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https://issues.apache.org/jira/browse/HBASE-17707?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15936530#comment-15936530
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Kahlil Oppenheimer commented on HBASE-17707:
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Sorry, I just realized it was unclear because the unit test was pre-existing
(before my patch) for the old table skew cost function, but now applies to the
new one. It is found at TestStochasticLoadBalancer::testTableSkewCost. Also it
*is* a hard guarantee that {{numMovesPerTable <= pathologicalNumMoves}}. I made
sure to be consistent with the other cost functions when creating this one.
The issue is that the old table skew cost function was fundamentally broken. It
did not change its cost estimate as the balancer proposed region moves/swaps,
meaning the table skew cost it estimated at the beginning of balancing was
often the same as at the end, which meant it actually played no role in the
balancing at all. I have a separate issue open HBASE-17706 that fixes the
behavior in the old TableSkewCostFunction if people would still like to use it.
But I can't merge that one until this one gets resolved. In any case, it does
not surprise me that this new cost function would alter behavior because we are
effectively having table skew considered for the first time in the balancing
process.
I'll go ahead and rebase/resubmit a new patch that includes the new table skew
stuff as well as the fix to the region replica host cost function.
> New More Accurate Table Skew cost function/generator
> ----------------------------------------------------
>
> Key: HBASE-17707
> URL: https://issues.apache.org/jira/browse/HBASE-17707
> Project: HBase
> Issue Type: New Feature
> Components: Balancer
> Affects Versions: 1.2.0
> Environment: CentOS Derivative with a derivative of the 3.18.43
> kernel. HBase on CDH5.9.0 with some patches. HDFS CDH 5.9.0 with no patches.
> Reporter: Kahlil Oppenheimer
> Assignee: Kahlil Oppenheimer
> Priority: Minor
> Fix For: 2.0
>
> Attachments: HBASE-17707-00.patch, HBASE-17707-01.patch,
> HBASE-17707-02.patch, HBASE-17707-03.patch, HBASE-17707-04.patch,
> HBASE-17707-05.patch, HBASE-17707-06.patch, HBASE-17707-07.patch,
> HBASE-17707-08.patch, HBASE-17707-09.patch, HBASE-17707-11.patch,
> HBASE-17707-11.patch, test-balancer2-13617.out
>
>
> This patch includes new version of the TableSkewCostFunction and a new
> TableSkewCandidateGenerator.
> The new TableSkewCostFunction computes table skew by counting the minimal
> number of region moves required for a given table to perfectly balance the
> table across the cluster (i.e. as if the regions from that table had been
> round-robin-ed across the cluster). This number of moves is computer for each
> table, then normalized to a score between 0-1 by dividing by the number of
> moves required in the absolute worst case (i.e. the entire table is stored on
> one server), and stored in an array. The cost function then takes a weighted
> average of the average and maximum value across all tables. The weights in
> this average are configurable to allow for certain users to more strongly
> penalize situations where one table is skewed versus where every table is a
> little bit skewed. To better spread this value more evenly across the range
> 0-1, we take the square root of the weighted average to get the final value.
> The new TableSkewCandidateGenerator generates region moves/swaps to optimize
> the above TableSkewCostFunction. It first simply tries to move regions until
> each server has the right number of regions, then it swaps regions around
> such that each region swap improves table skew across the cluster.
> We tested the cost function and generator in our production clusters with
> 100s of TBs of data and 100s of tables across dozens of servers and found
> both to be very performant and accurate.
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