Cache pollution

Cache pollution describes situations where an executing computer program loads data into CPU cache unnecessarily, thus causing other needed data to be evicted from the cache into lower levels of the memory hierarchy, potentially all the way down to main memory, thus causing a performance hit.

Example

Consider the following illustration: T [0] = T [0] + 1; for i in 0..SIZEOF(CACHE) C [i] = C [i] + 1; T [0] = T [0] + C [SIZEOF(CACHE)-1] ;

(The assumptions here are that the cache is composed of only one level, it is unlocked, the replacement policy is pseudo-LRU, all data is cacheable, the set associativity of the cache is N (where N > 1), and at most one processor register is available to contain program values).

Right before the loop starts, T [0] will be fetched from memory into cache, its value updated. However, as the loop executes, because the number of data elements the loop references requires the whole cache to be filled to its capacity, the cache block containing T [0] has to be evicted. Thus, the next time the program requests T [0] to be updated, the cache misses, and the cache controller has to request the data bus to bring the corresponding cache block from main memory again.

In this case the cache is said to be "polluted," because it is obvious that just by changing the pattern of data accesses (e.g. positioning the first update of T [0] between the loop and the second update) the inefficiency can be eliminated.

Solutions

Other solutions to this problem involve the use of specialized hardware instructions such as "lvxl" provided by PowerPC AltiVec. This instruction loads a 128 bit wide value into a register and marks the corresponding cache block as "least recently used," i.e. as the prime candidate for eviction upon a need to evict a block from its cache set. To appropriately use that instruction in the context of the above example, the data elements referenced by the loop would have to be loaded using this instruction. When implemented in this manner, cache pollution would not take place, since the execution of such loop would not cause premature eviction of T [0] from cache. This would be avoided because, as the loop would progress, the addresses of the elements in C would map to the same cache way, leaving the actually older (but not marked as "least recently used") data intact on the other way(s). Only the oldest data (not pertinent for the example given) would be evicted from cache, which T [0] is not a member of, since its update occurs right before the loop's start.

Yet other possible solutions involve the operating system. For example, the pages in main memory that correspond to the C data array can be marked as "caching inhibited" or, in other words, non-cacheable. However, in the above program's example, such manipulations do not appear to improve execution performance, since the resulting run time overhead is overwhelmingly larger than any gain achievable by cache pollution avoidance (unless the memory region has been non-cacheable to begin with).

Often in real life the cache is composed of more than one level (called the "L1," "L2," etc.). Therefore, "cache pollution" is well-defined only for situations where the term "cache" is unambiguous. Otherwise, it is imperative to specify which level of cache is involved.

Increasing importance

The reason why cache pollution control has been increasing in importance, is because the penalties caused by the so-called "memory wall" keep on growing. Chip manufacturers continue devising new tricks to overcome the ever increasing relative memory-to-CPU latency. They do that by increasing cache sizes and by providing useful ways for software engineers to control the way data arrives and stays at the CPU. Cache pollution control is one of the numerous devices available to the (mainly embedded) programmer. However, other methods, most of which are proprietary and highly hardware and application specific, are used as well.

Sometimes, however, even the most evolved and involved software is either insufficient or the very effort of software optimization crosses cost tolerance thresholds or profitability ratios as explained by the law of diminishing returns. In those cases the ball shifts back to the court of hardware engineers, and the "trick game" starts again. The relatively recent surge of interest in the system-on-a-chip concept (such as the Cell processor) is fueled by these issues.