Thread context switch Vs. process context switch

Process context switching involves switching the memory address space. This includes memory addresses, mappings, page tables, and kernel resources—a relatively expensive operation. On some architectures, it even means flushing various processor caches that aren't sharable across address spaces. For example, x86 has to flush the TLB and some ARM processors have to flush the entirety of the L1 cache!

Thread switching is context switching from one thread to another in the same process (switching from thread to thread across processes is just process switching).Switching processor state (such as the program counter and register contents) is generally very efficient.

The main distinction between a thread switch and a process switch is that during a thread switch, the virtual memory space remains the same, while it does not during a process switch. Both types involve handing control over to the operating system kernel to perform the context switch. The process of switching in and out of the OS kernel along with the cost of switching out the registers is the largest fixed cost of performing a context switch.

A more fuzzy cost is that a context switch messes with the processors cacheing mechanisms. Basically, when you context switch, all of the memory addresses that the processor "remembers" in its cache effectively become useless. The one big distinction here is that when you change virtual memory spaces, the processor's Translation Lookaside Buffer (TLB) or equivalent gets flushed making memory accesses much more expensive for a while. This does not happen during a thread switch.