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https://issues.apache.org/jira/browse/HBASE-17707?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15931834#comment-15931834
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Kahlil Oppenheimer commented on HBASE-17707:
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[~enis] [~tedyu] the new table skew cost function is actually guaranteed to be
within the [0-1] range (and this behavior is even unit tested!). The cost
function does not dominate over other cost functions because it is out of the
[0-1] range. Instead, I debugged the breaking test and found that the issue is
that the region replica host cost function can produce very small values when
there are a lot of regions. In my testing, I found that for some medium-large
cluster sizes, the cost function can produce values as small as 2.6 x 10^(-6).
Sadly, this means that even with a weight of 5000 (which is what is set in the
test), the "soft" requirement of having no two region replicas hosted on the
same machine when it is avoidable is not met because the cost function has too
small a contribution (even with this high weight). Instead, my latest patch
updates the region replica cost function to give it a minimum value (.1) for
any amount of co-hosted replicas. This makes it so that if two regions replicas
are placed on the same host, the cost will be at least .1 (whether or not there
are 5 or 1,000,000 regions in the cluster). This better enforces the "soft"
constraint as it makes sure that no other cost functions can overpower 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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