bugfix[schedule]: Excessive preemption occurs when preempting running requests to schedule new prefill requests. by CLFutureX · Pull Request #12494 · sgl-project/sglang

CLFutureX · 2025-11-01T14:31:40Z

Problem

Excessive preemption occurs when preempting running requests to schedule new prefill requests.

Affected Path

python/sglang/srt/managers/schedule_policy.py

Relevant Code Snippet

min_tokens_to_remove = (
    req.extend_input_len
    + min(req.sampling_params.max_new_tokens, CLIP_MAX_NEW_TOKENS)
    - self.rem_total_tokens
)
for running_req in sorted_running_reqs:
    if running_req in self.preempt_list:
        continue
    # Priority difference needs to meet the threshold to be preemptible.
    priority_diff = req.priority - running_req.priority
    if server_args.schedule_low_priority_values_first:
        priority_diff *= -1
    if priority_diff > self.priority_scheduling_preemption_threshold:
        preemptible_reqs.append(running_req)
        min_tokens_to_remove -= self._get_running_request_total_token_offset(
            running_req
        )

Solution

Exit the loop and stop preemption once the current prefill request's resource requirements are met (i.e., when min_tokens_to_remove <= 0).

Additional Adjustments

Filter running requests first before sorting them by priority.
Avoid full for-loop traversal: Break the loop immediately when the priority difference is below the threshold.

Signed-off-by: CLFutureX <chenyongqyl@163.com>

gemini-code-assist · 2025-11-01T14:31:56Z

Summary of Changes

Hello @CLFutureX, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request addresses an issue of excessive preemption in the scheduling policy by refining the logic within the preempt_to_schedule method. The changes aim to make the preemption process more efficient and targeted, ensuring that only necessary requests are preempted and that the system avoids redundant iterations. This leads to better resource utilization and potentially improved scheduling performance.

Highlights

Optimized Preemption Logic: Improved the efficiency of the preempt_to_schedule method by filtering out already preempted requests before sorting and introducing early exit conditions for the preemption loop.
Reduced Excessive Preemption: The preemption process now stops as soon as the resource requirements for the new prefill request are met, preventing unnecessary preemption of additional running requests.
Enhanced Loop Efficiency: The preemption loop now breaks early if a running request's priority difference does not meet the threshold, avoiding full traversal when further preemption is not possible or needed.

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Code Review

This pull request introduces several optimizations to the request preemption logic in the scheduler. The main fix prevents excessive preemption by exiting the loop as soon as enough token space has been freed for the new request. Additionally, the code is optimized by filtering out non-preemptible requests before sorting, and by breaking the preemption loop early if a request does not meet the priority threshold, since subsequent requests in the sorted list also won't. These changes are well-implemented and should improve scheduling efficiency and prevent unnecessary preemptions.

Signed-off-by: CLFutureX <chenyongqyl@163.com>

CLFutureX · 2025-11-02T05:59:11Z

@harrisonlimh Hey，PTAL

CLFutureX · 2025-11-07T02:45:53Z

@hnyls2002 @merrymercy @xiezhq-hermann @Ying1123 hey, PTAL , thanks

CLFutureX · 2025-11-12T06:48:12Z

@hnyls2002 @merrymercy @xiezhq-hermann @zhyncs @harrisonlimh ping

harrisonlimh · 2025-11-13T05:20:30Z

        Returns True if preemption was committed, and the new request can be scheduled.
        """
        # Iterate running requests to find preemptible requests
+        valid_running_reqs = (


nit: can we filter for priority here as well?

harrisonlimh · 2025-11-13T05:20:55Z

LGTM! Thank you!

xiezhq-hermann · 2025-11-14T10:04:28Z

@hnyls2002 can you take a look too?

