perf: reduce decode overhead in scheduler and executor

- Precompute page_table and decode_mask on KVCache once per step in PagePool.bind_tasks, instead of per-layer in CudaBackend/TorchNativeBackend
- Skip frequency penalty history tensor construction when all penalties are 0 in Executor.execute_decode
- Omit FrequencyPenaltyStrategy from sampling pipeline when penalty is 0
- Deduplicate get_active_tasks calls in scheduler loop (3 to 1), remove redundant sorted() on decode tasks
- Benchmark (L20, bf16, CUDA backend): B=1 9.48->9.40ms (+1%), B=4 10.73->9.89ms (+8.6%), B=8 10.77->10.13ms (+6.4%)

astrai/extension/attention_backend.py

@@ -272,8 +272,14 @@ class TorchNativeBackend(AttentionBackend):
             kv_cache.k_buffer[layer_id, kv_cache.out_cache_loc] = k
             kv_cache.v_buffer[layer_id, kv_cache.out_cache_loc] = v
 
-            max_len = kv_cache.seq_lens.max()
+            max_len = kv_cache.max_len
+            if kv_cache.page_table is not None:
+                indices = kv_cache.page_table
+            else:
                 indices = kv_cache.req_to_token[kv_cache.req_pool_indices, :max_len]
+            if kv_cache.decode_mask is not None:
+                pos_mask = kv_cache.decode_mask
+            else:
                 pos_mask = (
                     torch.arange(max_len, device=q.device)[None, :]
                     < kv_cache.seq_lens[:, None]
@@ -338,9 +344,11 @@ class CudaBackend(AttentionBackend):
         kv_cache.k_buffer[layer_id, kv_cache.out_cache_loc] = k
         kv_cache.v_buffer[layer_id, kv_cache.out_cache_loc] = v
 
-        seq_lens = kv_cache.seq_lens
         max_len = kv_cache.max_len
 
+        if kv_cache.page_table is not None:
+            page_table = kv_cache.page_table
+        else:
             page_table = kv_cache.req_to_token[kv_cache.req_pool_indices, :max_len]
 
         k_cache = kv_cache.k_buffer[layer_id].unsqueeze(1)
@@ -348,8 +356,13 @@ class CudaBackend(AttentionBackend):
 
         if q.size(0) == 1:
             mask = None
+        elif kv_cache.decode_mask is not None:
+            mask = kv_cache.decode_mask
         else:
-            mask = torch.arange(max_len, device=q.device)[None, :] < seq_lens[:, None]
+            mask = (
+                torch.arange(max_len, device=q.device)[None, :]
+                < kv_cache.seq_lens[:, None]
+            )
 
         out = attn_paged_decode(
             q,

astrai/inference/core/cache.py

@@ -203,6 +203,10 @@ class KVCache:
         seq_lens: [batch_size] — per-request total sequence lengths
         out_cache_loc: [batch, new_seq_len] or [batch, 1] — write indices
         max_len: max(seq_lens) as Python int — avoids GPU sync in decode
+        page_table: [batch, max_len] — precomputed gather indices for decode;
+            None for prefill or when not yet computed.
+        decode_mask: [batch, max_len] bool — precomputed position validity
+            mask for decode; None for prefill or single-batch decode.
     """
 
     k_buffer: Tensor
@@ -212,6 +216,8 @@ class KVCache:
     seq_lens: Tensor
     out_cache_loc: Tensor
     max_len: int = 0
+    page_table: Optional[Tensor] = None
+    decode_mask: Optional[Tensor] = None
 
 
 class PagePool:
@@ -428,11 +434,21 @@ class PagePool:
             out_cache_loc = self._req_pool.req_to_token[
                 req_pool_indices, start_pos:seq_len
             ]
+            page_table = None
+            decode_mask = None
         else:
             write_pos = seq_lens_t - 1
             out_cache_loc = self._req_pool.req_to_token[
                 req_pool_indices, write_pos
             ].unsqueeze(-1)
+            ml = max(seq_lens)
+            page_table = self._req_pool.req_to_token[req_pool_indices, :ml]
+            if len(task_ids) > 1:
+                decode_mask = (
+                    torch.arange(ml, device=device)[None, :] < seq_lens_t[:, None]
+                )
+            else:
+                decode_mask = None
 
         return KVCache(
             k_buffer=self._storage.k_buffer,
@@ -442,6 +458,8 @@ class PagePool:
             seq_lens=seq_lens_t,
             out_cache_loc=out_cache_loc,
             max_len=max(seq_lens),
+            page_table=page_table,
+            decode_mask=decode_mask,
         )
 
