Intel-based Macs: Inexplicable high processor loads when Windows Sharing is turned on

We are investigating a new issue where users of Intel-based Macs report extremely high processor loads when the "Windows Sharing" function is turned on in the Sharing pane of System Preferences.

MacFixIt reader Randy Howard writes:

"In the last couple of days (not sure I know exactly when it started unfortunately) my MacBook Pro suddenly has some mystery code running the CPU loads very high, and heating the box up like a pancake griddle. It will literally get almost painful to touch the chassis just about the function keys at the hinge point a few minutes after bootup.

"CPU load, with nothing loaded after login is averaging around 40% on the first core and 70% on the second [...] Sorting processes by CPU load, nothing really shows up in the list above 1% or so, and they come and go as you let it run. I have 'All Processes' selected, and nothing comes anywhere near to explaining the almost 60% load on the machine.

"Using top from terminal gives almost identical numbers. Using a regular login or 'sudo top -o cpu' gives identical results.

"With CHUD installed, and turning off the second processor, you get 100% CPU load continuously. Battery life is needless to say almost totally absent.

"I tried creating a fresh test account, rebooting and logging into the test " account, same problem.

"Nothing shows where this mystery CPU load is coming from, and it happens whether on AC power or running off of battery. It happens independent of power management settings, even when set to 'better energy savings' the system is running itself as if some hidden background job is gone haywire.

"I found the culprit. Simply turning the "Windows Sharing" option on/off in sysprefs->Sharing is the culprit. Something is badly broken. I don't even have to have an active network connection (turn the airport off and bluetooth off completely, no ethernet cable), and I still get some process and/or threads (inconveniently invisible or in the kernel) spinning out of control. Along with that is a very steady 262KB/sec of disk traffic reported in Activity monitor until I turn it back off again. Once I disable it, the disk I/O goes to zero and CPU load drops down to about 7%."

"My PowerPC G5 powermac does not have anything like that show up with Windows Sharing turned on."

If you are experiencing a similar issue (try toggling Windows Sharing on and off and while Activity Monitor [located in Applications/Utilities) is open and check for dramatic swings in processor usage), please let us know.

Note that in-house we were not able to reproduce this issue on a 1.83 GHz MacBook Pro, with CPU usage rising only 2-3% with Windows Sharing turned on.

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