L2 Hit Bound

Metric Description

The L2 is the last and longest-latency level in the memory hierarchy before the main memory (DRAM) or MCDRAM. While L2 hits are serviced much more quickly than hits in DRAM or MCDRAM, they can still incur a significant performance penalty. This metric also includes coherence penalties for shared data. The L2 Hit Bound metric shows a ratio of cycles spent handling L2 hits to all cycles. The cycles spent handling L2 hits are calculated as L2 CACHE HIT COST * L2 CACHE HIT COUNT where L2 CACHE HIT COST is a constant measured as typical L2 access latency in cycles.

Possible Issues

A significant proportion of cycles is being spent on data fetches that miss the L1 but hit the L2. This metric includes coherence penalties for shared data.


1. If contested accesses or data sharing are indicated as likely issues, address them first. Otherwise, consider the performance tuning applicable to an L2-missing workload: reduce the data working set size, improve data access locality, consider blocking or partitioning your working set so that it fits into the L1, or better exploit hardware prefetchers.

2. Consider using software prefetchers, but note that they can interfere with normal loads, potentially increasing latency, as well as increase pressure on the memory system.

For more complete information about compiler optimizations, see our Optimization Notice.
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