Optimizes CUDA kernel block sizes for better occupancy

[LoserCheems]

Reduces block dimensions across multiple head sizes to improve GPU occupancy and memory efficiency. Updates block configurations for head dimensions 32, 64, 96, 128, 192, and 256 to achieve better CTA (Cooperative Thread Array) utilization per streaming multiprocessor.

Adds debug comments for occupancy analysis and updates memory usage calculations in comments to reflect the new configurations.

[Copilot]

Pull Request Overview

Optimizes CUDA kernel block dimensions across different head sizes to improve GPU occupancy and memory efficiency, while adding debug placeholders and updating related comments.

• Adjust block configurations for head dimensions 32, 64, 96, 128, 192, and 256.
• Insert commented debug printf and occupancy analysis snippets.
• Update memory usage comments to reflect new block sizes and shared memory calculations.

Comments suppressed due to low confidence (3)

csrc/src/flash_fwd_launch_template.h:161

• This historical segfault comment no longer aligns with the updated kBlockN logic. Consider removing or updating it to reflect the current block size thresholds.

    // TD [2023-08-28]: nvcc segfaults for headdim 96 with block size 64 x 256,

csrc/src/flash_fwd_launch_template.h:181

• This performance comment refers to a 64×128 block size, but the code now launches with 64×64. Please update the comment to match the current configuration.

            // Using block size (64 x 128) is 27% slower for seqlen=2k

csrc/src/flash_fwd_launch_template.h:229

• The comment suggests a 64×64 configuration for alignment, but the is_sm8x branch actually uses 64×32. Align the comment to reflect the code or vice versa.

            // Use block configuration (kBlockM = 64, kBlockN = 64) for better memory alignment

[Copilot]

[nitpick] Consider removing or wrapping this large block of commented-out debug code in a dedicated debug macro to reduce noise in the main dispatch path.

Suggested change

	// auto kernel = &flash_fwd_splitkv_kernel<Kernel_traits, Is_causal, false, true, Split, Append_KV>;
	// auto kernel = &flash_fwd_splitkv_kernel<Kernel_traits, Is_causal, false, IsEvenKConst>;
	// printf("Split = %d, Append_KV = %d, Is_causal = %d, IsEvenMNConst = %d, IsEvenKConst = %d, Is_softcap = %d\n", int(Split), int(Append_KV), int(Is_causal), int(IsEvenMNConst), int(IsEvenKConst), int(Is_softcap));
	DEBUG_PRINT("Split = %d, Append_KV = %d, Is_causal = %d, IsEvenMNConst = %d, IsEvenKConst = %d, Is_softcap = %d\n", int(Split), int(Append_KV), int(Is_causal), int(IsEvenMNConst), int(IsEvenKConst), int(Is_softcap));

[Copilot]

[nitpick] The threshold expression uses raw magic numbers. Extract these calculations into named constants or add a brief comment explaining the chosen factors for clarity.

Suggested change

	if (max_smem_per_block >= 2 * Headdim * (128 + 2 * 64) && max_smem_per_sm < 4 * Headdim * (64 + 2 * 64)) {
	// Constants for shared memory calculations
	constexpr static int SMEM_BASE = 128; // Base shared memory size
	constexpr static int SMEM_MULTIPLIER = 64; // Multiplier for shared memory size
	constexpr static int SMEM_FACTOR = 2; // Factor for additional shared memory
	if (max_smem_per_block >= SMEM_FACTOR * Headdim * (SMEM_BASE + SMEM_FACTOR * SMEM_MULTIPLIER) &&
	max_smem_per_sm < 4 * Headdim * (SMEM_MULTIPLIER + SMEM_FACTOR * SMEM_MULTIPLIER)) {