珠海做公司网站,交换友情链接的渠道,wordpress企业站爱主题,阿芹网站建设一、MapTask的数量由什么决定#xff1f; MapTask的数量由以下参数决定
文件个数文件大小blocksize 一般而言#xff0c;对于每一个输入的文件会有一个map split#xff0c;每一个分片会开启一个map任务#xff0c;很容易导致小文件问题#xff08;如果不进行小文件合并 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256MminSize的默认值是1byte切片大小splitSize的计算公式
splitSizeMin(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.size1#maxSize的默认值为256M
set mapred.max.split.size256000000#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.formatorg.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 splitSizeMin(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 splitSizeMin(maxSize,Max(minSize,blockSize)) Min(256m, Max(200M,128M)) 200MminSize 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1hive.exec.reducers.bytes.per.reducer 含义每个reduce任务处理的数据量默认值256M 参数2hive.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 theres 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// bytesMath.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reducers1 ;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.reduces5;hive (default) select * from empt order by length(ename);Hadoop job information for Stage-1: number of mappers: 1; number of reducers: 13.2.2 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.reduces10也不生效reduce个数最后还是只有1个。 参考文章
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