add alternating window size patterns for the GPT layers, following GPT-3. Experimented a bit and found the pattern SSSL to work well - 3 short, 1 long alternating. This is now the new default and the plots look quite a bit better on flops vs. bpb

dev/LOG.md

@@ -4,6 +4,22 @@ A running summary documenting some experiments and findings. Started ~Jan 7 2026
 
 ---
 
+## 2026-01-11: Sliding Window Attention
+
+Added configurable sliding window attention, inspired by GPT-3's alternating short/long pattern.
+
+**Pattern string configuration:**
+- New `--window_pattern` CLI arg and `GPTConfig.window_pattern` field
+- Pattern is tiled across layers (e.g., `SSSL` for 20 layers → `SSSLSSSLSSSLSSSLSSSL`)
+- Final layer always forced to L (full context) regardless of pattern
+- Short window = `sequence_len // 2`
+- Long window = `sequence_len` (full context)
+- All previous models so far have been simply `L` and checkpoint loading is modified accordingly to fill in this param for old models, see `_patch_missing_config_keys`
+
+Quick experiments showed `SSSL` (every 4th layer is long) works well - provides a good balance between compute savings and model quality. This is now the default.
+
+---
+
 ## 2026-01-11: Flash Attention 3 Integration
 
 Replaced PyTorch's `scaled_dot_product_attention` (FA2) with Flash Attention 3 for training and inference.

nanochat/checkpoint_manager.py

@@ -20,6 +20,12 @@ def log0(message):
     if int(os.environ.get('RANK', 0)) == 0:
         logger.info(message)
 
+def _patch_missing_config_keys(model_config_kwargs):
+    """Add default values for new config keys missing in old checkpoints."""
+    # Old models were trained with full context (no sliding window)
+    if "window_pattern" not in model_config_kwargs:
+        model_config_kwargs["window_pattern"] = "L"
+
 def _patch_missing_keys(model_data, model_config):
     """Add default values for new parameters that may be missing in old checkpoints."""
     n_layer = model_config.n_layer
@@ -84,6 +90,7 @@ def build_model(checkpoint_dir, step, device, phase):
     # Hack: fix torch compile issue, which prepends all keys with _orig_mod.
     model_data = {k.removeprefix("_orig_mod."): v for k, v in model_data.items()}
     model_config_kwargs = meta_data["model_config"]
+    _patch_missing_config_keys(model_config_kwargs)
     log0(f"Building model with config: {model_config_kwargs}")
     model_config = GPTConfig(**model_config_kwargs)
     _patch_missing_keys(model_data, model_config)

nanochat/gpt.py

@@ -39,6 +39,10 @@ class GPTConfig:
     n_head: int = 6 # number of query heads
     n_kv_head: int = 6 # number of key/value heads (GQA)
     n_embd: int = 768
+    # Sliding window attention pattern string, tiled across layers. Final layer always L.
+    # Characters: L=long (full context), S=short (half context)
+    # Examples: "L"=all full context, "SL"=alternating, "SSL"=two short then one long
+    window_pattern: str = "L"
 
 
 def norm(x):
@@ -69,7 +73,7 @@ class CausalSelfAttention(nn.Module):
         self.c_v = nn.Linear(self.n_embd, self.n_kv_head * self.head_dim, bias=False)
         self.c_proj = nn.Linear(self.n_embd, self.n_embd, bias=False)
 
-    def forward(self, x, cos_sin, kv_cache):
+    def forward(self, x, cos_sin, window_size, kv_cache):
         B, T, C = x.size()
 
