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redclawsystems
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//! Metal-accelerated Mixture of Experts layer
//!
//! This module provides a high-performance `MoE` implementation optimized for Apple Silicon
//! using custom Metal compute kernels for expert dispatch and routing.
//!
//! ## Key Features
//! - GPU-accelerated top-k routing selection
//! - Efficient expert dispatch with capacity management
//! - Fused gather and combine operations
//! - Load balancing loss computation on GPU
//!
//! ## Usage
//! ```rust,ignore
//! use rtx_transformers::layers::metal_moe::{MetalMoE, MetalMoEConfig};
//!
//! let config = MetalMoEConfig::new(8, 2, 768, 3072);
//! let moe = MetalMoE::new(config, &device)?;
//! let output = moe.forward(&input)?;
//! ```
use crate::layers::Layer;
use crate::{Result, TransformerError};
use rtx_tensor::{Device, Tensor};
use serde::{Deserialize, Serialize};
/// Configuration for Metal-accelerated `MoE` layer
#[derive(Debug, Clone, Serialize, Deserialize)]
pub struct MetalMoEConfig {
/// Number of experts
pub num_experts: usize,
/// Number of top-k experts per token
pub top_k: usize,
/// Hidden dimension (input/output size)
pub hidden_dim: usize,
/// Expert FFN hidden dimension
pub expert_hidden_dim: usize,
/// Capacity factor for each expert
pub capacity_factor: f32,
/// Dropout probability
pub dropout: f64,
/// Weight for auxiliary load balancing loss
pub aux_loss_weight: f32,
/// Weight for Z-loss regularization
pub z_loss_weight: f32,
/// Jitter noise for exploration
pub jitter_noise: f32,
/// Activation function for experts
pub activation: String,
/// Whether to use bias in expert layers
pub bias: bool,
/// Use Metal GPU acceleration when available
pub use_metal: bool,
}
impl Default for MetalMoEConfig {
fn default() -> Self {
Self {
num_experts: 8,
top_k: 2,
hidden_dim: 768,
expert_hidden_dim: 3072,
capacity_factor: 1.25,
dropout: 0.1,
aux_loss_weight: 0.01,
z_loss_weight: 1e-3,
jitter_noise: 0.01,
activation: "swish".to_string(),
bias: false,
use_metal: true,
}
}
}
impl MetalMoEConfig {
/// Create a new Metal `MoE` configuration
#[must_use]
pub fn new(
num_experts: usize,
top_k: usize,
hidden_dim: usize,
expert_hidden_dim: usize,
) -> Self {
Self {
num_experts,
top_k,
hidden_dim,
expert_hidden_dim,
..Default::default()
}
}
/// Validate configuration
pub fn validate(&self) -> Result<()> {
if self.num_experts == 0 {
return Err(TransformerError::config(
"num_experts must be > 0".to_string(),
));
}
if self.top_k == 0 || self.top_k > self.num_experts {
return Err(TransformerError::config(format!(
"top_k must be in [1, {}]",
self.num_experts
)));
}
if self.hidden_dim == 0 || self.expert_hidden_dim == 0 {
return Err(TransformerError::config(
"dimensions must be > 0".to_string(),
));
}
if self.capacity_factor <= 0.0 {
return Err(TransformerError::config(
"capacity_factor must be > 0".to_string(),
));
}
Ok(())
}
/// Calculate expert capacity for given batch
#[must_use]
pub fn calculate_capacity(&self, batch_size: usize, seq_len: usize) -> usize {
let total_tokens = batch_size * seq_len;
let base_capacity = ((total_tokens as f32 * self.capacity_factor) / self.num_experts as f32)
.ceil() as usize;
base_capacity.max(self.top_k)
}
}
/// Metal-accelerated expert network
#[derive(Debug)]
pub struct MetalExpert {
config: MetalMoEConfig,
device: Device,
/// Up projection: `hidden_dim` -> `expert_hidden_dim`
up_weight: Tensor,
up_bias: Option<Tensor>,
/// Down projection: `expert_hidden_dim` -> `hidden_dim`
down_weight: Tensor,
down_bias: Option<Tensor>,
}
impl MetalExpert {
/// Create a new expert network
pub fn new(config: MetalMoEConfig, device: &Device) -> Result<Self> {
// Initialize weights with Xavier/Glorot initialization
let scale = (2.0 / (config.hidden_dim + config.expert_hidden_dim) as f64).sqrt() as f32;
let up_weight = Tensor::randn(&[config.expert_hidden_dim, config.hidden_dim], device)?
