[Hardware][TPU] Support parallel sampling & Swapping (vllm-project#5855)

WoosukKwon · Robert Shaw · commit 165329368c20 · 2024-07-01T02:07:17.000Z
diff --git a/vllm/attention/backends/pallas.py b/vllm/attention/backends/pallas.py
@@ -28,21 +28,35 @@ def get_kv_cache_shape(
     ) -> Tuple[int, ...]:
         return (num_kv_heads, num_blocks, block_size, head_size)
 
+    @torch.compile(backend="openxla")
     @staticmethod
     def swap_blocks(
-        src_kv_cache: torch.Tensor,
-        dst_kv_cache: torch.Tensor,
-        src_to_dst: Dict[int, int],
+        src_kv_cache: Tuple[torch.Tensor, torch.Tensor],
+        dst_kv_cache: Tuple[torch.Tensor, torch.Tensor],
+        src_to_dst: Tuple[torch.Tensor, torch.Tensor],
     ) -> None:
-        raise NotImplementedError("swap_blocks is not implemented.")
+        src_k_cache, src_v_cache = src_kv_cache
+        dst_k_cache, dst_v_cache = dst_kv_cache
+        torch.ops.xla.dynamo_set_buffer_donor_(dst_k_cache, True)
+        torch.ops.xla.dynamo_set_buffer_donor_(dst_v_cache, True)
 
+        device = dst_k_cache.device
+        src_indices, dst_indices = src_to_dst
+        dst_k_cache[:, dst_indices] = src_k_cache[:, src_indices].to(device)
+        dst_v_cache[:, dst_indices] = src_v_cache[:, src_indices].to(device)
+
+    @torch.compile(backend="openxla")
     @staticmethod
     def copy_blocks(
-        kv_caches: List[torch.Tensor],
-        src_to_dists: Dict[int, List[int]],
+        kv_caches: List[Tuple[torch.Tensor, torch.Tensor]],
+        src_to_dists: Tuple[torch.Tensor, torch.Tensor],
     ) -> None:
-        # TODO(woosuk): Implement this.
-        raise NotImplementedError("copy_blocks is not implemented.")
+        src_indices, dst_indices = src_to_dists
+        for k_cache, v_cache in kv_caches:
+            torch.ops.xla.dynamo_set_buffer_donor_(k_cache, True)
+            k_cache[:, dst_indices] = k_cache[:, src_indices]
+            torch.ops.xla.dynamo_set_buffer_donor_(v_cache, True)
+            v_cache[:, dst_indices] = v_cache[:, src_indices]
 
 
 @dataclass
diff --git a/vllm/worker/tpu_model_runner.py b/vllm/worker/tpu_model_runner.py
@@ -22,6 +22,9 @@
 _PAD_SLOT_ID = 0  # FIXME(woosuk)
 # FIXME(woosuk): Temporarily disabled top-p sampling since it's too slow.
 _ENABLE_TOP_P = False
+# FIXME(woosuk): A temporary hack to support `n > 1`.
+# This can significantly affect the performance if too large.
+_MAX_NUM_SAMPLES = 128
 
 
 class TPUModelRunner:
@@ -143,8 +146,9 @@ def _dummy_run(
         p = torch.ones((batch_size, ), dtype=torch.float32, device=self.device)
 
         # Dummy run.
+        num_samples = _MAX_NUM_SAMPLES if is_prompt else 1
         self.model(token_ids, position_ids, kv_caches, attn_metadata,
-                   input_lens, t, p)
+                   input_lens, t, p, num_samples)
 
     def warmup_model(
         self,
@@ -268,14 +272,11 @@ def _prepare_decode(
         input_positions: List[List[int]] = []
         slot_mapping: List[List[int]] = []
         context_lens: List[int] = []
-        num_seq_groups = len(seq_group_metadata_list)
-        batch_size = _get_padded_batch_size(num_seq_groups)
 
