【大数据】Spark及SparkSQL数据倾斜现象和解决思路

数据倾斜

分类

join	其中一个表数据量小，key比较集中	分发到某一个或几个reduce的数据远高于平均值
	大表与小表，空值过多	这些空值都由一个reduce处理，处理慢
group by	group by 维度太少，某字段量太大	处理某值的reduce非常慢
count distinct	某些特殊值过多	处理此特殊值的reduce慢

数据倾斜原因分析

数据倾斜表现

• 任务日志进度长度为99%，在日志监控进度条显示只有几个reduce进度一直没有完成。
• 某一task处理时长 > 平均处理时长
• executor出现Java heap space、OutOfMemoryError、executor dead等

数据原因

• 主表驱动表应该选择分布均匀的表作为驱动表，并做好列裁剪。
• 大小表Join，需要记得使用map join,小表会先进入内存，在map端即会完成reduce.
• 此种情形最为常用！！！大表join大表时，关联字段存在大量空值null key
• 数据类型不匹配关联，先转换数据类型

常见shuffle算子

• 去重

def distinct()
def distinct(numPartitions: Int)

• 聚合

def reduceByKey(func: (V, V) => V, numPartitions: Int): RDD[(K, V)]
def reduceByKey(partitioner: Partitioner, func: (V, V) => V): RDD[(K, V)]
def groupBy[K](f: T => K, p: Partitioner):RDD[(K, Iterable[V])]
def groupByKey(partitioner: Partitioner):RDD[(K, Iterable[V])]
def aggregateByKey[U: ClassTag](zeroValue: U, partitioner: Partitioner): RDD[(K, U)]
def aggregateByKey[U: ClassTag](zeroValue: U, numPartitions: Int): RDD[(K, U)]
def combineByKey[C](createCombiner: V => C, mergeValue: (C, V) => C, mergeCombiners: (C, C) => C): RDD[(K, C)]
def combineByKey[C](createCombiner: V => C, mergeValue: (C, V) => C, mergeCombiners: (C, C) => C, numPartitions: Int): RDD[(K, C)]
def combineByKey[C](createCombiner: V => C, mergeValue: (C, V) => C, mergeCombiners: (C, C) =>

• 排序

def sortByKey(ascending: Boolean = true, numPartitions: Int = self.partitions.length): RDD[(K, V)]
def sortBy[K](f: (T) => K, ascending: Boolean = true, numPartitions: Int = this.partitions.length

• 重分区

def coalesce(numPartitions: Int, shuffle: Boolean = false, partitionCoalescer: Option[PartitionCoalescer] = Option.empty)
def repartition(numPartitions: Int)(implicit ord: Ordering[T] = null)

• 集合或者表操作

def intersection(other: RDD[T]): RDD[T]
def intersection(other: RDD[T], partitioner: Partitioner)(implicit ord: Ordering[T] = null): RDD[T]
def intersection(other: RDD[T], numPartitions: Int): RDD[T]
def subtract(other: RDD[T], numPartitions: Int): RDD[T]
def subtract(other: RDD[T], p: Partitioner)(implicit ord: Ordering[T] = null): RDD[T]
def subtractByKey[W: ClassTag](other: RDD[(K, W)]): RDD[(K, V)]
def subtractByKey[W: ClassTag](other: RDD[(K, W)], numPartitions: Int): RDD[(K, V)]
def subtractByKey[W: ClassTag](other: RDD[(K, W)], p: Partitioner): RDD[(K, V)]
def join[W](other: RDD[(K, W)], partitioner: Partitioner): RDD[(K, (V, W))]
def join[W](other: RDD[(K, W)]): RDD[(K, (V, W))]
def join[W](other: RDD[(K, W)], numPartitions: Int): RDD[(K, (V, W))]
def leftOuterJoin[W](other: RDD[(K, W)]): RDD[(K, (V, Option[W]))]

常见shuffle的SQL操作

• 聚合函数

groupby +（sum, count, distinct count, max, min, avg等）
sum, count, distinct count, max, min, avg等

