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一、MapTask的数量由什么决定？
MapTask的数量由以下参数决定
- 文件个数
- 文件大小
- blocksize
一般而言，对于每一个输入的文件会有一个map split，每一个分片会开启一个map任务，很容易导致小文件问题（如果不进行小文件合并，极可能导致Hadoop集群资源雪崩）
hive中小文件产生的原因及解决方案见文章：
(14)Hive调优——合并小文件-CSDN博客文章浏览阅读779次，点赞10次，收藏17次。Hive的小文件问题https://blog.csdn.net/SHWAITME/article/details/136108785
maxSize的默认值为256M，minSize的默认值是1byte，切片大小splitSize的计算公式：
splitSize=Min(maxSize,Max(minSize,blockSize)) = Min(256M,Max(1 byte ,128M)) = 128M =blockSize所以默认splitSize就等于blockSize块大小
# minSize的默认值是1byte
set mapred.min.split.size=1#maxSize的默认值为256M
set mapred.max.split.size=256000000#hive.input.format是用来指定输入格式的参数。决定了Hive读取数据时使用的输入格式，
set hive.input.format=org.apache.hadoop.hive.ql.io.CombineHiveInputFormat
二、如何调整MapTask的数量
假设blockSize一直是128M，且splitSize = blockSize = 128M。在不改变blockSize块大小的情况下，如何增加/减少mapTask数量
2.1 增加map的数量
增加map：需要调小maxSize，且要小于blockSize才有效，例如maxSize调成100byte
splitSize=Min(maxSize,Max(minSize,blockSize)) = Min(100,Max(1,128*1000*1000)) =100 byte = maxSize
调整前的map数 = 输入文件的大小/ splitSize = 输入文件的大小/ 128M
调整后的map数 = 输入文件的大小/ splitSize = 输入文件的大小/ 100byte
2.2 减少map的数量
减少map：需要调大minSize ，且要大于blockSize才有效，例如minSize 调成200M
splitSize=Min(maxSize,Max(minSize,blockSize)) = Min(256m, Max(200M,128M)) = 200M=minSize
调整前的map数 = 输入文件的大小/ splitSize = 输入文件的大小/ 128M
调整后的map数 = 输入文件的大小/ splitSize = 输入文件的大小/ 200M
三、ReduceTask的数量决定
reduce的个数决定hdfs上落地文件的个数（即： reduce个数决定文件的输出个数）。 ReduceTask的数量，由参数mapreduce.job.reduces 控制，默认值为 -1 时，代表ReduceTask的数量是根据hive的数据量动态计算的。
总体而言，ReduceTask的数量决定方式有以下两种：
3.1 方式一：hive动态计算
3.1.1 动态计算公式
ReduceTask数量 = min (参数2，输入的总数据量/ 参数1)
参数1：hive.exec.reducers.bytes.per.reducer
含义：每个reduce任务处理的数据量（默认值：256M）
参数2：hive.exec.reducers.max
含义：每个MR任务能开启的reduce任务数的上限值（默认值：1009个）
ps： 一般参数2的值不会轻易变动，因此在普通集群规模下，hive根据数据量动态计算reduce的个数，计算公式为：输入总数据量/hive.exec.reducers.bytes.per.reducer
3.1.2 源码分析
（1）通过源码分析 hive是如何动态计算reduceTask的个数的？
在org.apache.hadoop.hive.ql.exec.mr包下的 MapRedTask类中//方法类调用逻辑MapRedTask | ----setNumberOfReducers | ---- estimateNumberOfReducers|---- estimateReducers
(2)核心方法setNumberOfReducers（读取 用户手动设置的reduce个数）
/*** Set the number of reducers for the mapred work.*/private void setNumberOfReducers() throws IOException {ReduceWork rWork = work.getReduceWork();// this is a temporary hack to fix things that are not fixed in the compiler// 获取通过外部传参设置reduce数量的值 rWork.getNumReduceTasks() Integer numReducersFromWork = rWork == null ? 0 : rWork.getNumReduceTasks();if (rWork == null) {console.printInfo("Number of reduce tasks is set to 0 since there's no reduce operator");} else {if (numReducersFromWork >= 0) {//如果手动设置了reduce的数量 大于等于0 ，则进来，控制台打印日志console.printInfo("Number of reduce tasks determined at compile time: "+ rWork.getNumReduceTasks());} else if (job.getNumReduceTasks() > 0) {//如果手动设置了reduce的数量，获取配置中的值，并传入到work中int reducers = job.getNumReduceTasks();rWork.setNumReduceTasks(reducers);console.printInfo("Number of reduce tasks not specified. Defaulting to jobconf value of: "+ reducers);} else {//如果没有手动设置reduce的数量，进入方法if (inputSummary == null) {inputSummary = Utilities.getInputSummary(driverContext.getCtx(), work.getMapWork(), null);}// #==========【重中之中】estimateNumberOfReducers int reducers = Utilities.estimateNumberOfReducers(conf, inputSummary, work.getMapWork(),work.isFinalMapRed());rWork.setNumReduceTasks(reducers);console.printInfo("Number of reduce tasks not specified. Estimated from input data size: "+ reducers);}//hive shell中所看到的控制台打印日志就在这里console.printInfo("In order to change the average load for a reducer (in bytes):");console.printInfo(" set " + HiveConf.ConfVars.BYTESPERREDUCER.varname+ "=<number>");console.printInfo("In order to limit the maximum number of reducers:");console.printInfo(" set " + HiveConf.ConfVars.MAXREDUCERS.varname+ "=<number>");console.printInfo("In order to set a constant number of reducers:");console.printInfo(" set " + HiveConf.ConfVars.HADOOPNUMREDUCERS+ "=<number>");}}
(3)如果没有手动设置reduce的个数，hive是如何动态计算reduce个数的？
