Recently, I got a new PC with a 6-core HT CPU, i.e. 12 cores altogether.
I am running 2 GPUGRID tasks (using my 2 high-end GPUs), 4 ATLAS@home tasks, and now I had planned to add 4 tasks for WCG - in order to have 10 tasks running altogether.
The ressource share for GPUGRID is at 1000 (so that two such tasks are running in any case), the ressource share for ATLAS presently is at 200, the ressource share for WCG presently is at even 300.
In the BOINC computing preferences, CPU usage is at 80%, which means 10 tasks (for 12 cores).
When I added the WCG project, 2 GPUGRID tasks and 4 ATLAS tasks were running (for other reasons, by a config_app.xml I had limited ATLAS to maximum 4 tasks), so I expected WCG to download and run 4 tasks. However, only 1 task is running. When I go to "projects" and "update", the BOINC event log says:

17/04/2016 09:27:36 | World Community Grid | update requested by user
17/04/2016 09:27:39 | World Community Grid | Sending scheduler request: Requested by user.
17/04/2016 09:27:39 | World Community Grid | Not requesting tasks: don't need (CPU: job cache full; NVIDIA GPU: )
17/04/2016 09:27:41 | World Community Grid | Scheduler request completed

why so ??? CPU job cache is definitely NOT full with 7 tasks running - as said, before I had 10 task running with CPU usage set to 80% of my 12-core CPU.

Any comments / help ? I would definitely like to have 4 WCG task running, not only 1.

So how many tasks did you have in cache?

I did warn you about <max_concurrent> and work fetch not always working well together.

good question: there were several ATLAS tasks in "Ready to start" queue. Once I removed them, WCG tasks were downloaded and started. So, I had 2 GPUGRID tasks running, 4 ATLAS, 4 WCG. That's what I intended to happen.

I then left for a while, and when I came back, beside the 2 GPUGRID tasks, 8 WCG tasks were running, and the ATLAS tasks were in "Waiting to run" queue. Very nice :-(
Although I had set the ressource share to equal values for these both projects.

My target is to run concurrently:

2 GPUGRID tasks (which so far has always worked well, since this is the only project with GPU use, plus ressource share set to 1000).
4 ATLAS tasks
4 WCG tasks

So, I decided to write the following app_config.xml, like I have it for ATLAS (which works well):
<app_config>
<app>
<name>wcgrid</name>
<max_concurrent>4</max_concurrent>
</app>
</app_config>

However, it does not work.
after loading the app into the WCG project folder, the BOINC event log says:

17/04/2016 17:51:09 | World Community Grid | Your app_config.xml file refers to an unknown application 'wcgrid'. Known applications: 'mcm1', 'ugm1', 'fahb'

That's what I was afraid of :-(

Any idea what an app_config.xml has to look like in order the limit of WCG tasks to 4 running concurrently?

One thing that seems clear is: the ressource share thing will not help much in this case, as it weighs other parameters also, like task finishing deadline, and what not.

17/04/2016 17:51:09 | World Community Grid | Your app_config.xml file refers to an unknown application 'wcgrid'. Known applications: 'mcm1', 'ugm1', 'fahb'

The message gives you a clue as to what the application name should look like. WCG has multiple applications all with their own name.

But if in doubt you can check in the client_state.xml file what the application name is. And remember that you can make one app_config.xml with lines for all the different application names if necessary.

Any idea what an app_config.xml has to look like in order the limit of WCG tasks to 4 running concurrently?

We have a user manual wiki at http://boinc.berkeley.edu/wiki/User_manual which answers quite a bit of things. In this case, look at http://boinc.berkeley.edu/wiki/Client_configuration#Application_configuration

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One thing that seems clear is: the ressource share thing will not help much in this case, as it weighs other parameters also, like task finishing deadline, and what not.

Yep, resource shares set a long term goal.