Parallel files may be stored and accessed
in great many ways that depend on the operating system,
particular devices used for storage and any middleware that may live in between.
In order to optimize file access the programmer may wish to provide
additional information to MPI, in hope that MPI would know
what to do with it. Such information is referred to as
hints and there is a special MPI construct
the info object
that is supposed to collect all the hints. Once constructed the info object can be
It should be understood though that any hints you may wish to
give MPI this way are only advisory and what MPI is going to do
with them is implementation dependent.
The info object is opaque. MPI provides functions for its manipulation
and inspection, but this is the only way you can (or should)
deal with it. Information contained in the info object
consists of (key, value) pairs - both are strings.
Their maximum length allowed by an MPI implementation is given
which can be looked up on
mpi.h. Their values on the AVIDD cluster are:
gustav@bh1 $ grep MAX_INFO mpi.h #define MPI_MAX_INFO_KEY 255 #define MPI_MAX_INFO_VAL 1024 gustav@bh1 $
An empty info object can be created with the call to MPI_Info_create, whose synopsis is simply:
int MPI_Info_create(MPI_Info *info)Once created, it can be populated with (key, value) pairs by calling MPI_Info_set whose synopsis is
int MPI_Info_set(MPI_Info info, char *key, char *value)
An info object obtained, e.g., from an MPI file, can be interrogated in various ways. First you can find how many (key, value) pairs there are in the object by calling function MPI_Info_get_nkeys which returns the number of keys currently defined within the object in its second argument:
int MPI_Info_get_nkeys(MPI_Info info, int *nkeys)Once you know how many (key, value) pairs there are in it, you can inspect them by calling first MPI_Info_get_nthkey in order to get the string that corresponds to the nth key - the synopsis of this function is:
int MPI_Info_get_nthkey(MPI_Info info, int n, char *key)and then MPI_Info_get to get the value associated with the key. The synopsis of
int MPI_Info_get(MPI_Info info, char *key, int valuelen, char *value, int *exists)where
existsis a Boolean flag that is set to
TRUE(1) if the corresponding (key, value) pair exists and to
An info object can be duplicated and it can be also erased by freeing it. Specific (key, value) pairs may be deleted from an existing info object too.
MPI reserves certain hints, which are described in
section 126.96.36.199, Reserved File Hints,
of the MPI-2 Standard. Unfortunately the current MPICH2 implementation of MPI-2
ignores most of them, including
file_perm, which would come handy
in some of our example programs. The reserved hints cover parameters such
as striping factor, striping unit, number of IO nodes (there
are systems where not all nodes have IO capability),
list of IO devices to store the file on, various chunking
parameters, the size of buffers to be used for reading and writing,
and the like.
Because the GPFS on the AVIDD system is accessed only through its UFS interface (MPI doesn't really know that GPFS, is GPFS), few of the reserved hints would be of much use anyway, even if MPICH2 did recognize them.
When a file is opened with the
parameter passed in place of an info object, a default info is used
by MPI. The following program retrieves this info object and
queries it. This shows us some of the info (key, value) pairs
that are implemented by MPICH2 on the UFS at present.