[Enhancement] Skip rebalancing scan ranges for hdfs backend selector by default when using datacache. #51996
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…by default when using datacache. Signed-off-by: GavinMar <[email protected]>
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Smith-Cruise
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Oct 16, 2024
[Java-Extensions Incremental Coverage Report]✅ pass : 0 / 0 (0%) |
[FE Incremental Coverage Report]✅ pass : 8 / 10 (80.00%) file detail
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zombee0
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Oct 18, 2024
| boolean enableDataCache = ConnectContext.get() != null ? ConnectContext.get().getSessionVariable(). | ||
| isEnableScanDataCache() : false; | ||
| // If force-rebalancing is not specified and cache is used, skip the rebalancing directly. | ||
| if (!forceReBalance && enableDataCache) { |
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maybe we need add user guide that only when cache enabled, forceReBalance does it's work.
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maybe we need add user guide that only when cache enabled, forceReBalance does it's work.
Ok，set it invisible currently, and if it is necessary to expose it to users, we will add relevant documentation to explain it
dirtysalt
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Oct 18, 2024
Youngwb
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Oct 18, 2024
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@Mergifyio backport branch-3.3 |
✅ Backports have been created
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…by default when using datacache. (#51996) Signed-off-by: GavinMar <[email protected]> (cherry picked from commit fe00c0b)
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wanpengfei-git
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…by default when using datacache. (backport #51996) (#52072) Co-authored-by: Gavin <[email protected]>
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Why I'm doing:
Now we use consistent hash algorithm to select backend for hdfs scan ranges, which cannot make sure the scan ranges will be evenly distributed among all backends. So, we rebalance the scan range from one backend to another one if the data distribution on the former exceeds 10% of the average bytes.
However, this may cause random cache miss because the same scan range may be rebalanced to a different one. So, even if the same query is executed multiple times, it still cannot fully hit the cache each time. This will lead to significant performance degradation in many scenarios.
What I'm doing:
Considering with the help of so many virtual nodes, consistent hashing usually does not result in significant deviations in data distribution. So, we skip rebalancing scan ranges by default when using datacache.
Also, we add a session variable to change this default behavior in some special cases.
What type of PR is this:
Does this PR entail a change in behavior?
If yes, please specify the type of change:
Checklist:
Bugfix cherry-pick branch check: