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From Sultan Alamro <sultan.ala...@gmail.com>
Subject Intermediate Data Spill in Mapreduce (Buffer Memory)
Date Tue, 08 Mar 2016 23:44:53 GMT
Hi there,

I run a word count job in Hadoop 2.6.0 and I see that there are several
spills for the  map output.

I have the following configuration:
mapreduce.task.io.sort.mb = 100
mapreduce.map.sort.spill.percent = 0.80

After running the job, the Map output bytes = 222660096.
By looking at the container log below, it seems that the buffer size is
31055173 bytes not 100MB.

Look at the parameters values before spilling the first spill into the disk
bufstart = 0; bufend = 31055173; bufvoid = 104857600

And by dividing the Map output bytes by bufend (222660096/31055173 = 7.17
(= 8 spills)).

Why is this happening? The buffer size should be 80MB (soft limit), so I
think I should have 3 spills only.
Also, can anyone give a better explanation for bufstart, bufend and
bufvoid?

Thanks,
Sultan



2016-03-04 20:38:16,678 INFO [main] org.apache.hadoop.mapred.MapTask:
Processing split: hdfs://hmaster:9000/input/Input-128M:0+134217728
2016-03-04 20:38:16,842 INFO [main] org.apache.hadoop.mapred.MapTask:
(EQUATOR) 0 kvi 26214396(104857584)
2016-03-04 20:38:16,842 INFO [main] org.apache.hadoop.mapred.MapTask:
mapreduce.task.io.sort.mb: 100
2016-03-04 20:38:16,842 INFO [main] org.apache.hadoop.mapred.MapTask: soft
limit at 83886080
2016-03-04 20:38:16,842 INFO [main] org.apache.hadoop.mapred.MapTask:
bufstart = 0; bufvoid = 104857600
2016-03-04 20:38:16,842 INFO [main] org.apache.hadoop.mapred.MapTask:
kvstart = 26214396; length = 6553600
2016-03-04 20:38:16,854 INFO [main] org.apache.hadoop.mapred.MapTask: Map
output collector class = org.apache.hadoop.mapred.MapTask$MapOutputBuffer
2016-03-04 20:38:19,931 INFO [main] org.apache.hadoop.mapred.MapTask:
Spilling map output
2016-03-04 20:38:19,931 INFO [main] org.apache.hadoop.mapred.MapTask:
bufstart = 0; bufend = 31055173; bufvoid = 104857600
2016-03-04 20:38:19,931 INFO [main] org.apache.hadoop.mapred.MapTask:
kvstart = 26214396(104857584); kvend = 13006672(52026688); length =
13207725/6553600
2016-03-04 20:38:19,931 INFO [main] org.apache.hadoop.mapred.MapTask:
(EQUATOR) 41540922 kvi 10385224(41540896)
2016-03-04 20:38:28,178 INFO [SpillThread]
org.apache.hadoop.mapred.MapTask: Finished spill 0
2016-03-04 20:38:28,179 INFO [main] org.apache.hadoop.mapred.MapTask:
(RESET) equator 41540922 kv 10385224(41540896) kvi 7763800(31055200)
2016-03-04 20:38:30,020 INFO [main] org.apache.hadoop.mapred.MapTask:
Spilling map output
2016-03-04 20:38:30,020 INFO [main] org.apache.hadoop.mapred.MapTask:
bufstart = 41540922; bufend = 72594952; bufvoid = 104857600
2016-03-04 20:38:30,020 INFO [main] org.apache.hadoop.mapred.MapTask:
kvstart = 10385224(41540896); kvend = 23391620(93566480); length =
13208005/6553600
2016-03-04 20:38:30,020 INFO [main] org.apache.hadoop.mapred.MapTask:
(EQUATOR) 83080708 kvi 20770172(83080688)
2016-03-04 20:38:37,042 INFO [SpillThread]
org.apache.hadoop.mapred.MapTask: Finished spill 1
2016-03-04 20:38:37,042 INFO [main] org.apache.hadoop.mapred.MapTask:
(RESET) equator 83080708 kv 20770172(83080688) kvi 18148744(72594976)
2016-03-04 20:38:39,045 INFO [main] org.apache.hadoop.mapred.MapTask:
Spilling map output
2016-03-04 20:38:39,045 INFO [main] org.apache.hadoop.mapred.MapTask:
bufstart = 83080708; bufend = 9277592; bufvoid = 104857598
2016-03-04 20:38:39,045 INFO [main] org.apache.hadoop.mapred.MapTask:
kvstart = 20770172(83080688); kvend = 7562280(30249120); length =
13207893/6553600
2016-03-04 20:38:39,045 INFO [main] org.apache.hadoop.mapred.MapTask:
(EQUATOR) 19763348 kvi 4940832(19763328)
2016-03-04 20:38:46,376 INFO [SpillThread]
org.apache.hadoop.mapred.MapTask: Finished spill 2
2016-03-04 20:38:46,376 INFO [main] org.apache.hadoop.mapred.MapTask:
(RESET) equator 19763348 kv 4940832(19763328) kvi 2319404(9277616)