     # ---- internals ----

astrai/inference/core/executor.py

@@ -105,6 +105,8 @@ class Executor:
             [t.frequency_penalty for t in tasks], device=self.device
         )
 
+        has_freq = bool((freq_penalties != 0).any())
+        if has_freq:
             history_lists = []
             history_lens = []
             for t in tasks:
@@ -123,8 +125,13 @@ class Executor:
             )
             for i, h in enumerate(history_lists):
                 L = history_lens[i]
-            padded_ids[i, :L] = torch.as_tensor(h, dtype=torch.long, device=self.device)
+                padded_ids[i, :L] = torch.as_tensor(
+                    h, dtype=torch.long, device=self.device
+                )
                 padded_mask[i, :L] = True
+        else:
+            padded_ids = None
+            padded_mask = None
 
         with torch.inference_mode():
             outputs = self.model(

astrai/inference/core/scheduler.py

@@ -109,9 +109,11 @@ class InferenceScheduler:
                     self._task_mgr.wait_for_tasks(timeout=1.0)
                     continue
 
+                active = self._task_mgr.get_active_tasks()
+
                 to_prefill = [
                     t
-                    for t in self._task_mgr.get_active_tasks()
+                    for t in active
                     if t.output_tokens == 0
                     and cache.task_cached(t.task_id) < len(t.prompt_ids)
                 ]
@@ -137,10 +139,10 @@ class InferenceScheduler:
                                 t.task_id, t.prompt_ids, start_logical_page
                             )
 
-                decode_tasks = self._task_mgr.get_active_tasks()
+                decode_tasks = active
 
                 valid: List[Task] = []
-                for t in sorted(decode_tasks, key=lambda t: t.task_id):
+                for t in decode_tasks:
                     if cache.task_extend(t.task_id, t.next_pos):
                         valid.append(t)
                     else:

astrai/inference/sample.py

@@ -343,6 +343,10 @@ def sample(
     When **temperature** is exactly 0 (scalar or single-element tensor)
     the function short-circuits to ``argmax`` for deterministic decode.
 
+    When **frequency_penalty** is 0 (the common decode case), the entire
+    frequency penalty computation — including the O(batch * vocab) count
+    tensor allocation — is skipped.
+
     Args:
         logits: Raw logits ``[batch, vocab_size]``.
         frequency_penalty: Penalty per occurrence for repeated tokens
@@ -359,14 +363,39 @@ def sample(
         ``True`` — a ``(token_ids, chosen_logprobs)`` tuple where
         ``chosen_logprobs`` has shape ``[batch]``.
     """
-    return SamplingPipeline(
-        [
+    greedy = (
+        (
+            isinstance(temperature, Tensor)
+            and temperature.numel() == 1
+            and temperature.item() == 0
+        )
+        if isinstance(temperature, Tensor)
+        else temperature == 0
+    )
+
+    if greedy:
+        tokens = logits.argmax(dim=-1)
+        if not return_logprobs:
+            return tokens
+        log_probs = torch.log_softmax(logits.float(), dim=-1)
+        chosen = torch.gather(log_probs, -1, tokens.unsqueeze(-1)).squeeze(-1)
+        return tokens, chosen
+
+    has_freq = (
+        (isinstance(frequency_penalty, Tensor) and (frequency_penalty != 0).any())
+        if isinstance(frequency_penalty, Tensor)
+        else frequency_penalty != 0
+    )
+
+    strategies: List[BaseSamplingStrategy] = [
         TemperatureStrategy(temperature),
         TopKStrategy(top_k),
         TopPStrategy(top_p),
-            FrequencyPenaltyStrategy(frequency_penalty),
     ]
-    ).sample(
+    if has_freq:
+        strategies.append(FrequencyPenaltyStrategy(frequency_penalty))
+
+    return SamplingPipeline(strategies).sample(
         logits,
         filter_value=filter_value,
         input_ids=input_ids,