How do I flag a process to avoid hyperthreading (SMT)?

Suppose I have an 18 core CPU with SMT enabled, and there are 18 long runnning processes, all using ~100% of a thread. If I start a 19th process, the kernel will schedule it to run on one of the cores, as a hypersibling of one of the 18 other processes. If that 19th process is something I want to have complete faster without interrupting the first 18, I would want to ask the kernel to "give that one an entire core", and push one of the rest to be a hypersibling. In other words, I want to flag the 19th process to never be scheduled on a thread which has a hypersibling in use by another process. Using nice -20 does not seem to work for this purpose.