Is there any way to set affinity (which processors it runs on) for one project?

Use Process Lasso.

But why?

If you set a certain science to 2 cores with process lasso, say affinity to 6-7 and the BOINC scheduler decides to start a 3rd of that science, you might be seeing a little headscratcher. The 3rd copy will run concurrent on either core 6 or 7 and share half the time that with HT crunching is already half time. BOINC being ignorant, just letting run a maximum of 8, will use 5 of 6 in the 0-5 cores range i.e 1 core-thread idles. Maybe you have to see it to believe it... I did.

We still need, and age old wish list item, the ability to tell BOINC how many of X or Y are allowed to run concurrent. wish(ful) list thinking. WCG worked it's way around on one heavy app (CEP2) and allows to set how many can be assigned to a client at any one time. Then, if you run a zero cache, you almost always have the number specified running concurrent. At least if you specify 2 for a quad, you never have more than 2 running concurrent.

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