[jira] [Commented] (HBASE-26311) Balancer gets stuck in cohosted replica distribution

["Clara Xiong (Jira)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/HBASE-26311?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17424743#comment-17424743 ] 

Clara Xiong commented on HBASE-26311:
-------------------------------------

When balancer has to satisfy other constraints, even region count distribution just cannot be guaranteed, as in existing test case TestStochasticLoadBalancerRegionReplicaWithRacks. Because replica distribution has much higher weight than region count skew, the rack with fewer servers tend to get more regions than those with more servers.

In this test case, server 0 and 1 are on the same rack while 2 and 3 are on each's rack because servers cannot be place completely evenly. The resulted region count distribution can be [2,2, 4, 4] or be[1, 3, 4, 4]so that we have no replicas of the same region on the first rack. So we have to have fewer regions per server on the first two servers. With the current algorithm, the costs of two plan are the same for region count skew because only linear deviation to ideal average is considered. It can get much more extreme when we have 5 servers for this test case: [1,3,3,3,5]or [2,2,3,3,5] depending on the random walk. But since the algorithm says they are the same expense for region count skew, balancer can be stuck at the former. The more servers we have, as long as the RS counts are not completely even, which happens all the time, the more variation of results we will see depending the random walk. But once we reach the extreme case, balancer is stuck because the cost function says moving doesn't gain.

I am proposing using the sum of square of deviation for load functions, inline with replica cost functions. we don't need standard deviation so we can keep it simple and fast. see https://issues.apache.org/jira/browse/HBASE-25625

> Balancer gets stuck in cohosted replica distribution
> ----------------------------------------------------
>
>                 Key: HBASE-26311
>                 URL: https://issues.apache.org/jira/browse/HBASE-26311
>             Project: HBase
>          Issue Type: Bug
>          Components: Balancer
>            Reporter: Clara Xiong
>            Assignee: Clara Xiong
>            Priority: Major
>
> In production, we found a corner case where balancer cannot make progress when there is cohosted replica. This is repro'ed on master branch using test added in HBASE-26310. The two cost functions isn't provide proper evaluation so balancer could make progress. 
>  
> Another observation is the imbalance weight is not updated by the cost functions properly during plan generation. The subsequent run reports much high imbalance.
> {quote}2021-09-24 22:26:56,039 INFO org.apache.hadoop.hbase.master.balancer.S*tocha*sticLoadBalancer: Finished computing new moving plan. Computation took 2400001 ms to try 1284702 different iterations.  Found a solution that moves 6941 regions; Going from a computed imbalance of 6389.260497305375 to a new imbalance of 21.03904901349833. 
> 2021-09-24 22:33:40,961 INFO org.apache.hadoop.hbase.master.balancer.S*tocha*sticLoadBalancer: Running balancer because at least one server hosts replicas of the same region.
> 2021-09-24 22:33:40,961 INFO org.apache.hadoop.hbase.master.balancer.S*tocha*sticLoadBalancer: Start S*tocha*sticLoadBalancer.balancer, initial weighted average imbalance=6726.357026325619, functionCost=RegionCountSkewCostFunction : (multiplier=500.0, imbalance=0.07721156356401288); PrimaryRegionCountSkewCostFunction : (multiplier=500.0, imbalance=0.06298215530179263); MoveCostFunction : (multiplier=7.0, imbalance=0.0, balanced); ServerLocalityCostFunction : (multiplier=25.0, imbalance=0.463289517245148); RackLocalityCostFunction : (multiplier=15.0, imbalance=0.25670928199727017); TableSkewCostFunction : (multiplier=500.0, imbalance=0.4378048676389543); RegionReplicaHostCostFunction : (multiplier=100000.0, imbalance=0.05809798270893372); RegionReplicaRackCostFunction : (multiplier=10000.0, imbalance=0.061018251681075886); ReadRequestCostFunction : (multiplier=5.0, imbalance=0.08235908576054465); WriteRequestCostFunction : (multiplier=5.0, imbalance=0.09385090828285425); MemStoreSizeCostFunction : (multiplier=5.0, imbalance=0.1327376982847744); StoreFileCostFunction : (multiplier=5.0, imbalance=0.07986594927573858);  computedMaxSteps=5579331200
> {quote}
>  



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