User Guide


Bus Lock

Metric Description

Intel processors provide a LOCK# signal that is asserted automatically during certain critical memory operations to lock the system bus or equivalent link. While this output signal is asserted, requests from other processors or bus agents for control of the bus are blocked. This metric measures the ratio of bus cycles, during which a LOCK# signal is asserted on the bus. The LOCK# signal is asserted when there is a locked memory access due to uncacheable memory, locked operation that spans two cache lines, and page-walk from an uncacheable page table.

Possible Issues

Bus locks have a very high performance penalty. It is highly recommended to avoid locked memory accesses to improve memory concurrency.


Examine the BUS_LOCK_CLOCKS.SELF event in the source/assembly view to determine where the LOCK# signals are asserted from. If they come from themselves, look at Back-end issues, such as memory latency or reissues. Account for skid.

Product and Performance Information


Performance varies by use, configuration and other factors. Learn more at