xpu_model_runner.py 24 KB
Newer Older
1
# SPDX-License-Identifier: Apache-2.0
2
# SPDX-FileCopyrightText: Copyright contributors to the vLLM project
3

4
5
6
import dataclasses
import time
import weakref
7
from collections import defaultdict
8
from dataclasses import dataclass
9
10
from typing import (TYPE_CHECKING, Any, Callable, Dict, List, Optional, Tuple,
                    Type, TypeVar)
11
12
13
14
15

import torch
import torch.nn as nn

from vllm.attention import get_attn_backend
16
from vllm.config import VllmConfig
17
from vllm.distributed import get_pp_group
18
from vllm.forward_context import set_forward_context
19
from vllm.inputs import INPUT_REGISTRY, InputRegistry
20
from vllm.logger import init_logger
21
from vllm.model_executor import SamplingMetadataCache
22
from vllm.model_executor.layers.sampler import SamplerOutput, get_sampler
23
from vllm.model_executor.model_loader import get_model
24
from vllm.multimodal import (MULTIMODAL_REGISTRY, BatchedTensorInputs,
25
                             MultiModalKwargs, MultiModalPlaceholderMap,
26
                             MultiModalRegistry)
27
from vllm.sampling_params import SamplingParams
28
from vllm.sequence import IntermediateTensors, SequenceGroupMetadata
29
from vllm.utils import DeviceMemoryProfiler, GiB_bytes, make_tensor_with_pad
30
from vllm.worker.model_runner import AttentionMetadata, SamplingMetadata
31
from vllm.worker.model_runner_base import (
32
    ModelRunnerBase, ModelRunnerInputBase, ModelRunnerInputBuilderBase,
33
34
35
36
37
38
39
    _add_attn_metadata_broadcastable_dict,
    _add_sampling_metadata_broadcastable_dict,
    _init_attn_metadata_from_tensor_dict,
    _init_sampling_metadata_from_tensor_dict)

if TYPE_CHECKING:
    from vllm.attention.backends.abstract import AttentionBackend
40
41
42
43
44

logger = init_logger(__name__)

_PAD_SLOT_ID = -1

45
46
TModelInputForXPU = TypeVar('TModelInputForXPU', bound="ModelInputForXPU")

47

48
49
50
51
52
53
54
55
@dataclass(frozen=True)
class ModelInputForXPU(ModelRunnerInputBase):
    """
    Used by the NeuronModelRunner.
    """
    input_tokens: Optional[torch.Tensor] = None
    input_positions: Optional[torch.Tensor] = None
    attn_metadata: Optional["AttentionMetadata"] = None
56
    multi_modal_kwargs: Optional[BatchedTensorInputs] = None
57
58
59
    virtual_engine: Optional[int] = None
    seq_lens: Optional[List[int]] = None
    query_lens: Optional[List[int]] = None
60
    async_callback: Optional[Callable] = None
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88

    def as_broadcastable_tensor_dict(self) -> Dict[str, Any]:
        tensor_dict = {
            "input_tokens": self.input_tokens,
            "input_positions": self.input_positions,
        }
        _add_attn_metadata_broadcastable_dict(tensor_dict, self.attn_metadata)

        return tensor_dict

    @classmethod
    def from_broadcasted_tensor_dict(
        cls: Type[TModelInputForXPU],
        tensor_dict: Dict[str, Any],
        attn_backend: Optional["AttentionBackend"] = None,
    ) -> TModelInputForXPU:
        if attn_backend is not None:
            tensor_dict = _init_attn_metadata_from_tensor_dict(
                attn_backend, tensor_dict)
        return cls(**tensor_dict)


@dataclass(frozen=True)
class ModelInputForXPUWithSamplingMetadata(ModelInputForXPU):
    """
    Used by the ModelRunner.
    """
    sampling_metadata: Optional["SamplingMetadata"] = None
89

90
    def as_broadcastable_tensor_dict(self) -> Dict[str, Any]:
91
92
93
94
95
96
97
98
99
100
101
        tensor_dict = {
            "input_tokens": self.input_tokens,
            "input_positions": self.input_positions,
        }
        _add_attn_metadata_broadcastable_dict(tensor_dict, self.attn_metadata)
        _add_sampling_metadata_broadcastable_dict(tensor_dict,
                                                  self.sampling_metadata)
        return tensor_dict