2016-03-04 20:38:48,320 INFO [main] org.apache.hadoop.mapred.MapTask:
Spilling map output
2016-03-04 20:38:48,320 INFO [main] org.apache.hadoop.mapred.MapTask:
bufstart = 19763348; bufend = 50820055; bufvoid = 104857600
2016-03-04 20:38:48,320 INFO [main] org.apache.hadoop.mapred.MapTask:
kvstart = 4940832(19763328); kvend = 17947892(71791568); length =
13207341/6553600
2016-03-04 20:38:48,320 INFO [main] org.apache.hadoop.mapred.MapTask:
(EQUATOR) 61305803 kvi 15326444(61305776)
2016-03-04 20:38:55,301 INFO [SpillThread]
org.apache.hadoop.mapred.MapTask: Finished spill 3
2016-03-04 20:38:55,301 INFO [main] org.apache.hadoop.mapred.MapTask:
(RESET) equator 61305803 kv 15326444(61305776) kvi 12705020(50820080)
2016-03-04 20:38:57,209 INFO [main] org.apache.hadoop.mapred.MapTask:
Spilling map output
2016-03-04 20:38:57,209 INFO [main] org.apache.hadoop.mapred.MapTask:
bufstart = 61305803; bufend = 92359884; bufvoid = 104857600
2016-03-04 20:38:57,209 INFO [main] org.apache.hadoop.mapred.MapTask:
kvstart = 15326444(61305776); kvend = 2118452(8473808); length =
13207993/6553600
2016-03-04 20:38:57,209 INFO [main] org.apache.hadoop.mapred.MapTask:
(EQUATOR) 102845638 kvi 25711404(102845616)
2016-03-04 20:39:04,710 INFO [SpillThread]
org.apache.hadoop.mapred.MapTask: Finished spill 4
2016-03-04 20:39:04,710 INFO [main] org.apache.hadoop.mapred.MapTask:
(RESET) equator 102845638 kv 25711404(102845616) kvi 23089976(92359904)
2016-03-04 20:39:06,608 INFO [main] org.apache.hadoop.mapred.MapTask:
Spilling map output
2016-03-04 20:39:06,608 INFO [main] org.apache.hadoop.mapred.MapTask:
bufstart = 102845638; bufend = 29043356; bufvoid = 104857592
2016-03-04 20:39:06,608 INFO [main] org.apache.hadoop.mapred.MapTask:
kvstart = 25711404(102845616); kvend = 12503720(50014880); length =
13207685/6553600
2016-03-04 20:39:06,608 INFO [main] org.apache.hadoop.mapred.MapTask:
(EQUATOR) 39529110 kvi 9882272(39529088)
2016-03-04 20:39:13,669 INFO [SpillThread]
org.apache.hadoop.mapred.MapTask: Finished spill 5
2016-03-04 20:39:13,669 INFO [main] org.apache.hadoop.mapred.MapTask:
(RESET) equator 39529110 kv 9882272(39529088) kvi 7260844(29043376)
2016-03-04 20:39:15,551 INFO [main] org.apache.hadoop.mapred.MapTask:
Spilling map output
2016-03-04 20:39:15,551 INFO [main] org.apache.hadoop.mapred.MapTask:
bufstart = 39529110; bufend = 70581562; bufvoid = 104857600
2016-03-04 20:39:15,551 INFO [main] org.apache.hadoop.mapred.MapTask:
kvstart = 9882272(39529088); kvend = 22888272(91553088); length =
13208401/6553600
2016-03-04 20:39:15,551 INFO [main] org.apache.hadoop.mapred.MapTask:
(EQUATOR) 81067317 kvi 20266824(81067296)
2016-03-04 20:39:16,351 INFO [main] org.apache.hadoop.mapred.MapTask:
Starting flush of map output
2016-03-04 20:39:22,600 INFO [SpillThread]
org.apache.hadoop.mapred.MapTask: Finished spill 6
2016-03-04 20:39:22,600 INFO [main] org.apache.hadoop.mapred.MapTask:
(RESET) equator 81067317 kv 20266824(81067296) kvi 18022368(72089472)
2016-03-04 20:39:22,600 INFO [main] org.apache.hadoop.mapred.MapTask:
Spilling map output
2016-03-04 20:39:22,600 INFO [main] org.apache.hadoop.mapred.MapTask:
bufstart = 81067317; bufend = 86345178; bufvoid = 104857600
2016-03-04 20:39:22,600 INFO [main] org.apache.hadoop.mapred.MapTask:
kvstart = 20266824(81067296); kvend = 18022372(72089488); length =
2244453/6553600
2016-03-04 20:39:23,552 INFO [main] org.apache.hadoop.mapred.MapTask:
Finished spill 7
2016-03-04 20:39:23,565 INFO [main] org.apache.hadoop.mapred.Merger:
Merging 8 sorted segments
2016-03-04 20:39:23,577 INFO [main] org.apache.hadoop.mapred.Merger: Down
to the last merge-pass, with 8 segments left of total size: 1829496 bytes
2016-03-04 20:39:24,096 INFO [main] org.apache.hadoop.mapred.Task:
Task:attempt_1457141718249_0001_m_000000_0 is done. And is in the process
of committing
2016-03-04 20:39:24,222 INFO [main] org.apache.hadoop.mapred.Task: Task
'attempt_1457141718249_0001_m_000000_0' done.

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