[Users] Enabling CUDA/OpenCL thorns by default in Simfactory?

[Ian Hinder]

On 27 May 2014, at 17:23, Erik Schnetter <schnetter at cct.lsu.edu> wrote:

> On Tue, May 27, 2014 at 10:51 AM, Frank Loeffler <knarf at cct.lsu.edu> wrote:
> On Tue, May 27, 2014 at 09:46:29AM -0500, Frank Loeffler wrote:
> > Wouldn't that mean that every checkout on such a machine fails, because
> > simfactory would unconditionally add these thorns to the thornlist,
> > regardless of whether they are actually present or not?
> 
> Replying to myself: Is this really what Simfactory does - or does it
> only "enable" these thorns when they are present in the thornlist, but
> commented out? If the latter is the case I think it should be fine,
> although it might still confuse people that don't even look at the
> simfactory configuration of the machine, and use a thornlist with such a
> thorn commented out, but not present. Could we somehow let these users
> give a hint why Cactus fails with 'thorn not found', when the original
> thornlist clearly commented it out?
> 
> As per our previous discussion, the standard ET thorn list will (and this is the current state)
> 1. Check out all CUDA and OpenCL thorns, on ALL machines
> 2. Disable all CUDA and OpenCL thorns when building Cactus, on ALL machines
> If you are using this standard thorn list directly, then this is the only reasonable way to go; other choices don't make sense.
> 
> Simfactory pre-processes thorn lists, in the same way it pre-processes option lists and parameter files, expanding variables. It is important to have such a pre-processing step, since it avoids having to do this manually. This is how Simfactory "fixes" things for various machines, it is an important part of how it works.
> 
> Blue Waters supports both CUDA and OpenCL. We now have two options:
> 1. Use the standard ET thorn list there, i.e. disabling CUDA and OpenCL by default
> 2. Automatically enable CUDA and OpenCL thorns
> 
> As others mentioned, it does not make sense to enable CUDA and OpenCL everywhere, since it would not be supported everywhere. Enabling the thorns but adding a mechanism that turns these thorns into dummy thorns that do nothing also doesn't make sense -- think of a BSSN thorn written in CUDA; turning it into a dummy thorn that does nothing on a non-CUDA machine would just mean that BSSN evolution doesn't happen there. There are machines that support CUDA and there are machines that don't, and if you want to use CUDA then you will need to get an error message when you try to use a machine that doesn't support CUDA.
> 
> So -- how do we want to support CUDA on Blue Waters?
> 
> 1. Ask people to manually change the thorn list, in effect asking them to maintain one thorn list per machine
> 2. Let Simfactory do this, since it already knows what works on what machine and what doesn't
> 
> I don't think there are any other choices.
> 
> In case you are wondering: What I propose won't break checking out thorns on any machine, and won't break the build on any machine, and won't require people to do special things for special machines. I regularly build on many machines, and I do so in an automated manner, and I don't maintain per-machine thorn lists or parameter files. Simfactory has an MDB that encodes all the machines' peculiarities, and the information there is sufficient to use all of our machines efficiently, and in an automated manner.
> 
> There is the argument of "this would surprise people". If we are worried about people being surprised that a CUDA thorn can't be activated on a non-CUDA machine, then maybe we need to raise our expectations a bit.

I have been "surprised" in the past when I saw a thorn commented out in the thornlist, and Cactus trying to build it because SimFactory has enabled it on a particular machine.  I had no idea what was going on, and couldn't believe that simfactory was doing this when I found that it was.  I think if something is commented out, it should be ignored.  

Maybe we could add a feature to the Cactus thornlist language for "included if available"?  e.g.

? Arrangement/Thorn

SimFactory could expand this, and Cactus would ignore it by default.  This is similar to the existing "!" comments which are used by GetComponents, and ignored by Cactus.  At least this way, it doesn't look like it has been commented out.

-- 
Ian Hinder
http://numrel.aei.mpg.de/people/hinder

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