    @classmethod
    def from_broadcasted_tensor_dict(
102
        cls,
103
104
        tensor_dict: Dict[str, Any],
        attn_backend: Optional["AttentionBackend"] = None,
105
    ) -> "ModelInputForXPUWithSamplingMetadata":
106
107
108
109
110
111
112
        tensor_dict = _init_sampling_metadata_from_tensor_dict(tensor_dict)
        if attn_backend is not None:
            tensor_dict = _init_attn_metadata_from_tensor_dict(
                attn_backend, tensor_dict)
        return cls(**tensor_dict)


113
114
115
116
117
118
119
120
121
122
123
124
125
class ModelInputForXPUBuilder(ModelRunnerInputBuilderBase[ModelInputForXPU]):

    def __init__(self,
                 runner: "XPUModelRunner",
                 finished_requests_ids: Optional[List[str]] = None) -> None:
        super().__init__()
        self.runner = runner
        self.model_input_cls = self.runner._model_input_cls
        self.attn_backend = self.runner.attn_backend
        self.sliding_window = self.runner.sliding_window
        self.block_size = self.runner.block_size
        self.device = self.runner.device

126
127
128
129
    def prepare(self,
                finished_requests_ids: Optional[List[str]] = None) -> None:
        self.seq_group_metadata_list: List[SequenceGroupMetadata] = []

130
131
132
133
134
135
136
137
138
139
140
141
142
143
    def add_seq_group(self, seq_group_metadata: SequenceGroupMetadata):
        self.seq_group_metadata_list.append(seq_group_metadata)

    def build(self) -> ModelInputForXPU:
        is_prompt = self.seq_group_metadata_list[0].is_prompt
        # Prepare input tensors.
        if is_prompt:
            (input_tokens, input_positions, attn_metadata, seq_lens,
             multi_modal_kwargs) = self._prepare_prompt(
                 self.seq_group_metadata_list)
        else:
            (input_tokens, input_positions,
             attn_metadata) = self._prepare_decode(
                 self.seq_group_metadata_list)
144
            seq_lens = None
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
            multi_modal_kwargs = None

        return self.model_input_cls(
            input_tokens=input_tokens,
            input_positions=input_positions,
            attn_metadata=attn_metadata,
            multi_modal_kwargs=multi_modal_kwargs,
            seq_lens=seq_lens,
            query_lens=seq_lens,
        )

    def _prepare_prompt(
        self,
        seq_group_metadata_list: List[SequenceGroupMetadata],
    ) -> Tuple[torch.Tensor, torch.Tensor, AttentionMetadata, List[int],
               BatchedTensorInputs]:
        assert len(seq_group_metadata_list) > 0
        input_tokens: List[int] = []
        input_positions: List[int] = []
        slot_mapping: List[int] = []
        seq_lens: List[int] = []
166
        multi_modal_kwargs_list: List[MultiModalKwargs] = []
167
168
169
        multi_modal_placeholder_maps: Dict[
            str,
            MultiModalPlaceholderMap] = defaultdict(MultiModalPlaceholderMap)
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187

        for seq_group_metadata in seq_group_metadata_list:
            assert seq_group_metadata.is_prompt
            seq_ids = list(seq_group_metadata.seq_data.keys())
            assert len(seq_ids) == 1
            seq_id = seq_ids[0]

            seq_data = seq_group_metadata.seq_data[seq_id]
            prompt_tokens = seq_data.get_token_ids()
            computed_len = seq_data.get_num_computed_tokens()
            seq_len = len(prompt_tokens)

            seq_lens.append(seq_len)  # Prompt token num
            input_tokens.extend(prompt_tokens)  # Token ids

