capistrant opened a new pull request, #19328: URL: https://github.com/apache/druid/pull/19328
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Don't reference the issue in the title of this pull-request. --> <!-- If you are a committer, follow the PR action item checklist for committers: https://github.com/apache/druid/blob/master/dev/committer-instructions.md#pr-and-issue-action-item-checklist-for-committers. --> ### Description <!-- Describe the goal of this PR, what problem are you fixing. If there is a corresponding issue (referenced above), it's not necessary to repeat the description here, however, you may choose to keep one summary sentence. --> <!-- Describe your patch: what did you change in code? How did you fix the problem? --> <!-- If there are several relatively logically separate changes in this PR, create a mini-section for each of them. For example: --> Arguably could be `fix` commit type I guess. But I think `feat` is better since it is a behavior change that comes from learnings running this code in Druid clusters and may have been more of a gap in the initial design spec. When a historical is in turbo mode it is going to be eagerly loading segments on more threads than normal runtime. This can tie up CPU and cause contention for queries. This contention is called out in our docs so it was initially acknowledged as being a reality of using turbo loading. Upon reflection, I am proposing that the Broker takes turbo loading nodes and artificially de-prioritizes them in its historical selector structure. This will allow queries to target non-turbo nodes whenever such hosts contain the targeted segments for a query. This impl sends turbo nodes to dead last priority across all tiers. This means that if you have 1 replica on hot and one on a slower cold. if that hot is in turbo mode you will query cold. An argument could be made that this is wrong (called out below in design decisions) #### Design Decisions ##### Sending turbo nodes to dead last priority You could argue that you may only want a turbo node to be the least priority server within its own tier or existing priority level. That way if you have cold storage type historicals that are super low priority you'd still query the turbo hot replica over that one, thinking that it may still be better faster. The reason I did not go this direction is a feel that it was unnecessary complexity. Turbo mode is designed to be transient and I feel it is tolerable to have the simpler solution of dead last priority for some (hopefully) short period of time where your queries may hit slower nodes if that is where replicas live. If the turbo nodes truly are pegging CPU it is hopefully better to do it my proposed way anyways. <!-- In each section, please describe design decisions made, including: - Choice of algorithms - Behavioral aspects. 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