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<H2><A NAME="SECTION04536000000000000000">Performance Bottlenecks in the Expert Symmetric
Eigenproblem Drivers</A></H2>
<A NAME="subsecperfsyevx"> </A>
<P>
Large clusters of eigenvalues in the input matrix may
cause poor performance in the expert symmetric
eigenproblem drivers, PSSYEVX<A NAME="4239"> </A>/PDSYEVX<A NAME="4240"> </A>.
If the execution
time observed for the
ScaLAPACK drivers
PxSYGVX and
PxSYEVX is more than double
the estimate in
section <A HREF="node123.html#subsecestim">5.3.3</A>
and
more than the minimum LWORK is provided,
we recommend that
this part of the
code be retimed after relaxing
the orthogonalization
requirements. This can
be achieved either by
setting the value of
the formal parameter
LWORK to the minimum
allowed by the driver
as specified in the
leading comments of
the source code,
or by calling the driver
with the value of the
parameter ORTOL set to
the machine epsilon
multiplied by the norm
of the matrix. These
last two values may
be obtained by calling
respectively the
ScaLAPACK routines
PxLAMCH and
PxLANSY. If the
execution time obtained
for the driver after
relaxing the orthogonalization
requirements is substantially
reduced, it is likely that
the spectrum of the matrix
or matrix pencil has a
large cluster of eigenvalues
that the driver attempts to
reorthogonalize. Otherwise,
it is likely that the
performance bottleneck
is caused by other
factors as mentioned
in section <A HREF="node125.html#subsecperfid">5.3.5</A>.
If the matrix or
matrix pencil has
a large cluster of
eigenvalues, we
recommend using
the corresponding
simple driver PxSYEV, instead.
If
the application
can tolerate loss
of orthogonality, the
drivers PxSYGVX and
PxSYEVX may achieve
good performance by
relaxing the orthogonalization
requirements using the
method suggested above.
Please check
the value of the INFO
parameter returned by
these and all ScaLAPACK drivers.
<P>
<BR> <HR>
<P><ADDRESS>
<I>Susan Blackford <BR>
Tue May 13 09:21:01 EDT 1997</I>
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