-        for i, seq_group_metadata in enumerate(seq_group_metadata_list):
+        batch_idx = 0
+        for seq_group_metadata in seq_group_metadata_list:
             assert not seq_group_metadata.is_prompt
-
             seq_ids = list(seq_group_metadata.seq_data.keys())
-
             for seq_id in seq_ids:
                 seq_data = seq_group_metadata.seq_data[seq_id]
                 generation_token = seq_data.get_last_token_id()
@@ -288,14 +289,16 @@ def _prepare_decode(
 
                 assert seq_group_metadata.block_tables is not None
                 block_table = seq_group_metadata.block_tables[seq_id]
-                self.block_tables[i, :len(block_table)] = block_table
+                self.block_tables[batch_idx, :len(block_table)] = block_table
+                batch_idx += 1
 
                 block_number = block_table[position // self.block_size]
                 block_offset = position % self.block_size
                 slot = block_number * self.block_size + block_offset
                 slot_mapping.append([slot])
 
-        num_paddings = batch_size - num_seq_groups
+        batch_size = _get_padded_batch_size(batch_idx)
+        num_paddings = batch_size - batch_idx
         input_tokens = input_tokens + [[0]] * num_paddings
         input_positions = input_positions + [[0]] * num_paddings
         slot_mapping = slot_mapping + [[_PAD_SLOT_ID]] * num_paddings
@@ -333,14 +336,13 @@ def _prepare_sample(
         self,
         seq_group_metadata_list: List[SequenceGroupMetadata],
         padded_batch_size: int,
-    ) -> Tuple[torch.Tensor, torch.Tensor]:
+    ) -> Tuple[torch.Tensor, torch.Tensor, List[int]]:
         assert len(seq_group_metadata_list) > 0
         t = []
         p = []
+        best_of = []
         for seq_group_metadata in seq_group_metadata_list:
-            assert seq_group_metadata.sampling_params is not None
             sampling_params = seq_group_metadata.sampling_params
-
             # NOTE(woosuk): Here we mimic argmax sampling by applying a very
             # low temperature. This is not accurate.
             t.append(sampling_params.temperature
@@ -354,10 +356,11 @@ def _prepare_sample(
                 raise NotImplementedError(
                     "Top-k sampling is currently disabled for the TPU backend "
                     "due to performance issues.")
-            if sampling_params.best_of > 1:
+            if sampling_params.best_of > _MAX_NUM_SAMPLES:
                 raise NotImplementedError(
-                    "best_of > 1 is not currently supported by the TPU "
+                    f"Best of > {_MAX_NUM_SAMPLES} is not supported by the TPU "
                     "backend.")
+            best_of.append(sampling_params.best_of)
             if sampling_params.use_beam_search:
                 raise NotImplementedError(
                     "Beam search is not supported by the TPU backend.")
@@ -369,13 +372,19 @@ def _prepare_sample(
                     "prompt_logprobs is not currently supported by the TPU "
                     "backend.")
 
-        num_paddings = padded_batch_size - len(seq_group_metadata_list)
+            # Repeat the sampling params if the seq group has multiple seqs.
+            num_seqs = len(seq_group_metadata.seq_data)
+            t += [t[-1]] * (num_seqs - 1)
+            p += [p[-1]] * (num_seqs - 1)
+            best_of += [best_of[-1]] * (num_seqs - 1)
+
+        num_paddings = padded_batch_size - len(t)
         t += [1.0] * num_paddings
         p += [1.0] * num_paddings
 
         t = torch.tensor(t, dtype=torch.float32, device=self.device)
         p = torch.tensor(p, dtype=torch.float32, device=self.device)
-        return t, p
+        return t, p, best_of
 
     def _execute_model(
         self,
@@ -392,28 +401,41 @@ def _execute_model(
         else:
             inputs = self._prepare_decode(seq_group_metadata_list)
         padded_batch_size = inputs[0].shape[0]
-        t, p = self._prepare_sample(seq_group_metadata_list, padded_batch_size)
+        t, p, best_of = self._prepare_sample(seq_group_metadata_list,
+                                             padded_batch_size)
+        num_samples = _MAX_NUM_SAMPLES if is_prompt else 1
 
         # Execute the model.
         next_token_ids = self.model(inputs[0], inputs[1], kv_caches,
-                                    *inputs[2:], t, p)
+                                    *inputs[2:], t, p, num_samples)
         # Retrieve the outputs to CPU.
         next_token_ids = next_token_ids.cpu().tolist()
 
