way t o i m p l e m e n t MPI-IO portably is to implement it on top of the basic Unix I/O functions (open, lseek, read, write, a n d close), which a r e themselves portable. We argue that this approach has limitations in both functionality and performance. We instead advocate an implementation approach t h a t c o m bines a large portion of portable code and a small portion of code that is optimized separately for di erent m a c hines and le systems. We h a ve used such an approach to develop a high-performance, portable MPI-IO implementation, called ROMIO.In addition to basic I/O functionality, w e consider the issues of supporting other MPI-IO features, such as 64-bit le sizes, noncontiguous accesses, collective I/O, asynchronous I/O, consistency and atomicity s e m a n tics, user-supplied hints, shared le pointers, portable data representation, le preallocation, and some miscellaneous features. We describe how w e implemented each of these features on various machines and le systems. The machines we consider are the HP Exemplar, IBM SP, Intel Paragon, NEC SX-4, SGI Origin2000, and networks of workstations and the le systems we consider are HP HFS, IBM PIOFS, Intel PFS, NEC SFS, SGI XFS, NFS, and any general Unix le system (UFS).We also present our thoughts on how a le system can be designed to better support MPI-IO. We provide a list of features desired from a le system that would help in implementing MPI-IO correctly and with high performance.