What is the difference in logic and performance between LOCK XCHG and MOV+MFENCE? [duplicate]

Question

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Closed 3 years ago.

What is the difference in logic and performance between x86-instructions LOCK XCHG and MOV+MFENCE for doing a sequential-consistency store.

(We ignore the load result of the XCHG; compilers other than gcc use it for the store + memory barrier effect.)

Is it true, that for sequential consistency, during the execution of an atomic operation: LOCK XCHG locks only a single cache-line, and vice versa MOV+MFENCE locks whole cache-L3(LLC)?

Answer 1

The difference is in purpose of usage.

MFENCE (or SFENCE or LFENCE) is useful when we are locking a part of memory region accessible from two or more threads. When we atomically set the lock for this memory region we can after that use all non-atomic instruction, because there are faster. But we must call SFANCE (or MFENCE) one instruction before unlocking the memory region to ensure that locked memory is visible correctly to all other threads.

If we are changing only a single memory aligned variable, then we are using atomic instructions like LOCK XCHG so no lock of memory region is needed.