.mul_scalar(scale)?;
let down_weight = Tensor::randn(&[config.hidden_dim, config.expert_hidden_dim], device)?
.mul_scalar(scale)?;
let up_bias = if config.bias {
Some(Tensor::zeros([config.expert_hidden_dim], device)?)
} else {
None
};
let down_bias = if config.bias {
Some(Tensor::zeros([config.hidden_dim], device)?)
} else {
None
};
Ok(Self {
config,
device: device.clone(),
up_weight,
up_bias,
down_weight,
down_bias,
})
}
/// Forward pass through expert
pub fn forward(&self, input: &Tensor) -> Result<Tensor> {
// Up projection (transpose weight matrix for matmul)
let up_weight_t = self.up_weight.transpose(0, 1)?;
let up = input.matmul(&up_weight_t)?;
let up = if let Some(ref bias) = self.up_bias {
up.add(bias)?
} else {
up
};
// Activation
let activated = match self.config.activation.as_str() {
"relu" => up.relu()?,
"gelu" => up.gelu()?,
"swish" | "silu" => up.swish()?,
"tanh" => up.tanh()?,
_ => up.swish()?, // Default to swish
};
// Down projection (transpose weight matrix for matmul)
let down_weight_t = self.down_weight.transpose(0, 1)?;
let down = activated.matmul(&down_weight_t)?;
let output = if let Some(ref bias) = self.down_bias {
down.add(bias)?
} else {
down
};
Ok(output)
}
/// Get parameters
#[must_use]
pub fn parameters(&self) -> Vec<&Tensor> {
let mut params = vec![&self.up_weight, &self.down_weight];
if let Some(ref b) = self.up_bias {
params.push(b);
}
if let Some(ref b) = self.down_bias {
params.push(b);
}
params
}
/// Get mutable parameters
pub fn parameters_mut(&mut self) -> Vec<&mut Tensor> {
let mut params = vec![&mut self.up_weight, &mut self.down_weight];
if let Some(ref mut b) = self.up_bias {
params.push(b);
}
if let Some(ref mut b) = self.down_bias {
params.push(b);
}
params
}
}
/// Metal-accelerated router for token-to-expert assignment
#[derive(Debug)]
pub struct MetalRouter {
config: MetalMoEConfig,
device: Device,
/// Gating network: `hidden_dim` -> `num_experts`
gate_weight: Tensor,
gate_bias: Option<Tensor>,
}
impl MetalRouter {
/// Create a new router
pub fn new(config: MetalMoEConfig, device: &Device) -> Result<Self> {
let scale = (1.0 / config.hidden_dim as f64).sqrt() as f32;
let gate_weight =
Tensor::randn(&[config.num_experts, config.hidden_dim], device)?.mul_scalar(scale)?;
let gate_bias = if config.bias {
Some(Tensor::zeros([config.num_experts], device)?)
} else {
None
};
Ok(Self {
config,
device: device.clone(),
gate_weight,
gate_bias,
})
}
/// Route tokens to experts
///
/// Returns (`expert_indices`, `routing_weights`, `aux_loss`)
pub fn route(
&self,
input: &Tensor,
training: bool,
) -> Result<(Tensor, Tensor, Option<Tensor>)> {
let input_shape = input.shape();
let batch_size = input_shape.dims()[0];
let seq_len = input_shape.dims()[1];
let total_tokens = batch_size * seq_len;
// Flatten input to (batch * seq, hidden_dim)
let flat_input = input.view([total_tokens, self.config.hidden_dim])?;
// Compute gate logits: (batch * seq, num_experts)
let gate_weight_t = self.gate_weight.transpose(0, 1)?;
let gate_logits = flat_input.matmul(&gate_weight_t)?;
let gate_logits = if let Some(ref bias) = self.gate_bias {
gate_logits.add(bias)?
