Hardware Reference
In-Depth Information
with the le access phase. In Venkatesan et al.'s study [14], I/O aggrega-
tor placement methods are proposed to minimize the communication cost
in the request aggregation phase. Based on the file views, it calculates the
non-aggregator-to-aggregator communication matrix, divides processes with
high-volume communication into groups, and maps the process groups to the
underlying network topology.
13.4 MPI-IO Hints
MPI-IO hints are a mechanism for users to pass information, such as ac-
cess patterns, to the implementation so that it can help optimize file access.
This is done by setting an
MPIInfo
object and passing it to the functions
MPIFileopen
,
MPIFilesetview
, or
MPIFilesetinfo
. An MPI-IO im-
plementation may choose to ignore all hints, and the program would still be
functionally correct. Some of MPI predefined I/O hints are
cbbuffersize
(size of allowable buffer to be used by collective I/O),
cbnodes
(maximum
number of aggregators),
stripingfactor
(number of file system stripes),
and
stripingunit
(file stripe size). An implementation may define addi-
tional hints to make use of file system specific features. Readers are referred
to the ROMIO user guide for the full list of available hints [10].
13.5 Conclusions
This chapter describes several important MPI-IO features to allow users
to describe the data access patterns and the intent of their applications. The
MPI-IO functions convey such information to the implementation so that
better I/O strategies can be used to achieve high performance for parallel
applications. MPI-IO has become a building block for several high-level I/O
libraries, such as Parallel NetCDF (Chapter 15) and HDF5 (Chapter 16),
which are widely used in various scientific communities. There are other MPI-
IO features not covered in this chapter, including data consistency control,
portable data representation, and split collective I/O. For their detailed in-
formation and learning materials, readers are referred to the MPI-2 Standard
[6] and MPI tutorial topics, such as Using MPI-2 [3].
