Fix busy-waiting issue in ContinuousBatchingManager.get_result (#46516)
Author: shreyescodes
Pull request: yes
Labels: name: Code agent slop

Resolves #46516

Hi @remi-or,

I was going through the continuous batching code and noticed the issue you mentioned about `get_result` being quite wasteful. I spent some time debugging how the `output_queue` behaves when multiple concurrent requests are running. 

I observed that currently, all threads are polling the exact same shared `output_queue`. If a thread picks up an output meant for another request, it just puts it back into the queue. Under high load, this causes a heavy while loop (busy-waiting) across multiple threads, which spikes CPU usage and slows down the actual dispatching.

To fix this, I thought of separating the queues. I have added a dictionary of private queues (`sync_queues`) inside the `OutputRouter`. Here is how it works now:
- When a user calls `get_result(request_id)`, we create a dedicated queue for that specific request on the fly.
- The generation thread directly routes the output to the respective private queue during `deliver` and `deliver_batch`.
- Because of this, the consumer thread can now just safely block on its own queue using `.get(block=True)`. This completely stops the wasteful polling and brings down CPU usage.
- Once the generation is finished for that request, we pop the queue from the dictionary to clean up.

I have made sure to handle the locks properly so it remains completely thread-safe. 

Please review this approach and let me know your thoughts. I would be happy to make any changes if required!


Comments:
Hi @shreyescodes , I have taken a look at the PR and I like the idea of creating queues per request, that way each targeted `get_result` has a precise target. But I think there may be two issues with the way this is done right now:
1. the output_queue is never going to see results that have a sync_queue : if a sync queue is created for a request, the results for that request will never appear in the output queue again, so you might loop infinitely over the output queue waiting for a result that you should get with a targeted get_result. Might not happen, but that opens that possibility 
2. instead of creating the sync_queue when using a targeted get_result, why not always create the sync_queue (or whatever object we use to retrieve the result with a get_result) ?

So I am thinking maybe the output queue is not the best structure, maybe a dict would be better, plus a queue of most recent indexes so we can still get the "queue" behavior if needed. Wrapped in a lock to ensure thread safety. With a custom class, it could do the trick in a few lines. WDYT? I am happy with another kind of object as long as it gets the job done! 

Hi @remi-or,

Excellent catch! You are completely right, routing items into isolated sync queues breaks the global FIFO expectation for anyone polling output_queue.get().

I love the idea of a custom unified class! I'll refactor the OutputRouter to use a custom TargetedQueue internally. It will maintain both a deque for global arrival order and a dict for O(1) targeted retrieval. To keep it fully O(1) without doing  queue.remove() operations, I'll use a consumed_ids set to lazily drop items from the global queue if they were already popped via a targeted request. Thoughts ?



Or we keep the dictionary + queue and when an item is consumed in either source, we flip a mutable field to show this has already been consumed? And we drop those items when we encounter them. WDYT?

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Hi @remi-or,

Yes, that makes a lot of sense! Using a mutable flag is much simpler than maintaining a separate set.

So I am thinking we can just wrap the item in a small list like [output, False] when we put it in. Then, whenever get() is called (either targeted or global), we just check the front of the queue/dict. If the flag is already True, we just drop it and move to the next. When we find an item with False, we flip it to True, do a quick cleanup of the dict if it becomes empty so we don't leak memory, and return the item.

Does this logic look good to you? Let me know and I will go ahead, write the code, and update the PR.

Or just add a field to the output type? I think it's better if you first implement and then I can comment on the PR! Ty

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**[For maintainers]** Suggested jobs to run (before merge)

run-slow: clap, deit, depth_anything, dpt, gemma4_unified, ijepa, llama4, marian, mimi, minicpmv4_6, musicgen_melody, prompt_depth_anything, vit, vivit, whisper

Thank you for your contribution 🤗!

## CI Security Gate — automatic approval blocked

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Thanks @remi-or for the suggestions. I have updated the PR with the new approach.

Basically, I implemented the custom TargetedQueue inside OutputRouter and added a departed flag directly in the GenerationOutput dataclass like we discussed.

Here is how the logic works now:

Added a departed boolean flag to GenerationOutput.
Created a new TargetedQueue class that unifies both the global queue and the dictionary of targeted queues.
Whenever get_result() is called, we check the front of the queue. If we see departed == True, we just drop it and move to the next one. When we find a valid item, we flip its departed flag to True so the other queue knows to ignore it.
I also made sure to pop the request queue from the dictionary as soon as the request finishes to avoid memory leaks.
One small optimization I added: instead of using a standard deque for the global queue, I used collections.OrderedDict. The reason is, if a user only ever calls targeted get_result(request_id) and never polls the global queue, the global queue will keep growing infinitely with departed=True items. With OrderedDict, we can do an O(1) removal of the item from the global queue the moment it gets consumed by the targeted queue. This completely prevents the memory leak.

I've ran make style and tested everything locally, working fine now. Please review and let me know if any further changes are needed!

**CI Dashboard:** [View test results in Grafana](https://transformers-ci.lor-e.huggingface.cool/d/pytest-observability-by-pr/pytest-observability-branch?var-pr=46714)

Thanks for the discussion, but this is clearly a code agent PR. Marking as such and closing. Please read the rules about contributing.