            # Token position ids
            # NOTE(woosuk): Here we assume that the first token in the prompt
            # is always the first token in the sequence.
188
189
190
191
            positions_range = range(computed_len, seq_len)
            input_positions.extend(list(positions_range))

            if seq_group_metadata.multi_modal_data:
192
                # NOTE: mm_kwargs only includes the subset of multi-modal items
193
                # that intersect with the current prefill positions.
194
                mm_kwargs, placeholder_maps = MultiModalPlaceholderMap \
195
196
                    .from_seq_group(seq_group_metadata, positions_range)

197
                multi_modal_kwargs_list.append(mm_kwargs)
198
199
200
201

                for modality, placeholder_map in placeholder_maps.items():
                    multi_modal_placeholder_maps[modality].extend(
                        placeholder_map)
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241

            if seq_group_metadata.block_tables is None:
                # During memory profiling, the block tables are not initialized
                # yet. In this case, we just use a dummy slot mapping.
                slot_mapping.extend([_PAD_SLOT_ID] * seq_len)
                continue

            # Compute the slot mapping.
            block_table = seq_group_metadata.block_tables[seq_id]
            # Mask the [0, start_idx) tokens of the prompt with _PAD_SLOT_ID,
            # where start_idx is max(0, seq_len - sliding_window).
            # For example, if the prompt len is 10, sliding window is 8, and
            # block size is 4, the first two tokens are masked and the slot
            # mapping will be [-1, -1, 2, 3, 4, 5, 6, 7, 0, 1].
            start_idx = 0
            if self.sliding_window is not None:
                start_idx = max(0, seq_len - self.sliding_window)

            for i in range(computed_len, seq_len):
                if i < start_idx:
                    slot_mapping.append(_PAD_SLOT_ID)
                    continue

                block_number = block_table[i //
                                           self.block_size]  # type: ignore
                block_offset = i % self.block_size  # type: ignore
                slot = block_number * self.block_size + block_offset
                slot_mapping.append(slot)

        num_prompt_tokens = len(input_tokens)

        input_tokens = torch.tensor(input_tokens,
                                    dtype=torch.long,
                                    device=self.device)  # type: ignore
        input_positions = torch.tensor(input_positions,
                                       dtype=torch.long,
                                       device=self.device)  # type: ignore
        slot_mapping = torch.tensor(slot_mapping,
                                    dtype=torch.long,
                                    device=self.device)  # type: ignore
242
243
244
245
246
        placeholder_index_maps = {
            modality: placeholder_map.index_map()
            for modality, placeholder_map in
            multi_modal_placeholder_maps.items()
        }
247
248
249
250
251
252
253
254
255
256

        max_seqlen = max(seq_lens)
        tmp = [0]
        tmp.extend(seq_lens)
        seqlen = torch.tensor(tmp)
        seqlen_q = torch.cumsum(seqlen, dim=0).to(device=self.device)

        attn_metadata = self.attn_backend.make_metadata(
            is_prompt=True,
            slot_mapping=slot_mapping,
257
            multi_modal_placeholder_index_maps=placeholder_index_maps,
258
            enable_kv_scales_calculation=False,
259
260
261
262
263
264
265
266
267
268
269
            seq_lens=seq_lens,
            seqlen_q=seqlen_q,
            max_seqlen=max_seqlen,
            seq_lens_tensor=torch.tensor([]),
            max_decode_seq_len=0,
            num_prefills=len(seq_lens),
            num_prefill_tokens=num_prompt_tokens,
            num_decode_tokens=0,
            block_tables=torch.tensor([], device=self.device, dtype=torch.int),
        )

270
        multi_modal_kwargs = MultiModalKwargs.batch(multi_modal_kwargs_list)
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341

        return (input_tokens, input_positions, attn_metadata, seq_lens,
                multi_modal_kwargs)

    def _prepare_decode(
        self,
        seq_group_metadata_list: List[SequenceGroupMetadata],
    ) -> Tuple[torch.Tensor, torch.Tensor, AttentionMetadata]:
        assert len(seq_group_metadata_list) > 0
        input_tokens: List[int] = []
        input_positions: List[int] = []
        slot_mapping: List[int] = []
        seq_lens: List[int] = []
        block_tables: List[List[int]] = []

        for seq_group_metadata in seq_group_metadata_list:
            assert not seq_group_metadata.is_prompt
            assert seq_group_metadata.token_chunk_size == 1

