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use enable_gqa of pytorch sdpa, allows us to delete some code, didnt realize it's available

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Andrej Karpathy 2025-10-21 18:07:33 +00:00
parent 94ee507054
commit a088b7a6ec
2 changed files with 5 additions and 21 deletions
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24
nanochat/gpt.py
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@ -48,19 +48,6 @@ def apply_rotary_emb(x, cos, sin):
out = out.to(x.dtype) # ensure input/output dtypes match
return out
def repeat_kv(x, n_rep):
"""torch.repeat_interleave(x, dim=1, repeats=n_rep)"""
if n_rep == 1:
return x
bs, n_kv_heads, slen, head_dim = x.shape
return (
x[:, :, None, :, :]
.expand(bs, n_kv_heads, n_rep, slen, head_dim)
.reshape(bs, n_kv_heads * n_rep, slen, head_dim)
)
class CausalSelfAttention(nn.Module):
def __init__(self, config, layer_idx):
super().__init__()
@ -96,19 +83,16 @@ class CausalSelfAttention(nn.Module):
Tq = q.size(2) # number of queries in this forward pass
Tk = k.size(2) # number of keys/values in total (in the cache + current forward pass)
# Apply MQA: replicate the key/value heads for each query head
nrep = self.n_head // self.n_kv_head
k, v = repeat_kv(k, nrep), repeat_kv(v, nrep)
# Attention: queries attend to keys/values autoregressively. A few cases to handle:
enable_gqa = self.n_head != self.n_kv_head # Group Query Attention (GQA): duplicate key/value heads to match query heads if desired
if kv_cache is None or Tq == Tk:
# During training (no KV cache), attend as usual with causal attention
# And even if there is KV cache, we can still use this simple version when Tq == Tk
y = F.scaled_dot_product_attention(q, k, v, is_causal=True)
y = F.scaled_dot_product_attention(q, k, v, is_causal=True, enable_gqa=enable_gqa)
elif Tq == 1:
# During inference but with a single query in this forward pass:
# The query has to attend to all the keys/values in the cache
y = F.scaled_dot_product_attention(q, k, v, is_causal=False)
y = F.scaled_dot_product_attention(q, k, v, is_causal=False, enable_gqa=enable_gqa)
else:
# During inference AND we have a chunk of queries in this forward pass:
# First, each query attends to all the cached keys/values (i.e. full prefix)
@ -118,7 +102,7 @@ class CausalSelfAttention(nn.Module):
attn_mask[:, :prefix_len] = True
# Then, causal attention within this chunk
attn_mask[:, prefix_len:] = torch.tril(torch.ones((Tq, Tq), dtype=torch.bool, device=q.device))
y = F.scaled_dot_product_attention(q, k, v, attn_mask=attn_mask)
y = F.scaled_dot_product_attention(q, k, v, attn_mask=attn_mask, enable_gqa=enable_gqa)
# Re-assemble the heads side by side and project back to residual stream
y = y.transpose(1, 2).contiguous().view(B, T, -1)

2
scripts/base_train.py
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@ -85,7 +85,7 @@ print0(f"Vocab size: {vocab_size:,}")
num_layers = depth
model_dim = depth * 64 # aspect ratio 64 (usually this is varied from 64 -> 128 as model size increases)
num_heads = max(1, (model_dim + 127) // 128) # head dim 128 (the division here is ceil div)
num_kv_heads = num_heads # 1:1 MQA ratio
num_kv_heads = num_heads # default is 1:1 GQA (Group Query Attention) ratio (i.e. GQA is disabled)
print0(f"num_layers: {num_layers}")
print0(f"model_dim: {model_dim}")
print0(f"num_heads: {num_heads}")