silx.opencl: Avoid executing OpenCL kernels at import time

[t20100]

First raised in #4077 (comment) :

then there is most likely a design issue with the silx.opencl.common.py module and more specifically about the OpenCL class.
During import plenty of code will be executed. And this is what create a seg fault during import. Even if no-opencl is used when importing silx.opencl or other module linked.

Also, for advanced usage, 1it would be nice to be able to set the OpenCL platforms+devices to use to spare the probing of all platforms

[pierrepaleo]

+1

I really don't like this probing at import which silently creates contexts all over the place. I've had numerous issues in the past with it. The only advantage is that the "best" device is automatically picked (though "best" can be incorrect in certain cases).

Perhaps this probing could be made optional with an environment variable, eg. SILX_OPENCL="auto".

[t20100]

I would be in favor of a Python API rather than an env. var. to control this.

I'd keep the automatic selection as the default behavior (since it is a good default and it would otherwise break the compatibility).

[kif]

I am on it ... several path have been envisaged to mitigate this effect:

• do not instanciate the OpenCL in the module (lasy loading)
• make the OpenCL class a borg (easy)
• lasy test the devices only when any of the atomic feature is actually needed (not difficult).

Only the first one is really tricky...

[t20100]

BTW, there is silx.config which is used to custom some behaviors (mostly for plots), but it may be used here. Best to stick to built-in types there though.