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From "Kahlil Oppenheimer (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HBASE-17707) New More Accurate Table Skew cost function/generator
Date Thu, 23 Mar 2017 15:33:41 GMT

    [ https://issues.apache.org/jira/browse/HBASE-17707?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15938578#comment-15938578

Kahlil Oppenheimer commented on HBASE-17707:

bq. We cannot maintain two different cost functions for table skew. Let's remove the old one
from the code, and only have the new implementation in this patch. We cannot have dead code
lying around and rot. We can close HBASE-17706 as won't fix.
I will add the removal of this old cost function to my patch.

bq. The new candidate generator TableSkewCandidateGenerator is not added to the SLB::candidateGenerators
field which means that it is not used? I can only see the test using it. Is this intended?
It has to be enabled by default.
Good catch on the table skew candidate generator. I will also add that to the patch as well.
I was originally going to do it in a separate patch, but it makes much more sense to just
do it here.

bq. Did you intend to use the raw variable here instead of calling scale again:
Yup! Let's call R the range [0, 1]. We know that scale() maps values into R. We also know
that sqrt() maps values from R -> R. Lastly, we know that .9 * r + .1 for any r in R yields
another value in R. So can be sure the outcome is in R. No need to call scale 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, HBASE-17707-12.patch,
> 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

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