I recently turned hyperthreading off on a dual-processor Nehalem workstation. BOINC 6.10.18 restarted, claimed in the message log that it found 8 XEON processors, but is still running 16 project WU's (+1 GPU) simultaneously. As these complete it isn't dropping the number of simultaneously-executing WU's: it just starts up a new WU as if it still had 16 logical processors.

Is there some configuration item that needs to be manually changed?

BOINC will use, through the preference -use at maximum 100% of the multiprocessors- the maximum amount of CPUs that the operating system says there are. So how many does your OS show you there are?

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XP shows 8 processors in Device Manager.

BOINC says in the message log:

14-Mar-2010 21:54:27 		Processor: 8 GenuineIntel Intel(R) Xeon(R) CPU           E5540  @ 2.53GHz [x86 Family 6 Model 26 Stepping 5]

This is bizarre to say the least. Rebooting the machine a SECOND time solved this problem. BOINC still says 8 processors found but now it's only running 8 (+1 GPU) tasks instead of 16.

The only difference from the last reboot was that I manually shut down BOINC manager/client beforehand. Maybe something to investigate in more depth next time I have to enable hyperthreading on that box.

To revisit this issue, the same thing happened in reverse when hyperthreading was re-enabled. BOINC message log shows it saw 16 logical processors yet dispatched only 8 CPU tasks. The apparent solution is to re-run the CPU Benchmark from the Advanced menu. That changes the CPU count.