Re: [ublas] [bindings] LAPACK ?GESVD and minimum workspace size

Subject: Re: [ublas] [bindings] LAPACK ?GESVD and minimum workspace size
From: Rutger ter Borg (rutger_at_[hidden])
Date: 2011-04-07 04:41:27

On 04/07/2011 10:29 AM, sguazt wrote:
> Hi Rutger,
>
> On Thu, Apr 7, 2011 at 9:26 AM, Rutger ter Borg<rutger_at_[hidden]>  wrote:
>> On 04/06/2011 05:08 PM, sguazt wrote:
>>>
> [cut]
>>
>> You are saying that, in some cases, for gesvd, the minimum workspace size
>> returned may be larger than the queried optimum workspace size, and the
>> bindings are asserting on that error?
>>
>
> Exactly. Specifically, what the problem is in the assertion which
> involves the call to the min_size_work function.
>
>> Looking at the Fortran code, I see that there are not less than 10 code
>> paths for determining workspace size, but that the variation in minimum
>> seems to be limited to
>>
>> max( 4*n, 5*n )
>> max( 3*(m+n), 5*n )
>> max( 4*m, 5*n )
>>
>> Could you come up with a clever branch tree? Let's incorporate that in the
>> bindings. This is only for the real variants of gesvd, right?
>
> I've just updated my branch. It is at revision 71072.
>

Sorry, I meant if you could suggest the optimal conditions/structure of 
something like

if ( ... ) {
    return max( 3*(m+n) );
} else if ( ... ) {
    return ...;
} else {
    return ...;
}

for the min_size member function used in the gesvd driver.

Cheers,

Rutger