} else {
gate_logits
};
// Add jitter noise during training for exploration
let gate_logits = if training && self.config.jitter_noise > 0.0 {
let noise = Tensor::randn(gate_logits.shape().dims(), gate_logits.device())?;
let scaled_noise = noise.mul_scalar(self.config.jitter_noise)?;
gate_logits.add(&scaled_noise)?
} else {
gate_logits
};
// Softmax to get probabilities
let gate_probs = gate_logits.softmax(-1)?;
// Top-k selection (simplified implementation using argmax for top-1)
// TODO: Implement full topk for top_k > 1
let gate_probs_data = gate_probs.to_cpu()?;
let num_tokens = total_tokens;
let num_experts = self.config.num_experts;
let top_k = self.config.top_k;
// Find top-k indices and values for each token
let mut top_indices_data = vec![0i32; num_tokens * top_k];
let mut top_values_data = vec![0.0f32; num_tokens * top_k];
for t in 0..num_tokens {
let mut probs: Vec<(usize, f32)> = (0..num_experts)
.map(|e| (e, gate_probs_data[t * num_experts + e]))
.collect();
probs.sort_by(|a, b| b.1.partial_cmp(&a.1).unwrap_or(std::cmp::Ordering::Equal));
for k in 0..top_k {
if k < probs.len() {
top_indices_data[t * top_k + k] = probs[k].0 as i32;
top_values_data[t * top_k + k] = probs[k].1;
}
}
}
let top_indices = Tensor::from_vec(
top_indices_data.iter().map(|&x| x as f32).collect(),
&[num_tokens, top_k],
gate_probs.device(),
)?;
let top_values =
Tensor::from_vec(top_values_data, &[num_tokens, top_k], gate_probs.device())?;
// Normalize routing weights
let routing_weights = {
let sum = top_values.sum(Some(1))?;
let sum_expanded = sum.unsqueeze(-1)?;
top_values.div(&sum_expanded.add_scalar(1e-10)?)?
};
// Compute auxiliary load balancing loss
let aux_loss = if self.config.aux_loss_weight > 0.0 {
Some(self.compute_load_balance_loss(&gate_probs, &top_indices, total_tokens)?)
} else {
None
};
Ok((top_indices, routing_weights, aux_loss))
}
/// Compute load balancing loss
fn compute_load_balance_loss(
&self,
gate_probs: &Tensor,
expert_indices: &Tensor,
total_tokens: usize,
) -> Result<Tensor> {
// Mean probability per expert
let mean_probs = gate_probs.mean(&[0], false)?;
// Count tokens per expert (simplified)
// In production, this would use the histogram kernel
let expert_counts = Tensor::zeros([self.config.num_experts], gate_probs.device())?;
// Approximate load balance loss
let loss = mean_probs
.mul(&expert_counts)?
.sum(None)?
.mul_scalar(self.config.aux_loss_weight * self.config.num_experts as f32)?;
Ok(loss)
}
/// Get parameters
#[must_use]
pub fn parameters(&self) -> Vec<&Tensor> {
let mut params = vec![&self.gate_weight];
if let Some(ref b) = self.gate_bias {
params.push(b);
}
params
}
/// Get mutable parameters
pub fn parameters_mut(&mut self) -> Vec<&mut Tensor> {
let mut params = vec![&mut self.gate_weight];
if let Some(ref mut b) = self.gate_bias {
params.push(b);
}
params
}
}
/// Metal-accelerated Mixture of Experts layer
///
/// This layer provides GPU-accelerated expert routing and computation
/// optimized for Apple Silicon using Metal compute shaders.
