[MITgcm-devel] vectorizing seaice and exf

[Martin Losch]

Hi Patrick,

in
/u/u0/mlosch/scratch/MITgcm/pkg/exf/exf_bulkformulae.F
you can find a version of exf_bulkformulae that does vectorize  
(haven't checked the vector operation ratio yet, but the compiler no  
vectorizes the i-loop(s)). Tell me what you think (I get identical  
results, but had to make 8 fields two dimensional: tstar, ustar,  
qstar, tau, rdn, t0, delq, deltap).

now your questions (i'll refer to the NEC sxf90, which is relevant  
for anyone running at DKRZ as well):
you can make the sxf90 inline functions automatically, but only those  
that are < 50 lines, and whose code are in the same file, so for  
exf_bulkformulae you'd have to put these functions into the same *.F  
file (for automatic inlining). This works for, eg. exf_interp.F and  
function lagran (which is part of exf_interp.F).
you can also make sxf90 inline functions > 50 lines and from other  
files by explicitly specifying at compile time, e.g. '-pi  
exp=timestep_tracer expin=timestep_tracer.F',
BUT that's complicated and does not work with genmake2 (because the  
compiler is looking for the specified files and crashes if it cannot  
find them, see my earlier email to Jean-Michel. My solution to this,  
an extra flag, is not general either). You'd have to modify your  
makefile each time you run make makefile for this to work. So until  
this is not solved I would like to rely on automatic inlining only.

statements like
if ( atemp(i,j,bi,bj) .ne. 0. _d 0 ) then
don't seem to be a problem for the compiler

Martin


On 27 Sep 2007, at 14:46, Patrick Heimbach wrote:

>
> Hi Martin,
>
> On Sep 27, 2007, at 8:14 AM, Martin Losch wrote:
>
>> Hi,
>>
> ...
>
>> The bottleneck (from a vectorization point of view) is in  
>> exf_bulkformulae (and exf_bulk_largeyeager04, I expect), because  
>> of the iteration as the innermost loop. Jens-Olaf's solution is  
>> unsatifactory: he noticed that niter_bulk = 2 always (per  
>> parameter definition) and just copied the loop body twice, thus  
>> removing the iteratioin loop. I am still looking for a directive  
>> that can do that automatically, since the loop range is know at  
>> compile time. But I am afraid that the only real solution would be  
>> "loop pushing": move the iteration out of the ij-loops. This means  
>> redefining a few fields (the ones that are updated in the  
>> iteration, which are they?) as 2D.
>
> First, quick question:
> Do you know whether your compiler can inline function calls
> to improve vectorization?
> I've removed the three exf_bulk functions about a month ago
> to help with vecorization, but would be inclined to put some back in
> (to be able to switch between different drag coeff. schemes).
> Good compilers would just inline them.
>
> A second thing to check is the
>           if ( atemp(i,j,bi,bj) .ne. 0. _d 0 ) then
> statement which likely hinders good vectorization.
>
>> I would like to do this, but what's the best way: have two  
>> versions of exf_bulkformlulae, one as it is and one for vector  
>> machines? Or can we live with one version that is vectorizable at  
>> the cost of having a few extra 2d fields?
>
> A few that are currently non-fields:
> tau
> tstar
> ustar
> qstar
>
> -p.
>
>
>
>>
>> Martin
>>
>>
>>
>> _______________________________________________
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>
> ---
> Patrick Heimbach | heimbach at mit.edu | http://www.mit.edu/~heimbach
> MIT | EAPS 54-1518 | 77 Massachusetts Ave | Cambridge MA 02139 USA
> FON +1-617-253-5259 | FAX +1-617-253-4464 | SKYPE patrick.heimbach
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