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From "Enis Soztutar (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HBASE-17707) New More Accurate Table Skew cost function/generator
Date Sat, 18 Mar 2017 02:40:42 GMT

    [ https://issues.apache.org/jira/browse/HBASE-17707?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15931018#comment-15931018
] 

Enis Soztutar commented on HBASE-17707:
---------------------------------------

[~ted_yu] please revert the patch. 

I think the problem might be that new cost function is not generated in the [0-1] range. If
you notice that all of the cost functions are in that range so that they will play well with
each other. The problem seems to be that the new cost function dominates over the other cost
functions including region replicas. The reason region replicas have a very high cost associated
is that we are implementing a "soft" requirement on replica colocating. In a plan, where there
are N replicas and N servers, the super high cost ensures that all replicas are immediately
moved to separate servers. However, in cases where N replicas, but <N servers, the replicas
can be co-located (because this is the only option). 

I've also noticed that {{OldTableSkewCostFunction}} is just left as deprecated with no usage.
SLB is purely internal class, no need to deprecate and have dead code lying around. We should
just remove it if not used. 

> 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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