Maximum Zeal ~ Emphatic prose on indulged fascinations

OpenCL Cookbook: Running remote multi-gpu OpenCL computations over SSH on Ubuntu or Debian using AMD Catalyst drivers

Continuing on in the OpenCL cookbook series here I present a post not about code but about environmental setup further diversifying the scope of the cookbook. Here is a no-nonsense guide to running OpenCL programs via ssh remote login into linux. This is intended as a follow up to my previous guide to compiling OpenCL on linux so have a look at that if you haven’t done so already but this account should be fairly self contained nevertheless.

Important: Note that this is not a guide to setting OpenCL on linux in a headless fashion. It requires GUI linux, it requires X and it requires that X be running on screen. Although there are reports of people getting headless setups to work I haven’t yet had time to try these things out so I cannot comment on them.

Install Linux

I went for Ubuntu and Debian for their easy and well known package management. I installed them in adjacent partitions so that I could test my setup on both and compare them easily. Eventually I settled on Debian due to strongly disliking unity and the presence of several known issues with raid on boot in Ubuntu. Regarding desktop environments/window managers I installed an assortment of them because I wanted fully featured ones for development and lightweight ones for benchmarking. I also wanted to test OpenCL in a variety of environments. I installed GNome, KDE, Blackbox, Fluxbox, XFCE and E17 on each distro. It’s good to have options. :) Note that you should NOT install any drivers automatically through the ubuntu/debian package management tools!

Install AMD Catalyst 12.11 beta drivers and AMD APP SDK 2.7

Installing the above is as simple as downloading the linux tarballs, extracting them and running the executable script within each as root. I don’t bother building distro specific packages. Just install the damn things. They work. If you want a bit more elaboration on installing and setting these up look at my previous guide. The reason I’m suggesting the 12.11 beta drivers is because the 12.10 stable driver doesn’t work with X on ubuntu or debian whereas 12.11 beta does. Plus I haven’t noticed any instability with 12.11 beta at all. It seems to work just fine.

Use amdconfig to generate a new Xorg configuration using all adapters

Amdconfig (also aliased as aticonfig) is a powerful command line tool provided by the AMD catalyst driver that you can use for an assortment of tasks such as listing your adapters, querying clocks and temperatures of your gpus, setting overclocks and most importantly for our purposes generating a variety of Xorg configurations with different settings. Here we need to generate an X configuration that will enable all our adapters in Xorg.

sudo amdconfig --initial -f --adapter=all

This will back up your existing xorg.conf in /etc/X11/xorg.conf and create a new one in its place containing device and screen sections for all your gpus. Though if you’ve just installed ubuntu or debian you won’t have an xorg config to begin with anyway.

Reboot to start using the new AMD fglrx driver and your new X configuration

A reboot is required to unload the old open source driver and load fresh the new proprietary amd driver fglrx which cannot be done cleanly by simply using modprobe -r to remove old drivers and modprobe to load new drivers into the kernel and then restarting X. A reboot is recommended and essential. Once you’ve rebooted you’ll also start to use your new X configuration with all adapters enabled. When you see a login manager screen (gdm/xdm/kdm/slim) log into it.

Test your new local OpenCL environment

Start up a terminal. First check that amdconfig sees all your adapters. If you have multiple gpus installed you should see all of them with one of them (typically the first) selected as default.

dhruba@debian:~$ amdconfig --list-adapters
* 0. 05:00.0 AMD Radeon HD 7900 Series  
  1. 06:00.0 AMD Radeon HD 7900 Series  
  2. 09:00.0 AMD Radeon HD 7900 Series  
  3. 0a:00.0 AMD Radeon HD 7900 Series  
  4. 85:00.0 AMD Radeon HD 7900 Series  
  5. 86:00.0 AMD Radeon HD 7900 Series  

* - Default adapter

Next, check that all your devices are being picked up for computation by the OpenCL runtime.As you can see below on my workstation it shows 7 devices – 6 of them being gpus and 1 being the cpu. I’m using 3 7990s each of which are a dual 7970.

dhruba@debian:~$ clinfo | grep 'Number of devices'
Number of devices:				 7

If you see all your devices in the count above your environment should be ready to compile and run OpenCL programs. The next step is get it ready for remote ssh multi-gpu OpenCL computations.

Enable multiple GPUs in the OpenCL runtime

At this point you may find that although amdconfig lists all your adapters the OpenCL runtime only sees one gpu or sees fewer gpus than you have installed. You may also find that some window managers/desktop environments see all devices and others see only one. To enable multiple gpus you may need to do two things.

First you need to disable crossfire. You can do this either from within the catalyst control panel which you can start by running sudo amdcccle or you can use the powerful amdconfig tool to do it by running sudo amdconfig --crossfire=off. See my post about amdconfig for more detail on how to use this amazing tool.

Secondly you may need to pass a hint to the OpenCL runtime to tell it to use the current display for computation as follows.

export COMPUTE=:0

As an alternative to this you can also do the following.

unset DISPLAY

However I’d recommend the first as it is an override flag that does not require any change to the existing DISPLAY variable. Implementing the above two tweaks should enable multi-gpu support in your OpenCL programs. Check either by using clinfo | grep 'Number of devices' or by using a C program as in my previous article.

Install SSH and set up your server for remote computations

Install SSH on ubuntu/debian by running sudo apt-get install openssh-server. This should install the ssh server and start it automatically. Login to your OpenCL server from a remote client and run clinfo again to check number of devices. If this is showing all your gpus then you’re done. However chances are that it won’t yet show all your devices. It will show only your cpu and no gpus at all. The reason for this is that the remote client has not yet been granted access to the X runtime. You can grant access as follows.

dhruba@debian:~$ xhost +
access control disabled, clients can connect from any host

This disables access control altogether. However if you are concerned about security you can be more restrictive and enable it only for specific usernames, hostnames, ip addresses, networks or protocols. See man xhost for details. Basically you append the hostname, username or ip to the plus sign. Once you’ve done the above run clinfo again from your ssh session and now you should see all your gpus and your system is ready to remote OpenCL computations.

Automating setup of remote computation environment

In a true datacentre environment you don’t really want to physically login to X and set environment variables particularly because your server may get rebooted. What you really want is for it to be ready to run OpenCL via remote sessions on a fresh boot without any manual intervention. If you wish to do this there is a guide available on the amd forum for ubuntu specifically which I suppose could be adapted to Debian. I’ve tried this on ubuntu and it does work though it’s a bit hackish. I’ll leave this guide at your discretion.

AMD OpenCL Limitations

Note that this is not a headless setup. In this guide we installed a GUI linux, ran X, logged into X and kept X displayed on screen. All of these are essential. If you either do not run X or run X but switch away from it to one of the other virtual terminals you will not be able to run OpenCL either from the virtual terminals or remotely. If you try to run OpenCL without X running and showing on screen the execution will just hang indefinitely until you start X and display it on screen at which point that hanging computation will continue and complete.

This seems to be an inherent limitation of the AMD driver. AMD say they’re working on separating X from the OpenCL runtime but who knows when this will actually be done. There are quite a few reports of people succeeding at running truly headless OpenCL setups – without any peripherals attached – but I have not had time to research these methods yet so I cannot comment on them.

3 Responses to OpenCL Cookbook: Running remote multi-gpu OpenCL computations over SSH on Ubuntu or Debian using AMD Catalyst drivers

  1. Could you elaborate on “quite a few reports of people succeeding at running truly headless OpenCL setups”? Links please?

Leave a Reply