Storage Efficiency - 2.6 English

Ternary CAM Search v2.6 LogiCORE IP Product Guide (PG318)

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2.6 English

For basic TCAM operation, a certain number of RAM accesses are required. This is an initial cost independent of TCAM depth. Due to this, the storage efficiency for small TCAMs is lower. For larger TCAMs, the initial cost gets amortized by a larger number of entries and the storage efficiency increases. For very large TCAMs, the storage efficiency approaches 90% (320-bit key widths). For small TCAMs with low storage efficiency, it is beneficial to use narrow priority + response since this memory is otherwise unused. For example, if the TCAM has 256 entries, priority+response = 128 bits and the storage efficiency is 25%, (1-0.25) *256*128 =24KB will be unused. It might be better to store an indirection pointer as response, and use a second table where the actual final response is well packed.