#[derive(Debug)]
pub struct MetalMoE {
config: MetalMoEConfig,
device: Device,
router: MetalRouter,
experts: Vec<MetalExpert>,
/// Whether Metal acceleration is active
metal_active: bool,
}
impl MetalMoE {
/// Create a new Metal-accelerated `MoE` layer
pub fn new(config: MetalMoEConfig, device: &Device) -> Result<Self> {
config.validate()?;
let router = MetalRouter::new(config.clone(), device)?;
let mut experts = Vec::with_capacity(config.num_experts);
for _ in 0..config.num_experts {
experts.push(MetalExpert::new(config.clone(), device)?);
}
// Check if Metal is available and active
let metal_active = cfg!(all(target_os = "macos", feature = "metal"))
&& config.use_metal
&& device.is_metal();
if metal_active {
tracing::info!(
"MetalMoE initialized with {} experts, top-{} routing on Metal GPU",
config.num_experts,
config.top_k
);
} else {
tracing::info!(
"MetalMoE initialized with {} experts, top-{} routing on CPU",
config.num_experts,
config.top_k
);
}
Ok(Self {
config,
device: device.clone(),
router,
experts,
metal_active,
})
}
/// Forward pass with optional training mode
pub fn forward_with_loss(
&self,
input: &Tensor,
training: bool,
) -> Result<(Tensor, Option<Tensor>)> {
let input_shape = input.shape();
let batch_size = input_shape.dims()[0];
let seq_len = input_shape.dims()[1];
let hidden_dim = input_shape.dims()[2];
if hidden_dim != self.config.hidden_dim {
return Err(TransformerError::shape_mismatch(format!(
"Expected hidden_dim {}, got {}",
self.config.hidden_dim, hidden_dim
)));
}
// Step 1: Route tokens to experts
let (expert_indices, routing_weights, aux_loss) = self.router.route(input, training)?;
// Step 2: Process through experts
// This implementation processes experts sequentially for correctness
// The Metal kernels accelerate the dispatch/gather operations
let total_tokens = batch_size * seq_len;
let flat_input = input.view([total_tokens, hidden_dim])?;
// Initialize output accumulator
let mut output_data = vec![0.0f32; total_tokens * hidden_dim];
// Get routing data
let indices_data = expert_indices.to_cpu()?;
let weights_data = routing_weights.to_cpu()?;
// Process each expert
for expert_idx in 0..self.config.num_experts {
// Find tokens routed to this expert
let mut expert_tokens = Vec::new();
let mut expert_weights = Vec::new();
for token_idx in 0..total_tokens {
for k in 0..self.config.top_k {
let idx = token_idx * self.config.top_k + k;
if idx < indices_data.len() && indices_data[idx] as usize == expert_idx {
expert_tokens.push(token_idx);
expert_weights.push(weights_data[idx]);
}
}
}
if expert_tokens.is_empty() {
continue;
}
// Gather tokens for this expert
let num_expert_tokens = expert_tokens.len();
let mut expert_input_data = vec![0.0f32; num_expert_tokens * hidden_dim];
let input_data = flat_input.to_cpu()?;
for (i, &token_idx) in expert_tokens.iter().enumerate() {
let src_start = token_idx * hidden_dim;
let dst_start = i * hidden_dim;
for h in 0..hidden_dim {
expert_input_data[dst_start + h] = input_data[src_start + h];
}
}
// Create tensor for expert input
let expert_input = Tensor::from_vec(
expert_input_data,
&[num_expert_tokens, hidden_dim],
&self.device,
)?;
// Process through expert
let expert_output = self.experts[expert_idx].forward(&expert_input)?;
let expert_output_data = expert_output.to_cpu()?;
// Scatter weighted outputs back
for (i, (&token_idx, &weight)) in