         # Project the input to get queries, keys, and values
@@ -85,9 +89,10 @@ class CausalSelfAttention(nn.Module):
 
         # Attention with Flash Attention 3
         # FA3 handles GQA automatically when n_kv_heads < n_heads
+        # window_size is (left, right) tuple: (N, 0) for causal, (-1, 0) for full context
         if kv_cache is None:
-            # Training: simple causal attention
+            # Training: causal attention with optional sliding window
-            y = flash_attn.flash_attn_func(q, k, v, causal=True)
+            y = flash_attn.flash_attn_func(q, k, v, causal=True, window_size=window_size)
         else:
             # Inference: use flash_attn_with_kvcache which handles cache management
             k_cache, v_cache = kv_cache.get_layer_cache(self.layer_idx)
@@ -96,6 +101,7 @@ class CausalSelfAttention(nn.Module):
                 k=k, v=v,
                 cache_seqlens=kv_cache.cache_seqlens,
                 causal=True,
+                window_size=window_size,
             )
             # Advance position after last layer processes
             if self.layer_idx == kv_cache.n_layers - 1:
@@ -126,8 +132,8 @@ class Block(nn.Module):
         self.attn = CausalSelfAttention(config, layer_idx)
         self.mlp = MLP(config)
 
-    def forward(self, x, cos_sin, kv_cache):
+    def forward(self, x, cos_sin, window_size, kv_cache):
-        x = x + self.attn(norm(x), cos_sin, kv_cache)
+        x = x + self.attn(norm(x), cos_sin, window_size, kv_cache)
         x = x + self.mlp(norm(x))
         return x
 
@@ -141,11 +147,14 @@ class GPT(nn.Module):
         """
         super().__init__()
         self.config = config
-        # For DDP, we want vocab_size divisible by world_size. Also, there are potential performance benefits, see:
+        # Compute per-layer window sizes for sliding window attention
+        # window_size is (left, right) tuple: (-1, 0) for full context, (N, 0) for sliding window
+        self.window_sizes = self._compute_window_sizes(config)
+        # Pad vocab for efficiency (DDP, tensor cores). This is just an optimization - outputs are cropped in forward().
         # https://huggingface.co/docs/transformers/main_classes/model#transformers.PreTrainedModel.resize_token_embeddings
         padded_vocab_size = ((config.vocab_size + pad_vocab_size_to - 1) // pad_vocab_size_to) * pad_vocab_size_to
         if padded_vocab_size != config.vocab_size:
-            print0(f"Padding vocab_size from {config.vocab_size} to {padded_vocab_size} to be divisible by {pad_vocab_size_to}")
+            print0(f"Padding vocab_size from {config.vocab_size} to {padded_vocab_size} for efficiency")
         self.transformer = nn.ModuleDict({
             "wte": nn.Embedding(padded_vocab_size, config.n_embd),
             "h": nn.ModuleList([Block(config, layer_idx) for layer_idx in range(config.n_layer)]),
@@ -228,6 +237,35 @@ class GPT(nn.Module):
         cos, sin = cos[None, :, None, :], sin[None, :, None, :] # add batch and head dims for later broadcasting
         return cos, sin
 
+    def _compute_window_sizes(self, config):
+        """
+        Compute per-layer window sizes for sliding window attention.
+
+        Returns list of (left, right) tuples for FA3's window_size parameter:
+        - left: how many tokens before current position to attend to (-1 = unlimited)
+        - right: how many tokens after current position to attend to (0 for causal)
+
+        Pattern string is tiled across layers. Final layer always gets L (full context).
+        Characters: L=long (full context), S=short (half context)
+        """
+        pattern = config.window_pattern.upper()
+        assert all(c in "SL" for c in pattern), f"Invalid window_pattern: {pattern}. Use only S and L."
+        # Map characters to window sizes
+        long_window = config.sequence_len
+        short_window = long_window // 2
+        char_to_window = {
+            "L": (long_window, 0),
+            "S": (short_window, 0),
+        }
+        # Tile pattern across layers
+        window_sizes = []
+        for layer_idx in range(config.n_layer):
+            char = pattern[layer_idx % len(pattern)]
+            window_sizes.append(char_to_window[char])
+        # Final layer always gets full context
+        window_sizes[-1] = (long_window, 0)
+        return window_sizes
+
     def get_device(self):
         return self.transformer.wte.weight.device
 
