As of Arnold 126.96.36.199, you can specify a negative number of threads, and Arnold will leave that many threads free. So, for example, if you want to leave two threads free for Maya while the IPR view is running, you would set the thread count to -2.
On a machine with 4 cores and 8 logical cores (aka threads):
| running on StephenBlair-PC, pid=16888 | 1 x Intel(R) Xeon(R) CPU E3-1240 V2 @ 3.40GHz (4 cores, 8 logical) with 16338MB | Windows 7 Professional Service Pack 1 (version 6.1, build 7601)
You would then see that Arnold uses only 6 logical cores:
| starting 6 bucket workers of size 64x64 ...
The advantage of the negative thread count is that you don’t have to know how many logical cores your machine has; you just need to know how many you want to keep free.