            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()
                input_tokens.append(generation_token)

                seq_len = seq_data.get_len()
                position = seq_len - 1
                input_positions.append(position)

                seq_len = seq_len if self.sliding_window is None else min(
                    seq_len, self.sliding_window)
                seq_lens.append(seq_len)

                block_table = seq_group_metadata.block_tables[seq_id]
                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)

                if self.sliding_window is not None:
                    sliding_window_blocks = (self.sliding_window //
                                             self.block_size)
                    block_table = block_table[-sliding_window_blocks:]
                block_tables.append(block_table)

        max_decode_seq_len = max(seq_lens)

        input_tokens = torch.tensor(input_tokens,
                                    dtype=torch.long,
                                    device=self.device)
        input_positions = torch.tensor(input_positions,
                                       dtype=torch.long,
                                       device=self.device)
        slot_mapping = torch.tensor(slot_mapping,
                                    dtype=torch.long,
                                    device=self.device)
        seq_lens_tensor = torch.tensor(seq_lens,
                                       dtype=torch.int,
                                       device=self.device)

        block_tables = make_tensor_with_pad(
            block_tables,
            pad=0,
            dtype=torch.int,
            device=self.device,
        )

        attn_metadata = self.attn_backend.make_metadata(
            is_prompt=False,
            slot_mapping=slot_mapping,
342
            multi_modal_placeholder_index_maps=None,
343
            enable_kv_scales_calculation=False,
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
            seq_lens=seq_lens,
            seqlen_q=torch.tensor([]),
            max_seqlen=0,
            seq_lens_tensor=seq_lens_tensor,
            max_decode_seq_len=max_decode_seq_len,
            num_prefill_tokens=0,
            num_decode_tokens=len(input_tokens),
            num_prefills=0,
            block_tables=block_tables,
        )
        return (
            input_tokens,
            input_positions,
            attn_metadata,
        )


class XPUModelRunner(ModelRunnerBase[ModelInputForXPUWithSamplingMetadata]):
    _model_input_cls: Type[ModelInputForXPUWithSamplingMetadata] = (
        ModelInputForXPUWithSamplingMetadata)
    _builder_cls: Type[ModelInputForXPUBuilder] = ModelInputForXPUBuilder
365
366
367

    def __init__(
        self,
368
        vllm_config: VllmConfig,
369
370
        kv_cache_dtype: Optional[str] = "auto",
        is_driver_worker: bool = False,
371
        return_hidden_states: bool = False,
372
373
        input_registry: InputRegistry = INPUT_REGISTRY,
        mm_registry: MultiModalRegistry = MULTIMODAL_REGISTRY,
374
    ):
375
376
377
378

        ModelRunnerBase.__init__(self, vllm_config=vllm_config)
        model_config = self.model_config
        cache_config = self.cache_config
379
        self.is_driver_worker = is_driver_worker
380
        self.return_hidden_states = return_hidden_states
381
382
383
384

        self.device = self.device_config.device

        self.kv_cache_dtype = kv_cache_dtype
385
        self.sliding_window = model_config.get_sliding_window()
386
387
388
389
390
391
392
        self.block_size = cache_config.block_size

        self.attn_backend = get_attn_backend(
            self.model_config.get_head_size(),
            self.model_config.dtype,
            self.kv_cache_dtype,
            self.block_size,
393
            self.model_config.is_attention_free,
394
395
        )

396
        # Multi-modal data support
397
398
        self.input_registry = input_registry
        self.mm_registry = mm_registry
399

400
401
        # Lazy initialization.
        self.model: nn.Module  # Set after init_Model
402
        self.sampler = get_sampler()
403

404
405
406
407
        self.sampling_metadata_cache: SamplingMetadataCache = \
              SamplingMetadataCache() \
                if self.parallel_config.pipeline_parallel_size == 1 else None

408
409
        self.builder = self._builder_cls(weakref.proxy(self))

