Running multiple tasks concurrently per GPU - How to revert to just one task.

For the benefit of anyone else who has a similar problem and finds this 'solution', it really isn't necessary to do that.  There are better solutions.

There are two ways to set up for running more than one GPU task concurrently on a discrete GPU.  Both have advantages and disadvantages.  The important thing to understand is that they work in different ways and it's not a good idea to have a mixture of both unless you really do understand the properties of each and how to 'undo' a configuration if necessary.

First, the simplest method is to go to your account page and select your project preferences (account -> preferences -> project).  Scroll down to find the section on GPU Utilization Factor.  There are 3 different settings - BRP (Binary Radio Pulsar) apps (no longer available for discrete GPUs) FGRP (Fermi Gamma Ray Pulsar) apps (the current GPU app) and GW (Gravitational Wave) apps (tested recently but not yet available, possibly for quite a while).  Only change the factor for the type of app you wish to use.  As of now that would be FGRP.

The number you enter is the fraction of a GPU that a task will use.  To run 2 concurrent tasks, each needs 0.5 of a GPU.  Make sure of two things.  (1) You change a value that applies to the 'location' you have assigned your computer to (default, home, work, school) and (2) you 'save' any changes you make.  If you haven't set up to use locations, you won't need to worry about (1).

This setting cannot act immediately.  The ONLY way your computer will know about it is when it receives a new task with the new settings encoded.  Even if you click 'update' in BOINC Manager, your computer won't be told until it receives new work.  This will also happen if you want to reverse the change at a later stage.  Your computer won't immediately revert to previous settings until new work is received that contain those previous settings.  This 'time delay' for changed settings to apply is one of the disadvantages of the simple method.  Another is that there is no ability to also configure the fraction of a CPU to be budgeted for GPU support duties.  Please note that "budgeting" is definitely not the same as what fraction of a CPU core that a task will actually use.

The second method involves you setting up a suitable app_config.xml file as documented here.  There are two particular advantages.  The changes are handled locally and don't require contact with the project.  In fact they will override any changes subsequently made on the website.  Local changes are applied immediately by just clicking 'reread config files' in BOINC Manager.  The second advantage is that you can customise both the fraction of a GPU and the fraction of a CPU that are budgeted for supporting a GPU task.

When you set up an app_config.xml file, there are disadvantages you need to be aware of - quite apart from understanding the syntax of the file itself.  The contents of the file are installed inside the state file (client_state.xml) which is the major piece of infrastructure that allows BOINC to do its work.  When you change the config file locally, the changed values will also cause the values in the state file to be updated.  If you decide not to use a config file any more, the stuff in the state file doesn't get removed, even if you delete the config file.

To go back to the original default values, you need to edit the config file to contain those values and then 'reread config files'.  To stop having to edit this file when things change, you can do what the OP did, or you could reset the project, both of which are like using a sledge hammer to crack an egg.  It should also be possible to manually remove the insertions in the state file with the client stopped first.  Not recommended unless you really know what you are doing.

Because an app_config.xml file always overrides GPU utilization settings supplied by the project, the OP could have reverted to default settings with the example he showed.  The max_concurrent is not needed because it will never be used when the cpu_usage and gpu_usage are both set to 1.  There can only be a single GPU task with default settings.  Here is the file with proper formatting to make it easy to see what includes what.

<app_config>
    <app>
        <name>hsgamma_FGRPB1G</name>
        <gpu_versions>
            <gpu_usage>1</gpu_usage>
            <cpu_usage>1</cpu_usage>
        </gpu_versions>
    </app>
</app_config>

One question often asked about these files is, "Where do I find the correct <name> to use?"  The documentation linked above shows a couple of different ways to find that.

If the OP had installed the file he showed in the proper place and clicked the 'reread config files' option, the crunching of GPU tasks should have reverted to the default of 1 task per discrete GPU. If that didn't happen, there must have been something wrong with the file, perhaps in the wrong place.  When clicking 'reread config files', always look in the event log to confirm that the file was found and that no syntax errors were reported.