         # NOTE(woosuk): Minimal code to construct the sampler outputs.
         # The TPU backend does not reuse the sampler, since the TPU backend
         # does not support the advanced sampling parameters such as logprobs.
-        i = 0
+        zero_logprob = Logprob(0.0)
+        batch_idx = 0
         sampler_outputs = []
         for seq_group_metadata in seq_group_metadata_list:
             seq_outputs = []
             seq_ids = list(seq_group_metadata.seq_data.keys())
-            for seq_id in seq_ids:
-                next_token_id = next_token_ids[i]
-                seq_outputs.append(
-                    SequenceOutput(seq_id, next_token_id,
-                                   {next_token_id: Logprob(0.0)}))
-                i += 1
+            if is_prompt:
+                assert len(seq_ids) == 1
+                seq_id = seq_ids[0]
+                for i in range(best_of[batch_idx]):
+                    next_token_id = next_token_ids[batch_idx][i]
+                    seq_outputs.append(
+                        SequenceOutput(seq_id, next_token_id,
+                                       {next_token_id: zero_logprob}))
+                batch_idx += 1
+            else:
+                for seq_id in seq_ids:
+                    next_token_id = next_token_ids[batch_idx][0]
+                    seq_outputs.append(
+                        SequenceOutput(seq_id, next_token_id,
+                                       {next_token_id: zero_logprob}))
+                    batch_idx += 1
             sampler_outputs.append(
                 CompletionSequenceGroupOutput(seq_outputs, None))
         return sampler_outputs
@@ -458,6 +480,7 @@ def forward(
         input_lens: torch.Tensor,
         t: torch.Tensor,
         p: torch.Tensor,
+        num_samples: int,
     ) -> torch.Tensor:
         """Executes the forward pass of the model and samples the next token.
 
@@ -520,8 +543,9 @@ def forward(
         if _ENABLE_TOP_P:
             logits = _apply_top_p(logits, p.unsqueeze(dim=1))
         probs = torch.softmax(logits, dim=-1, dtype=torch.float32)
-        # FIXME(woosuk): best_of > 1 is not supported.
-        next_token_ids = torch.multinomial(probs, num_samples=1).squeeze(dim=1)
+        next_token_ids = torch.multinomial(probs,
+                                           num_samples,
+                                           replacement=True)
         return next_token_ids
 
 
diff --git a/vllm/worker/tpu_worker.py b/vllm/worker/tpu_worker.py
@@ -1,5 +1,5 @@
 import os
-from typing import List, Optional, Tuple
+from typing import List, Optional, Tuple, Union
 
 import torch
 import torch_xla.core.xla_model as xm
@@ -117,19 +117,26 @@ def determine_num_available_blocks(self) -> Tuple[int, int]:
         # Synchronize before measuring the memory usage.
         xm.wait_device_ops()
 
+        dtype_btyes = get_dtype_size(self.cache_dtype)
+        block_size = self.cache_config.block_size
+        block_size_bytes = (dtype_btyes * block_size * num_layers * 2 *
+                            head_size * num_kv_heads)
+
+        # Calculate the TPU KV cache size based on profiling.
         m = xm.get_memory_info(self.device)
         total_memory_size = m["bytes_limit"]
         usable_memory_size = int(total_memory_size *
                                  self.cache_config.gpu_memory_utilization)
         profiled = m["bytes_used"]  # Weights + intermediate activations.
-        kv_cache_bytes = max(usable_memory_size - profiled, 0)
-        dtype_btyes = get_dtype_size(self.cache_dtype)
-        block_size = self.cache_config.block_size
-        num_tpu_blocks = (kv_cache_bytes //
-                          (dtype_btyes * block_size * num_layers * 2 *
-                           head_size * num_kv_heads))
+        tpu_kv_cache_bytes = max(usable_memory_size - profiled, 0)
+        num_tpu_blocks = tpu_kv_cache_bytes // block_size_bytes
         num_tpu_blocks = (num_tpu_blocks // 8) * 8  # Round down to 8.
-        return num_tpu_blocks, 0
+
+        # Calculate the CPU KV cache size based on the config.
+        num_cpu_blocks = (self.cache_config.swap_space_bytes //
+                          block_size_bytes)
+        num_cpu_blocks = (num_cpu_blocks // 8) * 8  # Round down to 8.
+        return num_tpu_blocks, num_cpu_blocks
 
     def initialize_cache(
         self,
@@ -145,15 +152,19 @@ def initialize_cache(
         num_kv_heads = self.model_config.get_num_kv_heads(self.parallel_config)
         head_size = self.model_config.get_head_size()
 