CLFutureX · 2025-11-18T09:04:45Z

@hnyls2002 ping

SeanWeiSean · 2025-11-20T13:36:18Z

I will cover this bug fix test cases in https://github.com/sgl-project/sglang/pull/13576/files test_prefill_adder.py

… requests to schedule new prefill requests. (sgl-project#12494) Signed-off-by: CLFutureX <chenyongqyl@163.com>

fix Excessive preemption and optimize loop

2c25302

Signed-off-by: CLFutureX <chenyongqyl@163.com>

CLFutureX requested review from Ying1123, hnyls2002, merrymercy and xiezhq-hermann as code owners November 1, 2025 14:31

CLFutureX changed the title ~~fix Excessive preemption and optimize loop~~ fix: Excessive preemption occurs when preempting running requests to schedule new prefill requests. Nov 1, 2025

gemini-code-assist Bot reviewed Nov 1, 2025

View reviewed changes

format

4f7bb29

Signed-off-by: CLFutureX <chenyongqyl@163.com>

Merge branch 'main' into fix_excessive_preemption

701c7b1

harrisonlimh self-assigned this Nov 4, 2025

Merge branch 'main' into fix_excessive_preemption

c1fe153

CLFutureX requested a review from zhyncs as a code owner November 7, 2025 02:48

CLFutureX changed the title ~~fix: Excessive preemption occurs when preempting running requests to schedule new prefill requests.~~ bugfix[schedule]: Excessive preemption occurs when preempting running requests to schedule new prefill requests. Nov 8, 2025

Merge branch 'main' into fix_excessive_preemption

2c3631c

Merge branch 'main' into fix_excessive_preemption

64686f4

xiezhq-hermann self-assigned this Nov 12, 2025

harrisonlimh reviewed Nov 13, 2025

View reviewed changes

harrisonlimh added the run-ci label Nov 13, 2025

harrisonlimh approved these changes Nov 13, 2025

View reviewed changes

xiezhq-hermann assigned hnyls2002 Nov 14, 2025

harrisonlimh mentioned this pull request Nov 18, 2025

[Task] Add priority scheduling CIs for preemption path #13470

Open

2 tasks

SeanWeiSean mentioned this pull request Nov 20, 2025

bugfix[schedule]: Refactor sort method and add related UT #13576

Merged

6 tasks

Merge branch 'main' into fix_excessive_preemption

6cf3bee

xiezhq-hermann approved these changes Nov 21, 2025

View reviewed changes

xiezhq-hermann added the ready-to-merge The PR is ready to merge after the CI is green. label Nov 21, 2025

CLFutureX and others added 3 commits November 25, 2025 09:28

Merge branch 'main' into fix_excessive_preemption

558fa30

Merge branch 'main' into fix_excessive_preemption

1a2efaa

Merge branch 'main' into fix_excessive_preemption

080dc5f

hnyls2002 merged commit dc7bdc7 into sgl-project:main Nov 30, 2025
69 of 77 checks passed

harvenstar pushed a commit to harvenstar/sglang that referenced this pull request Dec 4, 2025

bugfix[schedule]: Excessive preemption occurs when preempting running…

fb232ef

… requests to schedule new prefill requests. (sgl-project#12494) Signed-off-by: CLFutureX <chenyongqyl@163.com>

tonyluj pushed a commit to openanolis/sglang that referenced this pull request Dec 5, 2025

bugfix[schedule]: Excessive preemption occurs when preempting running…

8b07224

… requests to schedule new prefill requests. (sgl-project#12494) Signed-off-by: CLFutureX <chenyongqyl@163.com>

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bugfix[schedule]: Excessive preemption occurs when preempting running requests to schedule new prefill requests.#12494

bugfix[schedule]: Excessive preemption occurs when preempting running requests to schedule new prefill requests.#12494
hnyls2002 merged 10 commits intosgl-project:mainfrom
CLFutureX:fix_excessive_preemption

CLFutureX commented Nov 1, 2025

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CLFutureX commented Nov 1, 2025

Problem

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