There is one thread on this subject http://milkyway.cs.rpi.edu/milkyway/forum_thread.php?id=1534#36281 but not clear answer.

On my computer, one task is calculated in less than 5 minutes on my ATI HD4850, and about 6 hours on 1 core of my AMD Phenom ii x3 720 2.8ghz.

So I want to calculate only with gpu : 1 gpu taks and 0 cpu task

What is the good setting for that ?

In a new version, I suggest to add a limitation to % load of the gpu (to limit heat).[/url]

Go to your preferences here at Milkyway, then uncheck "use CPU",that should stop CPU tasks from running.

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Which line ?
It is not possible to set 'Use at most' to 0%.

With these settings, 1 core is used 100% :

Processor usage
Suspend work while computer is on battery power?
(matters only for portable computers) yes
Suspend work while computer is in use? no
Suspend GPU work while computer is in use?
Enforced by version 6.7+ yes
'In use' means mouse/keyboard activity in last 3 minutes
Suspend work if no mouse/keyboard activity in last
(Needed to enter low-power mode on some computers) --- minutes
Do work only between the hours of (no restriction)
Leave applications in memory while suspended?
(suspended applications will consume swap space if 'yes') no
Switch between applications every
(recommended: 60 minutes) 60 minutes
On multiprocessors, use at most 16 processors
On multiprocessors, use at most
Enforced by version 6.1+ 50 % of the processors
Use at most
(Can be used to reduce CPU heat) 100 percent of CPU time

http://milkyway.cs.rpi.edu/milkyway/prefs.php?subset=project
This is the milkyway preferences page. Click the edit preferences and the second one down uncheck the use CPU box and make sure the Gpu box is checked for the GPU you have.
Good luck, Jack

Thank you, I was in the boinc global config page http://milkyway.cs.rpi.edu/milkyway/prefs.php?subset=global