@@ -236,16 +274,24 @@ class GPT(nn.Module):
         Return the estimated FLOPs per token for the model (forward + backward).
         Each matmul weight parameter contributes 2 FLOPs (multiply *, accumulate +) in forward, and 2X that in backward => 2+4=6.
         Cleanest explanation of this: https://medium.com/@dzmitrybahdanau/the-flops-calculus-of-language-model-training-3b19c1f025e4
-        On top of that, the term 12 * l * h * q * t accounts for key @ query matmul flops inside attention.
+        On top of that, 12 * h * q * effective_seq_len accounts for key @ query matmul flops inside attention.
+        With sliding windows, effective_seq_len varies per layer (capped by window size).
         Ref: https://arxiv.org/abs/2204.02311 (PaLM paper).
         This is ~1% off from the exact formulas of Chinchilla paper, the difference is:
         - Chinchilla counts the embedding layer as flops (? weird, it's just a lookup => we ignore)
         - Chinchilla counts exp/sum/divide in attention softmax as flops (a little sus and very tiny => we ignore)
         """
         nparams = sum(p.numel() for p in self.parameters())
-        nparams_embedding = self.transformer.wte.weight.numel()
+        # Exclude non-matmul params: embeddings and per-layer scalars
-        l, h, q, t = self.config.n_layer, self.config.n_head, self.config.n_embd // self.config.n_head, self.config.sequence_len
+        nparams_exclude = self.transformer.wte.weight.numel() + self.resid_lambdas.numel() + self.x0_lambdas.numel()
-        num_flops_per_token = 6 * (nparams - nparams_embedding) + 12 * l * h * q * t
+        h, q, t = self.config.n_head, self.config.n_embd // self.config.n_head, self.config.sequence_len
+        # Sum attention FLOPs per layer, accounting for sliding window
+        attn_flops = 0
+        for window_size in self.window_sizes:
+            window = window_size[0]  # (left, right) tuple, we use left
+            effective_seq = t if window < 0 else min(window, t)
+            attn_flops += 12 * h * q * effective_seq
+        num_flops_per_token = 6 * (nparams - nparams_exclude) + attn_flops
         return num_flops_per_token
 
     def num_scaling_params(self):
@@ -311,7 +357,7 @@ class GPT(nn.Module):
         x0 = x  # save initial normalized embedding for x0 residual
         for i, block in enumerate(self.transformer.h):
             x = self.resid_lambdas[i] * x + self.x0_lambdas[i] * x0
-            x = block(x, cos_sin, kv_cache)
+            x = block(x, cos_sin, self.window_sizes[i], kv_cache)
         x = norm(x)
 
         # Forward the lm_head (compute logits)

scripts/base_train.py

@@ -42,6 +42,7 @@ parser.add_argument("--depth", type=int, default=20, help="depth of the Transfor
 parser.add_argument("--aspect_ratio", type=int, default=64, help="model_dim = depth * aspect_ratio")
 parser.add_argument("--head_dim", type=int, default=128, help="target head dimension for attention")
 parser.add_argument("--max_seq_len", type=int, default=2048, help="max context length")
+parser.add_argument("--window_pattern", type=str, default="L", help="sliding window pattern tiled across layers: L=full, S=half context (e.g. 'SSL')")
 # Training horizon (only one used, in order of precedence)
 parser.add_argument("--num_iterations", type=int, default=-1, help="explicit number of optimization steps (-1 = disable)")
 parser.add_argument("--target_flops", type=float, default=-1.0, help="calculate num_iterations to reach target_flops (-1 = disable)")
@@ -139,7 +140,7 @@ if args.depth != 12:
 # Initialize the Model
 
 # Create a new model with random weights
-model_config_kwargs = dict(sequence_len=args.max_seq_len, vocab_size=vocab_size, n_layer=num_layers, n_head=num_heads, n_kv_head=num_kv_heads, n_embd=model_dim)
+model_config_kwargs = dict(sequence_len=args.max_seq_len, vocab_size=vocab_size, n_layer=num_layers, n_head=num_heads, n_kv_head=num_kv_heads, n_embd=model_dim, window_pattern=args.window_pattern)
 with torch.device("meta"):
     # All tensors are created as meta tensors (they have shape/dtype but no data)
     model_config = GPTConfig(**model_config_kwargs)