MLA Cost Analysis & Regime Guide

Regime Selection

Crossover point: FlashAttention becomes faster than FlashMLA at approximately s=16-32 for DeepSeek-V3 parameters.

Cost Models (DeepSeek-V3-like: h=128, d=128, k=512, p=64)

FlashAttention

• FLOPs: 2bhst(2d + p) = 2bhst * 320
• Bytes: w bh(s+t)(2d + p) = w bh(s+t) * 320
• At s=1: AI ≈ 1 FLOP/byte (deeply memory-bound)
• At s=1024: AI ≈ 819 FLOP/byte (deeply compute-bound)

FlashMLA (latent-space attention via absorption)

• FLOPs: 2bhst(2k + p) = 2bhst * 1088
• Bytes: w * (bhs(2k+p) + bt(k+p))
• At s=1: AI ≈ 228 FLOP/byte (compute-bound even at decode)
• Problem: FLOPs grow linearly with s at k=512 instead of d=128 — 3.4x more FLOPs per token

MLAvar6+ (split latent + decompressed)

• FLOPs: 2bhstp + 4bhsnd + 4bhsok
• Bytes: w * (bhsp + bhsd + bhsk + bok + btp + bhnd + bhnd + bhsd + bhsk)
• Key: choosing n tunes operational intensity near roofline ridge point

Absorption Trick (FlashMLA)

Score:  Q @ K^T = Q @ (Z @ Wk)^T = (Q @ Wk^T) @ Z^T = Qz @ Z^T
Value:  softmax(A) @ V = softmax(A) @ (Z @ Wv) = (softmax(A) @ Z) @ Wv

Operate entirely in latent space (k-dim), with output decompression Olatent @ Wkvb2^T done by a separate kernel.

MLAvar6+ Design

Split KV cache into:

• o old tokens: latent Z (b, o, k) — compute-heavy, low bandwidth
• n new tokens: decompressed K,V (b, n, h, d) — bandwidth-heavy, low compute
• t = o + n

Interpolates: n=0 → FlashMLA-like, o=0 → FlashAttention-like.

Historical Baselines (RTX 5090, b=32, t=4096, bfloat16)

Note: These results are from RTX 5090 development. Reprofile on the current device before using as optimization targets. Ridge point, bandwidth, and compute ceilings differ across devices — see src/mla_var3/conf/devices.json.

MLAvar6+ V3 is the current best for speculative decoding (s=16), beating FlashMLA by 14%.

Tensor Shapes (DeepSeek-V3 defaults)

Tensor layouts:

• Q, Qc: [B, H, S, D] or [B, H, S, K]
• K, V: [B, T, H, D] (decompressed)
• Z: [B, T, K] (latent KV cache)
• QPE, KPE: [B, H, S, P] or [B, T, P]

Detailed Analysis

Full cost model derivations and roofline analysis: docs/cost-analysis.md