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Chunked Prefill VLM #3188

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@mht-sharma mht-sharma commented Apr 23, 2025

What does this PR do?

Models not supported:

  1. Gemma3 - [Due to biidirectional attention masks]
  2. Qwen2_VL, Qwen2.5_VL - The rust image_text_replacement and python are not consistent for different images

Fixes # (issue)

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  • This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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    Pull Request section?
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    to it if that's the case.
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    here are tips on formatting docstrings.
  • Did you write any new necessary tests?

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@mht-sharma mht-sharma changed the title Add vlm chunking optimized Chunked Prefill VLM Apr 23, 2025
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@@ -248,9 +248,18 @@ Options:
-p, --port <PORT>
The port to listen on

[env: PORT=]
[env: PORT=80]
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Did you change something ?

@@ -116,11 +116,10 @@ def __init__(self, prefix: str, config, weights, layer_id):
)
self.num_heads = config.num_attention_heads
self.hidden_size = config.hidden_size
if hasattr(config, "head_dim"):
if hasattr(config, "head_dim") and config.head_dim is not None:
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Suggested change
if hasattr(config, "head_dim") and config.head_dim is not None:
if getattr(config, "head_dim", None) is not None:

Nit

@@ -81,27 +112,13 @@ def forward(
image_sizes: Optional[torch.Tensor] = None,
adapter_data: Optional[torch.Tensor] = None,
image_grid_thw: Optional[torch.LongTensor] = None,
inputs_embeds: Optional[torch.Tensor] = None,
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Can we not have both, or either input_ids and input_embeds ?

Seems like it's an antipattern to accept both (since we don't know which one is valid.
It's totally fine to handle the embeddings before this step and only accept input_embeds imho

self,
input_ids: torch.Tensor,
vision_embeds: torch.Tensor = None,
**kwargs,
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Can we remove those kwargs everywhere they are not needed ?

vision_embeds: torch.Tensor = None,
pixel_values: torch.FloatTensor = None,
pixel_attention_mask: torch.BoolTensor = None,
**kwargs,
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No kwargs

][0]
length = tokens.numel()

assert (
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Why do we care, I thought that chunking inside an image would be fine (in the LLM part where we do not care about the results anyway, since we're going to replace more of the input_embeds with the image embedding anyway)


self.pixel_values = []

for i, (
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A small assert that all these have the same lengths wouldn't hurt imho

graph = torch.cuda.CUDAGraph()
self.cuda_graphs[bs] = {
"input_ids": input_ids,
"inputs_embeds": inputs_embeds,
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There's a big issue to adding that here.

With 1M context window, we're now reserving 1M * ~4k (hidden_dim) * 2 (f16) = 8GB of VRAM for every graph we save. This seems prohibitive.

Instead of doing that, couldn't we send pixel_embeds (which are just the image tokens, which are capped at max_batch_prefill_tokens) and some pixel_embed_positions (a sequence_length mask of 0, 1 which is simply 1M * 8bit so 1MB worth of storage ?). And within the modeling code, we simply have:

input_embeds[pixel_embed_positions] = pixel_embeds 

?

"inputs_embeds": inputs_embeds,
"position_ids": position_ids,
"kv_cache": self.kv_cache,
"block_tables": block_tables,
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Also duplicating this function in here, has a maintenance cost. I was happy we managed to avoid it until now (for sanity's sake).

I'm wondering if we could factor some stuff out, decompose this in a cleaner way.

This should definitely way for a subsequent PR in the current form (the current PR is already humonguous let's not make it even bigger), I'm just musing at how we could clean this up.

batch.input_ids, vision_embeds=vision_embeds
)

batch.inputs_embeds = inputs_embeds
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The function name is not correct, get_xx should always return something.
If you're not returning something but instead acting on some object, the name can be set_xxx.

Also let's keep consistency and use input_embeds everywhere instead of input_embeddings.

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3 participants