+        self.cpu_cache = []
         self.tpu_cache = []
         tpu_cache_shape = self.model_runner.attn_backend.get_kv_cache_shape(
             num_gpu_blocks, self.block_size, num_kv_heads, head_size)
         for _ in range(num_layers):
-            key_cache = torch.zeros(tpu_cache_shape,
-                                    dtype=dtype,
-                                    device=self.device)
-            value_cache = torch.zeros_like(key_cache)
-            self.tpu_cache.append((key_cache, value_cache))
+            tpu_k_cache = torch.zeros(tpu_cache_shape,
+                                      dtype=dtype,
+                                      device=self.device)
+            tpu_v_cache = torch.zeros_like(tpu_k_cache)
+            self.tpu_cache.append((tpu_k_cache, tpu_v_cache))
+            cpu_k_cache = torch.zeros_like(tpu_k_cache, device="cpu")
+            cpu_v_cache = torch.zeros_like(tpu_v_cache, device="cpu")
+            self.cpu_cache.append((cpu_k_cache, cpu_v_cache))
         self._warmup_model()
 
     def _warmup_model(self) -> None:
@@ -187,26 +198,68 @@ def execute_model(
         if not self.is_driver_worker:
             self._execute_model_non_driver()
             return []
-
         assert execute_model_req is not None
-        # Currently, TPUWorker does not support swapping.
-        # TODO(woosuk): Support block copying.
-        assert len(execute_model_req.blocks_to_swap_in) == 0, (
-            "Swapping is not supported for the TPU backend.")
-        assert len(execute_model_req.blocks_to_swap_out) == 0, (
-            "Swapping is not supported for the TPU backend.")
-        assert len(execute_model_req.blocks_to_copy) == 0
-
+        # Issue cache operations.
+        self.cache_swap(
+            execute_model_req.blocks_to_swap_in,
+            execute_model_req.blocks_to_swap_out,
+            execute_model_req.blocks_to_copy,
+        )
+        # Run the model.
         seq_group_metadata_list = execute_model_req.seq_group_metadata_list
         assert len(seq_group_metadata_list) > 0
         output = self.model_runner.execute_model(seq_group_metadata_list,
                                                  self.tpu_cache)
         return [output]
 
+    def cache_swap(
+        self,
+        blocks_to_swap_in: List[Tuple[int, int]],
+        blocks_to_swap_out: List[Tuple[int, int]],
+        blocks_to_copy: List[Tuple[int, int]],
+    ) -> None:
+        attn_backend = self.model_runner.attn_backend
+        num_layers = self.model_config.get_num_layers(self.parallel_config)
+
+        if blocks_to_swap_in:
+            # Swap from CPU to TPU.
+            src_to_dst = _make_src_to_dst(blocks_to_swap_in, "cpu",
+                                          self.device)
+            for i in range(num_layers):
+                attn_backend.swap_blocks(self.cpu_cache[i], self.tpu_cache[i],
+                                         src_to_dst)
+        if blocks_to_swap_out:
+            # Swap from TPU to CPU.
+            src_to_dst = _make_src_to_dst(blocks_to_swap_out, self.device,
+                                          "cpu")
+            for i in range(num_layers):
+                attn_backend.swap_blocks(self.tpu_cache[i], self.cpu_cache[i],
+                                         src_to_dst)
+        if blocks_to_copy:
+            src_to_dst = _make_src_to_dst(blocks_to_copy, self.device,
+                                          self.device)
+            attn_backend.copy_blocks(self.tpu_cache, src_to_dst)
+
     def start_worker_execution_loop(self) -> None:
         while self._execute_model_non_driver():
             pass
 
     def _execute_model_non_driver(self) -> bool:
         self.model_runner.execute_model(None, self.tpu_cache)
         return True
+
+
+def _make_src_to_dst(
+    mapping: List[Tuple[int, int]],
+    src_device: Union[torch.device, str],
+    dst_device: Union[torch.device, str],
+) -> Tuple[torch.Tensor, torch.Tensor]:
+    src_indices = [i for i, _ in mapping]
+    dst_indices = [i for _, i in mapping]
+    src_indices = torch.tensor(src_indices,
+                               device=src_device,
+                               dtype=torch.int64)
+    dst_indices = torch.tensor(dst_indices,
+                               device=dst_device,
+                               dtype=torch.int64)
+    return src_indices, dst_indices
