In OpenCL large memory objects, residing in the main memory of the host or the global memory at the accelerator/GPU, need special treatment. First reason is that these memories are relatively slow. Second reason is that the most times serial copy of objects between these two memories take time.

In this post I’d like to discuss all the flags for when creating memory objects, and what they can do to assist in this special treatment.

This is explained on this page of clCreateBuffer in the specifications, but I think it is not really clear. The function clCreateBuffer (and the alike functions for creating images, sub-buffers, etc) suggests that you create a special OpenCL-object to be given as argument to the kernel. What actually happens is that space is made available in main memory of the accelerator and optionally a link with host-memory is made.

The flags are divided over three groups: device access, host access and host pointer flags.

Read more …

You have run-time and compile-time of the C-code and of the OpenCL-code. It is very important to make clear when you talk about compile-time of the kernel as this can be confusing. Compile-time of the kernel is at run-time of the software after the compute-devices have been queried. The OpenCL-compiler can make better optimised code when you give as much information as possible. One of the methods is using Function Qualifiers. A function qualifier is notated as a kernel-attribute:

__kernel __attribute__((qualifier(qualification)))  void foo ( …. ) { …. }

There are three qualifiers described in OpenCL 1.x. Let’s walk through them one by one. You can also read them here in the official documentation, with more examples.

Read more …

Typos are a programmers worst nightmare, as they are bad for concentration. The code in your head is not the same as the code on the screen and therefore doesn’t have much to do with the actual problem solving. Code highlighting in the IDE helps, but better is to use the actual OpenCL compiler without running your whole software: an Online OpenCL Compiler. In short is just an OpenCL-program with a variable kernel as input, and thus uses the compilers of Intel, AMD, NVidia or whatever you have installed to try to compile the source. I have found two solutions, which both have to be built from source – so a C-compiler is needed.

• CLCC. It needs the boost-libraries, cmake and make to build. Works on Windows, OSX and Linux (needs possibly some fixes, see below).
• OnlineCLC. Needs waf to build. Seems to be Linux-only.

Read more …

In the series Basic Concepts I try to give an alternative description to what is said everywhere else. This time my eye fell on alternative convenience methods in two cases which were introduced there to be nice to devs with i.e. C/C++ and/or graphics backgrounds. But I see it explained too often from the convenience functions and giving the “preferred” functions as a sort of bonus which works for the cases the old functions don’t get it done. Below is the other way around and I hope it gives better understanding. I assume you have read another definition, so you see it from another view not for the first time.

Read more …

Time for some basic concepts of OpenCL. As I notice a growing number of visitors to this page, I notices I have actually not written much about coding and basics.

One of the first steps of an OpenCL program is selecting hosts and devices. If you program for a tablet, which has one chip and a screen, you don’t think of several devices. And if you log in on a server, your context is there is one host and that’s the one you logged into. If you have read my article about how to install all drivers on Ubuntu, you have gotten several clues. I added some tips&tricks, but not too many. If you know more stuff about this subject yourself, please share with others in the comments.

Read more …