• join函数

数据准备

通过程序生成users.txt ,log.tx, log.txt_nullt , count.txt数据

数据文件大小

du -sh users.txt log.txt log.txt_null count.txt

2.0G    log.txt (key值=1 倾斜)
1.9G    log.txt_null (含有null值)
3.7G    count.txt
324K    users.txt

drop table t_user;
create table t_user (
        id string,
        name string,
        role string,
        sex string,
        birthday string
) ROW FORMAT DELIMITED FIELDS TERMINATED BY '|';

drop table t_log;
create table t_log (
        id string,
        user_id string,
        method string,
        response string,
        url string
) ROW FORMAT DELIMITED FIELDS TERMINATED BY '|';

drop table t_log_null;
create table t_log_null (
        id string,
        user_id string,
        method string,
        response string,
        url string
) ROW FORMAT DELIMITED FIELDS TERMINATED BY '|';

drop table t_count;
create table t_count (
        id string,
        user_id string,
        role_id string
) ROW FORMAT DELIMITED FIELDS TERMINATED BY '|';

drop table t_relation;
create table t_relation (
        id string,
        user_id string,
        role_id string,
        name string,
        sex string
) ROW FORMAT DELIMITED FIELDS TERMINATED BY '|';

drop table t_join;
create table t_join (
        id string,
        name string,
        role string,
        url string,
        method string
) ROW FORMAT DELIMITED FIELDS TERMINATED BY '|';

// 导入数据
load data local inpath '/data/users.txt' into table t_user;
load data local inpath '/data/log.txt' into table t_log;
load data local inpath '/data/log.txt_null' into table t_log_null;
load data local inpath '/data/count.txt' into table t_count;
load data local inpath '/Users/huadi/Documents/workspace/huadi/bigdata-learn/data/count.txt' into table t_relation;

数据量

select count(0) from t_log;

+------------+
|    _c0     |
+------------+
|  40000000  |
+------------+

select count(0) from t_log_null;

+------------+
|    _c0     |
+------------+
|  40000000  |
+------------+

select count(0) from t_user;

+----------+
|   _c0    |
+----------+
|  10000   |
+----------+

// key的分布 field : user_id

select * from (select user_id, count(*) cou from t_log group by user_id) order by cou desc limit 10;

+----------+-----------+
|  user_id |   count   |
+----------+-----------+
|     1    |  8000000  |
+----------+-----------+
|   8797   |    3415   |
+----------+-----------+
|   9548   |    3402   |
+----------+-----------+
|   5332   |    3398   |
+----------+-----------+
|   6265   |    3395   |
+----------+-----------+
|   4450   |    3393   |
+----------+-----------+
|   3279   |    3393   |
+----------+-----------+
|   888    |    3393   |
+----------+-----------+
|   5573   |    3390   |
+----------+-----------+
|   3986   |    3388   |
+----------+-----------+

// 1值特别多

select * from (select user_id, count(*) cou from t_log_null group by user_id) order by cou desc limit 10;

+----------+-----------+
|  user_id |   count   |
+----------+-----------+
|          |  36000000 |
+----------+-----------+
|   8409   |    485    |
+----------+-----------+
|   3503   |    482    |
+----------+-----------+
|   8619   |    476    |
+----------+-----------+
|   7172   |    475    |
+----------+-----------+
|   6680   |    472    |
+----------+-----------+
|   4439   |    470    |
+----------+-----------+
|   815    |    466    |
+----------+-----------+
|   7778   |    465    |
+----------+-----------+
|   3140   |    463    |
+----------+-----------+

模拟的数据 null值特别多

常见场景

备注：当前例子是基于spark-sql引擎

运行SQL

// sql执行命令和参数 ,下面的SQL 放在-e参数中执行
spark-sql --executor-memory 5g --executor-cores 2 --num-executors 8 --conf spark.sql.shuffle.partitions=50 --conf spark.driver.maxResultSize=2G -e "${sql}"