410
    def load_model(self) -> None:
411
        with DeviceMemoryProfiler() as m:
412
            self.model = get_model(vllm_config=self.vllm_config)
413
414

        self.model_memory_usage = m.consumed_memory
415
416
        logger.info("Loading model weights took %.4f GiB",
                    self.model_memory_usage / GiB_bytes)
417

418
419
420
    def get_model(self) -> nn.Module:
        return self.model

421
422
423
424
425
426
427
428
429
430
431
432
433
434
    @property
    def vocab_size(self) -> int:
        return self.model_config.get_vocab_size()

    @torch.inference_mode()
    def profile_run(self) -> None:
        # Enable top-k sampling to reflect the accurate memory usage.
        sampling_params = SamplingParams(top_p=0.99, top_k=self.vocab_size - 1)
        max_num_batched_tokens = self.scheduler_config.max_num_batched_tokens
        max_num_seqs = self.scheduler_config.max_num_seqs

        # Profile memory usage with max_num_sequences sequences and the total
        # number of tokens equal to max_num_batched_tokens.
        seqs: List[SequenceGroupMetadata] = []
435
436
        # Additional GPU memory may be needed for multi-modal encoding, which
        # needs to be accounted for when calculating the GPU blocks for
437
438
439
440
        # vLLM blocker manager.
        # To exercise the worst scenario for GPU memory consumption,
        # the number of seqs (batch_size) is chosen to maximize the number
        # of images processed.
441
442
        max_mm_tokens = self.mm_registry.get_max_multimodal_tokens(
            self.model_config)
443
        if max_mm_tokens > 0:
444
445
446
447
448
449
450
451
452
453
            max_num_seqs_orig = max_num_seqs
            max_num_seqs = min(max_num_seqs,
                               max_num_batched_tokens // max_mm_tokens)
            if max_num_seqs < 1:
                expr = (f"min({max_num_seqs_orig}, "
                        f"{max_num_batched_tokens} // {max_mm_tokens})")
                logger.warning(
                    "Computed max_num_seqs (%s) to be less than 1. "
                    "Setting it to the minimum value of 1.", expr)
                max_num_seqs = 1
454

455
        batch_size = 0
456
457
458
        for group_id in range(max_num_seqs):
            seq_len = (max_num_batched_tokens // max_num_seqs +
                       (group_id < max_num_batched_tokens % max_num_seqs))
459
            batch_size += seq_len
460

461
            dummy_data = self.input_registry \
462
463
464
                .dummy_data_for_profiling(self.model_config,
                                          seq_len,
                                          self.mm_registry)
465

466
467
468
            seq = SequenceGroupMetadata(
                request_id=str(group_id),
                is_prompt=True,
469
                seq_data={group_id: dummy_data.seq_data},
470
471
472
                sampling_params=sampling_params,
                block_tables=None,
                lora_request=None,
473
474
                multi_modal_data=dummy_data.multi_modal_data,
                multi_modal_placeholders=dummy_data.multi_modal_placeholders)
475
476
            seqs.append(seq)

477
478
479
        finished_requests_ids = [seq.request_id for seq in seqs]
        model_input = self.prepare_model_input(
            seqs, finished_requests_ids=finished_requests_ids)
480
481
482
483
484
485
        intermediate_tensors = None
        if not get_pp_group().is_first_rank:
            intermediate_tensors = self.model.make_empty_intermediate_tensors(
                batch_size=batch_size,
                dtype=self.model_config.dtype,
                device=self.device)
486
        self.execute_model(model_input, None, intermediate_tensors)
487
488
489
        torch.xpu.synchronize()
        return

490
    def make_model_input_from_broadcasted_tensor_dict(
Mor Zusman's avatar
Mor Zusman committed
491
            self,
492
493
494
495
496
497
498
            tensor_dict: Dict[str,
                              Any]) -> ModelInputForXPUWithSamplingMetadata:
        return (
            ModelInputForXPUWithSamplingMetadata.from_broadcasted_tensor_dict(
                tensor_dict,
                attn_backend=self.attn_backend,
            ))
499

500
    def _prepare_model_input_tensors(
501
502
        self,
        seq_group_metadata_list: List[SequenceGroupMetadata],
503
504
505
506
507
508
509
        finished_requests_ids: Optional[List[str]] = None
    ) -> ModelInputForXPUWithSamplingMetadata:
        """Helper method to prepare the model input based on a given sequence
        group. Prepares metadata needed for the base model forward pass but not
        metadata for possible additional steps, e.g., sampling.

