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Equation 19 · Part 2 · Locality Is the Whole Game: The Memory Hierarchy and What a Kernel Does Not Read

Symbol N^3

Qmin⁡  ≈  23 N3M.Q_{\min} \;\approx\; 2\sqrt{3}\,\frac{N^3}{\sqrt{M}} .
N3N^3

What this part means

N3N^3 occurs above the fraction bar. The numerator is divided by the entire denominator below it.

Its job in the formula

N3N^3 occurs above the fraction bar. The numerator is divided by the entire denominator below it.

The passage around this formula

Taking b as large as the constraint allows gives Qmin⁡  ≈  23 N3MQ_{\min} \;\approx\; 2\sqrt{3}\,\frac{N^3}{\sqrt{M}} . Two things in that expression matter more than the constant. First, traffic falls linearly in b : doubling the tile halves the bytes moved, which is why tiling is the single highest-leverage transformation in dense linear algebra. Second, and less often noticed, traffic falls only as M−1/2M^{-1/2} . Quadrupling the fast store buys a factor of two in traffic, not a factor of four. That square root is the reason architects cannot simply spend their way out of the problem with more SRAM, and it is worth holding on to when reading any claim that a larger cache will fix a bandwidth-bound kernel.

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Learn the underlying idea

An exponent tells how a base is used in multiplication. In x³, x is the base and 3 is the exponent: x³ = x × x × x.

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See this notation across published equations →

Sources cited in the article section

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