The book I am reading suggests that in a many-to-one multi-threading model, the threads are unable to run in parallel in a multi-core system. I am thinking this is not the case because if we have many-to-one means that multiple user threads are mapped to one kernel thread. However, there might be many kernel threads running in a multi-core system . Am I right ? The threads sharing the same kernel thread might not be able to run in parallel among themselves but when a thread is accessing on kernel thread, another user thread, not sharing the same kernel can be accessing its own kernel thread. So there will still be some user threads that running in parallel in the system.