[TPU] kv cache update kernel doesn't need to be padded slices to multiple of num_slices_per_block (#22394)

Signed-off-by: Chengji Yao <chengjiyao@gmail.com>
Co-authored-by: Chengji Yao <chengjiyao@gmail.com>

Author: yaochengji

tests/v1/tpu/test_kv_cache_update_kernel.py

@@ -43,11 +43,6 @@ def test_kv_cache_update_kernel(page_size: int, combined_kv_head_num: int,
          np.cumsum(slice_lens[:-1])])
     slot_mapping = np.stack(
         [kv_cache_start_indices, new_kv_cache_indices, slice_lens], axis=1)
-    padded_size = (slot_mapping.shape[0] + num_slices_per_block -
-                   1) // num_slices_per_block * num_slices_per_block
-    slot_mapping = np.pad(slot_mapping,
-                          [[0, padded_size - slot_mapping.shape[0]], [0, 0]],
-                          constant_values=0)
     slot_mapping = np.transpose(slot_mapping)
     slot_mapping_cpu = torch.tensor(slot_mapping,
                                     device="cpu",

vllm/attention/ops/pallas_kv_cache_update.py

@@ -14,6 +14,7 @@ def _kv_cache_update_kernel(
     # Prefetch
     slices_ref,  # [3, padded_num_slices], list of (kv_cache_start,
     # new_kv_start, slice_len)
+    num_slices_ref,  # [1]
     # Input
     new_kv_hbm_ref,  # [num_tokens, num_combined_kv_heads, head_dim]
     kv_cache_hbm_ref,  # [total_num_pages * page_size, num_combined_kv_heads,
@@ -32,8 +33,10 @@ def _kv_cache_update_kernel(
     # Copy from new_kv_hbm_ref to scratch
     for i in range(num_slices_per_block):
         offset_i = i + block_idx * num_slices_per_block
-        new_kv_start = slices_ref[1, offset_i]
-        length = slices_ref[2, offset_i]
+        new_kv_start = jax.lax.select(offset_i < num_slices_ref[0],
+                                      slices_ref[1, offset_i], 0)
+        length = jax.lax.select(offset_i < num_slices_ref[0],
+                                slices_ref[2, offset_i], 0)
         async_copy = pltpu.make_async_copy(
             new_kv_hbm_ref.at[pl.ds(new_kv_start, length), ...],
             scratch.at[i, pl.ds(0, length), ...],
@@ -49,8 +52,10 @@ def _kv_cache_update_kernel(
     async_copies.clear()
     for i in range(num_slices_per_block):
         offset_i = i + block_idx * num_slices_per_block
-        kv_cache_start = slices_ref[0, offset_i]
-        length = slices_ref[2, offset_i]
+        kv_cache_start = jax.lax.select(offset_i < num_slices_ref[0],
+                                        slices_ref[0, offset_i], 0)
+        length = jax.lax.select(offset_i < num_slices_ref[0],
+                                slices_ref[2, offset_i], 0)
         async_copy = pltpu.make_async_copy(
             scratch.at[i, pl.ds(0, length), ...],
             kv_cache_hbm_ref.at[pl.ds(kv_cache_start, length), ...],
@@ -77,7 +82,6 @@ def kv_cache_update(
     page_size: int = 32,
     num_slices_per_block: int = 8,
 ):
-    assert slices.shape[1] % num_slices_per_block == 0
     _, num_combined_kv_heads, head_dim = new_kv.shape
     assert kv_cache.shape[1] == num_combined_kv_heads
     assert kv_cache.shape[2] == head_dim
@@ -93,7 +97,7 @@ def kv_cache_update(
     out_specs = [pl.BlockSpec(memory_space=pltpu.TPUMemorySpace.ANY)]
     out_shape = [jax.ShapeDtypeStruct(kv_cache.shape, dtype=kv_cache.dtype)]
 
-    scalar_prefetches = [slices]
+    scalar_prefetches = [slices, num_kv_update_slices]
     scratch = pltpu.VMEM(
         (num_slices_per_block, page_size, num_combined_kv_heads, head_dim),
         new_kv.dtype,

vllm/v1/worker/tpu_model_runner.py

@@ -745,7 +745,7 @@ def _prepare_inputs(self, scheduler_output: "SchedulerOutput",
         num_kv_update_slices = slot_mapping_metadata.shape[0]
         padded_num_slices = _get_padded_num_kv_cache_update_slices(
             padded_total_num_scheduled_tokens, self.max_num_reqs,
-            self.block_size, self._num_slices_per_kv_cache_update_block)
+            self.block_size)
         slot_mapping_metadata = np.pad(
             slot_mapping_metadata,
             [[0, padded_num_slices - len(slot_mapping_metadata)], [0, 0]],
@@ -1244,8 +1244,7 @@ def _dummy_run(self, num_tokens: int, num_reqs: int,
         position_ids = torch.zeros(num_tokens,
                                    dtype=torch.int32).to(self.device)
         padded_num_slices = _get_padded_num_kv_cache_update_slices(
-            num_tokens, self.max_num_reqs, self.block_size,
-            self._num_slices_per_kv_cache_update_block)
+            num_tokens, self.max_num_reqs, self.block_size)
         num_kv_update_slices = torch.tensor([padded_num_slices],
                                             dtype=torch.int32).to(self.device)
         slot_mapping = torch.zeros((3, padded_num_slices),
@@ -1963,17 +1962,17 @@ def copy_kv_blocks(
         _copy_fn(src_tensor, dst_tensor, src_indices, dst_indices)
 
 
-def _get_padded_num_kv_cache_update_slices(
-        num_tokens: int, max_num_reqs: int, page_size: int,
-        num_slices_per_kv_cache_update_block: int) -> int:
+def _get_padded_num_kv_cache_update_slices(num_tokens: int, max_num_reqs: int,
+                                           page_size: int) -> int:
     """Calculates the padded number of KV cache update slices to avoid
     recompilation."""
+    # NOTE(chengjiyao): let's say R_i is the token num for i-th request,
+    # so it occupies most 2 + R_i // page_size pages. The total maximum
+    # possible number of pages needed is sum(2 + R_i // page_size), which
+    # is <= 2 * max_num_reqs + sum(R_i) // page_size
+    # = 2 * max_num_reqs + num_tokens // page_size
     padded_num_slices = 2 * max_num_reqs + num_tokens // page_size
     padded_num_slices = min(padded_num_slices, num_tokens)
-    padded_num_slices = (
-        padded_num_slices + num_slices_per_kv_cache_update_block - 1
-    ) // num_slices_per_kv_cache_update_block * \
-        num_slices_per_kv_cache_update_block
     return padded_num_slices