about work space size

[neodreamer]

many lapack routines require a work space,
for the optimal work space lwork >= N*NB.

I dont want to use lwork=-1 to get the optimal size.
If I want to allocate sufficient work space to the routines
such as zgeqrf or zunmqr, Is lwork=N*N large enough?
I thought NB<=N, is this correct?
.

[Julien Langou]

You really should use LWORK = -1.That's definetely the best way to go.

Now taking LWORK = N*N should indeed give you a workspace bigger than
the optimal workspace (for very small N that might not work though). This means
that the LAPACK code will be able to block the operations (and so run fast), this also
means that you will waste space.