Data Stream Processing and Analytics Vasiliki (Vasia) Kalavri
 vkalavri@bu.edu Spring 2020 4/16: Skew mitigation ??? Vasiliki Kalavri | Boston University 2020 Key partitioning 2 w2 w1 w3 round-robin keys cause imbalance w2 w1 w3 ??? Vasiliki Kalavri | Boston University 2020 Addressing skew • To address skew, the system needs to track the frequencies of the partitioning key values. • We can then

Percolator model. Despite promising snap- shot isolation, TiDB 2.1.7 through 3.0.0-beta.1-40 allowed read skew and lost updates by default, thanks to two auto-retry mechanisms which blindly re-applied updates TiDB also supports a select ... for update statement which mostly, but not entirely, prevents write skew. We found a minor race condition in table creation, reduced durability & fault tolerance in fresh dependence on CLOCK_MONOTONIC_RAW for safety, but we have not yet observed anomalies due to clock skew, possibly due to tooling limitations. PingCAP has published a companion blog post to this report.

type of length-tuning pattern (single-track length, differential-pair length, and differential-pair skew). These defaults can be overridden in the properties of each tuning pattern added to the board. See section below. Tune length: These tools allow you to tune the length of single tracks or the length or skew of differential pairs, after they have been routed. See the Routing Tracks section for details. footprint pads when the footprint is dragged Allows tuning of track lengths and differential pair skew (phase) by inserting serpentine tuning shapes for designs with tight timing requirements By default

type of length-tuning pattern (single-track length, differential-pair length, and differential-pair skew). These defaults can be overridden in the properties of each tuning pattern added to the board. See section below. Tune length: These tools allow you to tune the length of single tracks or the length or skew of differential pairs, after they have been routed. See the Routing Tracks section for details. footprint pads when the footprint is dragged Allows tuning of track lengths and differential pair skew (phase) by inserting serpentine tuning shapes for designs with tight timing requirements By default

type of length-tuning pattern (single-track length, differential-pair length, and differential-pair skew). These defaults can be overridden in 17 the properties of each tuning pattern added to the board section below. Tune length: These tools allow you to tune the length of single tracks or the length or skew of differential pairs, after they have been routed. See the Routing Tracks section for details. routing functions: Route Single Track, Route Differential Pair, Tune length of a single track, and Tune skew of a differential pair. All of these are present in both the Route menu dropdown (individually) on

Orientation 262 Compression 263 Distributions 263 Local (Co-located) Joins 264 Data Skew 264 Processing Skew 265 VMware Greenplum v6.17 Documentation VMware, Inc. 11 Partitioning 267 Number of Checking for Data Distribution Skew 539 Viewing a Table's Distribution Key 539 Viewing Data Distribution 540 Checking for Query Processing Skew 540 Avoiding an Extreme Skew Warning 541 Viewing Metadata Benchmarks 928 Distribution and Skew 928 Distribution and Skew 0 Local (Co-located) Joins 929 Data Skew 929 Considerations for Replicated Tables 930 Processing Skew 930 Common Causes of Performance

Orientation 329 Compression 330 Distributions 330 Local (Co-located) Joins 331 Data Skew 331 Processing Skew 332 Partitioning 332 Number of Partition and Columnar Storage Files 333 Indexes 334 Checking for Data Distribution Skew 607 Viewing a Table's Distribution Key 607 Viewing Data Distribution 608 Checking for Query Processing Skew 608 Avoiding an Extreme Skew Warning 609 VMware Greenplum Benchmarks 996 Distribution and Skew 996 Distribution and Skew 0 Local (Co-located) Joins 997 Data Skew 997 Considerations for Replicated Tables 998 Processing Skew 998 Common Causes of Performance

Orientation 338 Compression 339 Distributions 339 Local (Co-located) Joins 340 Data Skew 340 Processing Skew 341 Partitioning 341 Number of Partition and Columnar Storage Files 342 Indexes 343 Checking for Data Distribution Skew 616 Viewing a Table's Distribution Key 616 Viewing Data Distribution 617 Checking for Query Processing Skew 617 Avoiding an Extreme Skew Warning 617 VMware Greenplum Benchmarks 1005 Distribution and Skew 1005 Distribution and Skew 0 Local (Co-located) Joins 1006 Data Skew 1006 Considerations for Replicated Tables 1007 Processing Skew 1007 Common Causes of Performance

SUM(balance) FROM accounts;
 SI: Bank Test SI: Bank Test •Write skew is valid in Snapshot Isolation •Solution? •SELECT FOR UPDATE 
 SI … Write skew? •Append unique numbers to key 0 and key 1 •Example: val FROM tb WHERE id = 0; •UPDATE tb SET val = "1,3,5,6,7" WHERE id = 1; 
 Write skew: Append Test Write skew w/o read-lock T1 {:f :txn, :value [[:append 0 28] [:r 1 [1 3 7 5 6 11 6} Write skew w/o read-lock •Append unique numbers to key 0 and key 1 •Example: •SELECT val FROM tb WHERE id = 0 FOR UPDATE; •UPDATE tb SET val = "1,3,5,6,7" WHERE id = 1; 
 Write skew gone w/ read-lock

Checking for Data Distribution Skew 1143 Viewing a Table's Distribution Key 1143 Viewing Data Distribution 1143 Checking for Query Processing Skew 1144 Avoiding an Extreme Skew Warning 1144 Checking for Performance Benchmarks 1174 Distribution and Skew 1174 Local (Co-located) Joins 1175 Data Skew 1175 Considerations for Replicated Tables 1176 Processing Skew 1176 Common Causes of Performance Issues Orientation 1243 Compression 1244 Distributions 1244 Local (Co-located) Joins 1245 Data Skew 1245 Processing Skew 1246 Partitioning 1246 Number of Partition and Columnar Storage Files 1247 Indexes 1247