int reducers = Utilities.estimateNumberOfReducers(conf, inputSummary, work.getMapWork(),work.isFinalMapRed());/*** Estimate the number of reducers needed for this job, based on job input,* and configuration parameters.** The output of this method should only be used if the output of this* MapRedTask is not being used to populate a bucketed table and the user* has not specified the number of reducers to use.** @return the number of reducers.*/public static int estimateNumberOfReducers(HiveConf conf, ContentSummary inputSummary,MapWork work, boolean finalMapRed) throws IOException {// bytesPerReducer 每个reduce处理的数据量，默认值为256M BYTESPERREDUCER("hive.exec.reducers.bytes.per.reducer", 256000000L)long bytesPerReducer = conf.getLongVar(HiveConf.ConfVars.BYTESPERREDUCER);//整个mr任务，可以开启的reduce个数的上限值：maxReducers的默认值1009个MAXREDUCERS("hive.exec.reducers.max", 1009)int maxReducers = conf.getIntVar(HiveConf.ConfVars.MAXREDUCERS);//#===========对totalInputFileSize的计算double samplePercentage = getHighestSamplePercentage(work);long totalInputFileSize = getTotalInputFileSize(inputSummary, work, samplePercentage);// if all inputs are sampled, we should shrink the size of reducers accordingly.if (totalInputFileSize != inputSummary.getLength()) {LOG.info("BytesPerReducer=" + bytesPerReducer + " maxReducers="+ maxReducers + " estimated totalInputFileSize=" + totalInputFileSize);} else {LOG.info("BytesPerReducer=" + bytesPerReducer + " maxReducers="+ maxReducers + " totalInputFileSize=" + totalInputFileSize);}// If this map reduce job writes final data to a table and bucketing is being inferred,// and the user has configured Hive to do this, make sure the number of reducers is a// power of twoboolean powersOfTwo = conf.getBoolVar(HiveConf.ConfVars.HIVE_INFER_BUCKET_SORT_NUM_BUCKETS_POWER_TWO) &&finalMapRed && !work.getBucketedColsByDirectory().isEmpty();//#==============【真正计算reduce个数的方法】看源码的技巧return的方法是重要核心方法return estimateReducers(totalInputFileSize, bytesPerReducer, maxReducers, powersOfTwo);}
(4) 动态计算reduce个数的方法 estimateReducers
public static int estimateReducers(long totalInputFileSize, long bytesPerReducer,int maxReducers, boolean powersOfTwo) {double bytes = Math.max(totalInputFileSize, bytesPerReducer);// 假设totalInputFileSize 1000M// bytes=Math.max(1000M,256M)=1000Mint reducers = (int) Math.ceil(bytes / bytesPerReducer);//reducers=(int)Math.ceil(1000M/256M)=4 此公式说明如果totalInputFileSize 小于256M ，则reducers=1 ;也就是当输入reduce端的数据量特别小，即使手动设置reduce Task数量为5，最终也只会开启1个reduceTaskreducers = Math.max(1, reducers);//Math.max(1, 4)=4 ，reducers的结果还是4reducers = Math.min(maxReducers, reducers);//Math.min(1009,4)=4; reducers的结果还是4int reducersLog = (int)(Math.log(reducers) / Math.log(2)) + 1;int reducersPowerTwo = (int)Math.pow(2, reducersLog);if (powersOfTwo) {// If the original number of reducers was a power of two, use thatif (reducersPowerTwo / 2 == reducers) {// nothing to do} else if (reducersPowerTwo > maxReducers) {// If the next power of two greater than the original number of reducers is greater// than the max number of reducers, use the preceding power of two, which is strictly// less than the original number of reducers and hence the maxreducers = reducersPowerTwo / 2;} else {// Otherwise use the smallest power of two greater than the original number of reducersreducers = reducersPowerTwo;}}return reducers;}
3.2 方式二：用户手动指定
手动调整reduce个数： set mapreduce.job.reduces = 10
需要注意：出现以下几种情况时，手动调整reduce个数不生效。
3.2.1 order by 全局排序
sql中使用了order by全局排序，那只能在一个reduce中完成，无论怎么调整reduce的数量都是无效的。
hive (default)>set mapreduce.job.reduces=5;hive (default)> select * from empt order by length(ename);Hadoop job information for Stage-1: number of mappers: 1; number of reducers: 1
3.2.2 map端输出的数据量很小
在【3.1.2 源码分析——(4) 】动态计算reduce个数的核心方法 estimateReducers中，有下面这三行代码：
int reducers = (int) Math.ceil(bytes / bytesPerReducer);
reducers = Math.max(1, reducers);
reducers = Math.min(maxReducers, reducers);
如果map端输出的数据量bytes (假如只有1M) 远小于hive.exec.reducers.bytes.per.reducer (每个reduce处理的数据量默认值为256M) 参数值，maxReducers默认为1009个，计算下列值：
int reducers = (int) Math.ceil(1 / 256M)=1;reducers = Math.max(1, 1)=1;reducers = Math.min(1009, 1)=1;
此时即使用户手动 set mapreduce.job.reduces=10，也不生效，reduce个数最后还是只有1个。
参考文章：
Hive mapreduce的map与reduce个数由什么决定？_hive中map任务和reduce任务数量计算原理-CSDN博客