OpenCL in the Clouds

Buzz-words are cool; they are loosely defined and are actually formed by the many implementation that use the label. Like Web 2.0 which is cool javascript for the one and interaction for the other. Now we have cloud-computing, which is cluster-computing with “something extra”. More than a year ago clouds were in the data-centre, but now we even have “private clouds”. So how to incorporate GPGPU? A cluster with native nodes to run our OpenCL-code with pre-distributed data is pretty hard to maintain, so what are the other solutions?

Distributed computing

Folding@home now has support for OpenCL to add the power of non-NVIDIA GPUs. While in clusters the server commands the clients what they have to do, here the clients ask the server for jobs. Disadvantage is that the clients are written for a specific job and are not really flexible to take different kind of jobs. There are several solutions for this code-distribution-problem, but still the solution is not suitable for smaller problems and small clusters.

Clusters: MPI

The project SHOC (Scalable HeterOgeneous Computing) is a collection of benchmark programs testing the performance and stability of systems using computing devices with non-traditional architectures for general purpose computing, and the software used to program them. While it is only a benchmark, it can be of great use when designing a cluster. For the rest I only  found CUDA MPI-solutions, which are not ported to OpenCL yet.

Also check out Hoopoe, which is a cloud-computing service to run your OpenCL-kernels in their cloud. It seems to be more limited to .NET and have better support for CUDA, but it is a start. In Europe there is a start-up offering a rent-model for OpenCL-computation-time; please contact us if you want to get in contact with them.

Clusters: OpenMP

MOSIX has added a “Many GPU Package” to their cluster management system, so it now allows applications to transparently use cluster-wide OpenCL devices. When “choosing devices” not only the local GPU pops up, but also all GPUs in the cluster.
It works disk-less, in the way no files are copied to the computation-clients and all stays in-memory. Disk-less computations have an advantage when cloud-computer are not fully trusted. Take note that on most cloud-computers the devices need to be virtualised (see next part).

Below is its layered model, VCL being the “Virtual OpenCL Layer”.

They have chosen to base it on OpenMP; while the kernels don’t need to be altered, some OpenMP-code needs to be added. They are very happy to tell it takes much less code to use openMP instead of MPI.

You see a speed-up between 2.19 and 3.29 on 4 nodes is possible. We see comparable cluster-speed-ups in an old cluster-study. The actual speed-up on clusters depends mostly on the amount of data that needs to be transferred.

The project references to a project called remote CUDA, which only works with NVIDIA-GPUs.

Device Virtualisation

Currently there is no good device virtualisation for OpenCL. The gVirtuS-project currently only supports CUDA, but they claim it is easily rewritten to OpenCL. Code needs to be downloaded with a Mercurius-client (comparable to GIT and in repositories of most Linux-distributions):
> hg clone http://osl.uniparthenope.it/hg/projects/gvirtus/gvirtus gvirtus
Or download it here (dated 7-Oct-2010).

Let me know when you ported it to OpenCL! Actually gVirtuS does not do the whole trick since you need to divide the host-devices between the different guest-OSes, but luckily there is an extension which provides sharing of devices, called fission. More about this later.

We can all agree there still needs to be done a lot in this area of virtualised devices to get OpenCL in the cloud. If you can’t wait, you can theoretically use MOSIX locally.

Afterword

A cloud is the best buzz-word to market a scalable solution to overcome limitations of internet connected personal devices. I personally think the biggest growth will be in personal clouds, so companies will have their own in-house cloud-server (read: clusters); people just want to have a feeling of control, comparable with preference of a daily traffic jam above public transport. But nevertheless shared clouds have potential if it comes to computation-intensive jobs which do not need to be done all year round.

The projects presented here are a start to have OpenCL-power at a larger scale for more demanding cases. Since we can have more power at our fingertips with one desktop-pc stuffed with high-end video-cards than a 4-year-old supercomputer-cluster, there is still time

Please send your comment if I missed a project or method.