        """
510
511
        builder = self.builder
        builder.prepare(finished_requests_ids)
512
        for seq_group_metadata in seq_group_metadata_list:
513
            builder.add_seq_group(seq_group_metadata)
514

515
        return builder.build()  # type: ignore
516

517
518
519
520
521
522
523
524
525
526
527
528
529
530
    def prepare_model_input(
        self,
        seq_group_metadata_list: List[SequenceGroupMetadata],
        virtual_engine: int = 0,
        finished_requests_ids: Optional[List[str]] = None
    ) -> ModelInputForXPUWithSamplingMetadata:
        """Prepare the model input based on a given sequence group, including
        metadata for the sampling step.

        """
        model_input = self._prepare_model_input_tensors(
            seq_group_metadata_list, finished_requests_ids)
        # Sampling metadata is only required for the final pp group
        generators = self.get_generators(finished_requests_ids)
531
532
533
534
535
536
537
538
        sampling_metadata = SamplingMetadata.prepare(
            seq_group_metadata_list,
            model_input.seq_lens,
            model_input.query_lens,
            self.device,
            pin_memory=False,
            generators=generators,
            cache=self.sampling_metadata_cache)
539
540
541
542

        return dataclasses.replace(model_input,
                                   sampling_metadata=sampling_metadata,
                                   virtual_engine=virtual_engine)
543
544
545
546

    @torch.inference_mode()
    def execute_model(
        self,
547
        model_input: ModelInputForXPUWithSamplingMetadata,
548
        kv_caches: List[torch.Tensor],
549
        intermediate_tensors: Optional[IntermediateTensors] = None,
550
551
552
553
554
555
        num_steps: int = 1,
    ) -> Optional[List[SamplerOutput]]:
        if num_steps > 1:
            raise ValueError(
                "XPUModelRunner does not support multi-step execution.")

556
        model_executable = self.model
557
558
559
        if (self.observability_config is not None
                and self.observability_config.collect_model_forward_time):
            model_forward_start_time = time.time()
560
561
562
563
564
565
        with set_forward_context(model_input.attn_metadata, self.vllm_config,
                                 model_input.virtual_engine):
            hidden_or_intermediate_states = model_executable(
                input_ids=model_input.input_tokens,
                positions=model_input.input_positions,
                intermediate_tensors=intermediate_tensors,
566
567
568
569
570
571
                **MultiModalKwargs.as_kwargs(
                    model_input.multi_modal_kwargs or {},
                    dtype=self.model_config.dtype,
                    device=self.device,
                ),
            )
572
573
574
575
        # Compute the logits in the last pipeline stage.
        if not get_pp_group().is_last_rank:
            return hidden_or_intermediate_states

576
577
578
        if (self.observability_config is not None
                and self.observability_config.collect_model_forward_time):
            model_forward_end_time = time.time()
579
580

        # Compute the logits.
581
        logits = self.model.compute_logits(hidden_or_intermediate_states,
582
                                           model_input.sampling_metadata)
583
584
585

        # Only perform sampling in the driver worker.
        if not self.is_driver_worker:
586
            return []
587

588
589
590
        if model_input.async_callback is not None:
            model_input.async_callback()

591
        # Sample the next token.
592
        output: SamplerOutput = self.sampler(
593
            logits=logits,
594
            sampling_metadata=model_input.sampling_metadata,
595
        )
596
597
598
599
600
601
602
603
604
605
        if (self.observability_config is not None
                and self.observability_config.collect_model_forward_time
                and output is not None):
            model_forward_time = (model_forward_end_time -
                                  model_forward_start_time)
            # If there are multiple workers, we are still tracking the latency
            # from the start time of the driver worker to the end time of the
            # driver worker. The model forward time will then end up covering
            # the communication time as well.
            output.model_forward_time = model_forward_time
606

607
        return [output]