LWPs are slower and more expensive to create than user threads. Whenever an LWP is created a system call must first be made to create a corresponding kernel thread, causing a switch to kernel mode. These mode switches would typically involve copying parameters between kernel and user space, also the kernel may need to have extra steps to verify the parameters to check for invalid behaviour. A context switch between LWPs means that the LWP that is being pre-empted has to save its registers, then go into kernel mode for the kernel thread to save its registers, and the LWP that is being scheduled must restore the kernel and user registers separately also.
For this reason, some user level thread libraries allow multiple user threads to be implemented on top of LWPs. User threads can be created, destroyed, synchronized and switched between entirely in user space without system calls and switches into kernel mode. This provides a significant performance improvement in thread creation time and context switches. However, there are difficulties in implementing a user level thread scheduler that works well together with the kernel.
Read more about this topic: Light-weight Process
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