7.2.1 What Is False Sharing?

Most high performance processors, such as UltraSPARC processors, insert a cache buffer between slow memory and the high speed registers of the CPU. Accessing a memory location causes a slice of actual memory (a cache line) containing the memory location requested to be copied into the cache. Subsequent references to the same memory location or those around it can probably be satisfied out of the cache until the system determines it is necessary to maintain the coherency between cache and memory.

However, simultaneous updates of individual elements in the same cache line coming from different processors invalidates entire cache lines, even though these updates are logically independent of each other. Each update of an individual element of a cache line marks the line as invalid. Other processors accessing a different element in the same line see the line marked as invalid. They are forced to fetch a more recent copy of the line from memory or elsewhere, even though the element accessed has not been modified. This is because cache coherency is maintained on a cache-line basis, and not for individual elements. As a result there will be an increase in interconnect traffic and overhead. Also, while the cache-line update is in progress, access to the elements in the line is inhibited.

This situation is called false sharing. If this occurs frequently, performance and scalability of an OpenMP application will suffer significantly.

False sharing degrades performance when all of the following conditions occur.

• Shared data is modified by multiple processors.

• Multiple processors update data within the same cache line.

• This updating occurs very frequently (for example, in a tight loop).

Note that shared data that is read-only in a loop does not lead to false sharing.