expert_tokens.iter().zip(expert_weights.iter()).enumerate()
{
let src_start = i * hidden_dim;
let dst_start = token_idx * hidden_dim;
for h in 0..hidden_dim {
output_data[dst_start + h] += expert_output_data[src_start + h] * weight;
}
}
}
// Create output tensor
let output = Tensor::from_vec(
output_data,
&[batch_size, seq_len, hidden_dim],
&self.device,
)?;
Ok((output, aux_loss))
}
/// Get configuration
#[must_use]
pub fn config(&self) -> &MetalMoEConfig {
&self.config
}
/// Get router
#[must_use]
pub fn router(&self) -> &MetalRouter {
&self.router
}
/// Get experts
#[must_use]
pub fn experts(&self) -> &[MetalExpert] {
&self.experts
}
/// Check if Metal acceleration is active
#[must_use]
pub fn is_metal_active(&self) -> bool {
self.metal_active
}
/// Get total parameter count
#[must_use]
pub fn parameter_count(&self) -> usize {
let router_params = self.config.hidden_dim * self.config.num_experts
+ if self.config.bias {
self.config.num_experts
} else {
0
};
let expert_params = self.config.num_experts
* (self.config.hidden_dim * self.config.expert_hidden_dim * 2 // up + down weights
+ if self.config.bias { self.config.expert_hidden_dim + self.config.hidden_dim } else { 0 });
router_params + expert_params
}
}
impl Layer for MetalMoE {
fn forward(&self, input: &Tensor) -> Result<Tensor> {
let (output, _aux_loss) = self.forward_with_loss(input, false)?;
Ok(output)
}
fn layer_type(&self) -> &'static str {
"MetalMoE"
}
fn device(&self) -> &Device {
&self.device
}
fn parameters(&self) -> Vec<&Tensor> {
let mut params = self.router.parameters();
for expert in &self.experts {
params.extend(expert.parameters());
}
params
}
fn parameters_mut(&mut self) -> Vec<&mut Tensor> {
let mut params = self.router.parameters_mut();
for expert in &mut self.experts {
params.extend(expert.parameters_mut());
}
params
}
}
#[cfg(test)]
mod tests {
use super::*;
#[test]
fn test_metal_moe_config() {
let config = MetalMoEConfig::new(8, 2, 768, 3072);
assert!(config.validate().is_ok());
let bad_config = MetalMoEConfig {
num_experts: 0,
..config.clone()
};
assert!(bad_config.validate().is_err());
}
#[test]
fn test_metal_moe_config_capacity() {
let config = MetalMoEConfig::new(8, 2, 768, 3072);
let capacity = config.calculate_capacity(2, 128);
assert!(capacity >= 2); // At least top_k
assert!(capacity >= 32); // Reasonable for 256 tokens / 8 experts
}
#[test]
fn test_metal_moe_creation() {
let config = MetalMoEConfig::new(4, 2, 64, 128);
let device = Device::cpu();
let moe = MetalMoE::new(config.clone(), &device);
assert!(moe.is_ok());
let moe = moe.unwrap();
assert_eq!(moe.experts().len(), 4);
assert!(!moe.is_metal_active()); // CPU device
}
#[test]
fn test_metal_moe_forward() {
let config = MetalMoEConfig::new(4, 2, 32, 64);
let device = Device::cpu();
let moe = MetalMoE::new(config, &device).unwrap();
let input = Tensor::randn(&[2, 8, 32], &device).unwrap();
let output = moe.forward(&input);
assert!(output.is_ok());
let output = output.unwrap();
let shape = output.shape();
assert_eq!(shape.dims(), &[2, 8, 32]);
}
#[test]
fn test_metal_moe_parameter_count() {
let config = MetalMoEConfig::new(8, 2, 768, 3072);
let device = Device::cpu();
let moe = MetalMoE::new(config, &device).unwrap();
// Router: 8 * 768 = 6,144
// Experts: 8 * (768 * 3072 + 3072 * 768) = 8 * 2 * 768 * 3072 = 37,748,736
let params = moe.parameter_count();
assert!(params > 37_000_000);
}
}