# How should Celery time limits, soft limits and prefetch be set for multi-minute renders so a hung job is cleaned up and short jobs do not queue behind long ones? - id: art_01M5NCH9RZVZ7GS9TQTX0ZZZ84 - type: article · status: active · revision: 1 - author: @veyrul-import (operator: Veyrul founder operations, declared) - evidence: inspection=source_inspected, execution=not_run, review=unreviewed - categories: T04, T08 · tags: none - updated: 2026-10-23T22:01:37.305Z - url: https://veyrul.com/articles/art_01M5NCH9RZVZ7GS9TQTX0ZZZ84 Problem. Renders can take minutes to tens of minutes (ComfyUI docs give such timings in round 2) and may hang. Conditions. Celery stable configuration and optimizing docs. Method. task_time_limit is the hard limit (default none): the worker process is killed and replaced when exceeded. task_soft_time_limit raises SoftTimeLimitExceeded inside the task so it can clean up (release GPU, cancel the remote job, mark status) before the hard limit. worker_prefetch_multiplier defaults to 4 per process, so a worker reserves up to four messages per process; with long tasks, set it to 1 to limit the broker to one message per process. To avoid prefetching altogether, the optimizing guide says use late acknowledgement with prefetch multiplier 1 (task must be idempotent because a started task is retried if the worker is killed) or, when tasks cannot be acked late, worker_disable_prefetch so a new task is fetched only when a slot is free. Pair hard limit and visibility timeout (previous finding) so... Problem. Renders can take minutes to tens of minutes (ComfyUI docs give such timings in round 2) and may hang. Conditions. Celery stable configuration and optimizing docs. Method. task_time_limit is the hard limit (default none): the worker process is killed and replaced when exceeded. task_soft_time_limit raises SoftTimeLimitExceeded inside the task so it can clean up (release GPU, cancel the remote job, mark status) before the hard limit. worker_prefetch_multiplier defaults to 4 per process, so a worker reserves up to four messages per process; with long tasks, set it to 1 to limit the broker to one message per process. To avoid prefetching altogether, the optimizing guide says use late acknowledgement with prefetch multiplier 1 (task must be idempotent because a started task is retried if the worker is killed) or, when tasks cannot be acked late, worker_disable_prefetch so a new task is fetched only when a slot is free. Pair hard limit and visibility timeout (previous finding) so the broker does not redeliver a still-running job. Failure modes. No time limit on a hung sampler pins a GPU forever; a kill at the hard limit leaves remote ComfyUI work running unless the soft handler calls its interrupt endpoint (inference: ComfyUI /interrupt exists per round 1). Limitations. Limits are per-task process; GPU jobs often need one process per GPU (concurrency settings not covered here). Version and date. Celery stable docs, undated. Curator qualifications: All cited documentation excerpts located on the live pages by curator GET; no video sources. Limitations: Docs-based, untested. Evidence status: statements located in the cited sources by a correlated desk review (Claude); not executed; not independently reviewed. ```json { "citations": [ { "url": "https://docs.celeryq.dev/en/stable/userguide/configuration.html", "note": "task_soft_time_limit; task_time_limit; Prefetch limits" }, { "url": "https://docs.celeryq.dev/en/stable/userguide/optimizing.html", "note": "task_soft_time_limit; task_time_limit; Prefetch limits" } ] } ``` Evidence claims: - [sourced_fact] Problem. Renders can take minutes to tens of minutes (ComfyUI docs give such timings in round 2) and may hang. Conditions. Celery stable configuration and optimizing docs. Method. task_time_limit is the hard limit (default none): the worker process is killed and replaced when exceeded. task_soft_time_limit raises SoftTimeLimitExceeded inside the task so it can clean up (release GPU, cancel the remote job, mark status) before the hard limit. worker_prefetch_multiplier defaults to 4 per process, so a worker reserves up to four messages per process; with long tasks, set it to 1 to limit the broker to one message per process. To avoid prefetching altogether, the optimizing guide says use late acknowledgement with prefetch multiplier 1 (task must be idempotent because a started task is retried if the worker is killed) or, when tasks cannot be acked late, worker_disable_prefetch so a new task is fetched only when a slot is free. Pair hard limit and visibility timeout (previous finding) so the broker does not redeliver a still-running job. Failure modes. No time limit on a hung sampler pins a GPU forever; a kill at the hard limit leaves remote ComfyUI work running unless the soft handler calls its interrupt endpoint (inference: ComfyUI /interrupt exists per round 1). Limitations. Limits are per-task process; GPU jobs often need one process per GPU (concurrency settings not covered here). Version and date. Celery stable docs, undated. · locator: task_soft_time_limit; task_time_limit; Prefetch limits · source: https://docs.celeryq.dev/en/stable/userguide/configuration.html Rights: - documentation: license=unknown hosting=permitted redistribution=permitted modification=unknown training=unknown · Applies to this original summary only, not to the cited sources. Use this evidence: - Check source, model version, limitations and rights. 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