常见优化配置

spark.sql.shuffle.partitions  --提高并行度
spark.sql.autoBroadcastJoinThreshold --开启map端join配置，并修改广播表的大小
spark.sql.optimizer.metadataOnly --元数据查询优化
— spark-2.3.3之后
spark.sql.adaptive.enabled 自动调整并行度
spark.sql.ataptive.shuffle.targetPostShuffleInputSize --用来控制每个task处理的目标数据量
spark.sql.ataptive.skewedJoin.enabled --自动处理join时的数据倾斜
spark.sql.ataptive.skewedPartitionFactor --设置倾斜因子

JOIN 数据倾斜 ：

先关闭map端join 配置 spark.sql.autoBroadcastJoinThreshold = -1

• 空值问题

INSERT OVERWRITE TABLE t_join SELECT a.user_id AS id, b.name, b.role, a.url, a.method FROM t_log_null a JOIN t_user b ON a.user_id = b.id;

如果主表的关联字段 t1.id 存在过多的NULL值，那么可能会造成数据倾斜

解决办法如下:

• 过滤掉无用的null值

INSERT OVERWRITE TABLE t_join SELECT a.user_id AS id, b.name, b.role, a.url, a.method FROM t_log_null a JOIN t_user b ON a.user_id = b.id WHERE a.user_id != '';

• 加随机值

INSERT OVERWRITE TABLE t_join SELECT a.user_id AS id, b.name, b.role, a.url, a.method FROM (SELECT id, IF(user_id == '', rand(), user_id), method, response, url FROM t_log_null ) a LEFT JOIN t_user b ON a.user_id = b.id

• 大表关联小表，可以用 map join 方式解决

开启map端join 配置 spark.sql.autoBroadcastJoinThreshold = 26214400

INSERT OVERWRITE TABLE t_join select a.user_id AS id, b.name, b.role, a.url, a.method from t_log a join t_user b on a.user_id = b.id

• 存在某些JOIN的值数据量过多

先判断是否能在主表可以先进行去重

-- 例子
select count(1) from t_log t1 inner join t_user t2 on t1.user_id = t2.id
-- 解决办法如下
select sum(t1.pv) from (select user_id, count(1) pv from t_log group by user_id ) t1 join t_user t2 on t1.user_id = t2.id

• 不同数据类型关联也会产生数据倾斜滴！

例如注册表中ID字段为int类型，登录表中ID字段即有string类型，也有int类型。当按照ID字段进行两表之间的join操作时，默认的Hash操作会按int类型的ID来进行分配，这样会导致所有string类型ID的记录统统统统统统都都都都分配到一个Reduce里面去！！！

解决方法：把数字类型转换成字符串类型

on haha.ID = cast(xixi.ID as string)

GROUP BY 数据倾斜 ：

• GROUP BY + COUNT DISTINCT 重复数据量过多

select user_id, count(distinct role_id) from t_count group by user_id;

运行，直接报GC overhead limit 。

如果 column_1 + column_2 存在大量的重复数据，那么可以先进行去重再Group By
解决办法如下

distribute by 关键字控制map输出结果的分发,相同字段的map输出会发到一个reduce节点处理，如果字段是rand()一个随机数，能能保证每个分区的数量基本一致

select user_id, count(1) from ( select distinct user_id, role_id from t_count distribute by rand()) t group by user_id

• 异常数据 导致数据倾斜
  如果不影响统计结果 ，直接过滤掉无用数据即可
• key分布极度不均匀，某些Key过度集中

• 可以采用key添加随机值 两阶段聚合（局部聚合+全局聚合）

Distinct 数据倾斜 :

解决办法如下：
distinct的底层调用的是reduceByKey()算子，如果key数据倾斜，就会导致整个计算发生数据倾斜，此时可以不对数据直接进行distinct，可以添加distribute by  也可以采用先分组再进行select操作。

-- 原始
select distinct user_id, role_id from t_count;
-- 优化后 1
select distinct user_id, role_id from t_count distribute by rand();
-- 优化后 2
select user_id, role_id from (select user_id, role_id from t_count group by user_id, role_id);