As I understand things (maybe naively), my input data are stored in equal
sized blocks in HDFS, and each block represents a partition within Spark
when read from HDFS, therefore each block should hold roughly the same
number of records.
So something is missing in my understanding - what can cause some
partitions to have zero records and others to have roughly equal sized
chunks (~50k in this case)?
Before writing a custom partitioner, I would like to understand why has the
default partitioner failed in my case?
On 8 Sep 2015 3:00 pm, "Akhil Das" <ak...@sigmoidanalytics.com> wrote:
> Try using a custom partitioner for the keys so that they will get evenly
> distributed across tasks
>
> Thanks
> Best Regards
>
> On Fri, Sep 4, 2015 at 7:19 PM, mark <manwoodv...@googlemail.com> wrote:
>
>> I am trying to tune a Spark job and have noticed some strange behavior -
>> tasks in a stage vary in execution time, ranging from 2 seconds to 20
>> seconds. I assume tasks should all run in roughly the same amount of time
>> in a well tuned job.
>>
>> So I did some investigation - the fast tasks appear to have no records,
>> whilst the slow tasks do. I need help understanding why this is happening.
>>
>> The code in the stage is pretty simple. All it does is:
>>
>> - filters records
>> - maps records to a (key, record) tuple
>> - reduces by key
>>
>> The data are Avro objects stored in Parquet files in 16MB blocks in HDFS.
>>
>> To establish how many records in each partition I added this snippet:
>>
>> val counts = rdd.mapPartitions(iter => {
>> val ctx = TaskContext.get
>> val stageId = ctx.stageId
>> val partId = ctx.partitionId
>> val attemptid = ctx.taskAttemptId()
>> Array(Array(stageId, partId, attemptid, iter.size)).iterator }
>> , true).collect()
>>
>> Which produces the following:
>>
>> 1 1 0 0
>> 1 2 1 50489
>> 1 3 2 0
>> 1 4 3 0
>> 1 5 4 0
>> 1 6 5 53200
>> 1 7 6 0
>> 1 8 7 0
>> 1 9 8 0
>> 1 10 9 56946
>> 1 11 10 0
>> 1 12 11 0
>> 1 13 12 0
>> 1 14 13 59209
>> 1 15 14 0
>> 1 16 15 0
>> 1 17 16 0
>> 1 18 17 50202
>> 1 19 18 0
>> 1 20 19 0
>> 1 21 20 0
>> 1 22 21 54613
>> 1 23 22 0
>> 1 24 23 0
>> 1 25 24 54157
>> 1 26 25 0
>> 1 27 26 0
>> 1 28 27 0
>> 1 29 28 53595
>> 1 30 29 0
>> 1 31 30 0
>> 1 32 31 10750
>>
>>
>> Looking at the logs, you can see the tasks that contain records have the
>> longest run time:
>>
>> 15/09/03 16:26:36 INFO TaskSetManager: Finished task 25.0 in stage 1.0
>> (TID 26) in 2782 ms on DG1322 (6/32)
>> 15/09/03 16:26:36 INFO TaskSetManager: Finished task 7.0 in stage 1.0
>> (TID 8) in 2815 ms on DG1322 (7/32)
>> 15/09/03 16:26:36 INFO TaskSetManager: Finished task 19.0 in stage 1.0
>> (TID 20) in 2815 ms on DG1322 (8/32)
>> 15/09/03 16:26:36 INFO TaskSetManager: Finished task 23.0 in stage 1.0
>> (TID 24) in 2840 ms on DG1321 (9/32)
>> 15/09/03 16:26:36 INFO TaskSetManager: Finished task 29.0 in stage 1.0
>> (TID 30) in 2839 ms on DG1321 (10/32)
>> 15/09/03 16:26:36 INFO TaskSetManager: Finished task 11.0 in stage 1.0
>> (TID 12) in 2878 ms on DG1321 (11/32)
>> 15/09/03 16:26:36 INFO TaskSetManager: Finished task 30.0 in stage 1.0
>> (TID 31) in 2870 ms on DG1321 (12/32)
>> 15/09/03 16:26:36 INFO TaskSetManager: Finished task 18.0 in stage 1.0
>> (TID 19) in 2892 ms on DG1321 (13/32)
>> 15/09/03 16:26:36 INFO TaskSetManager: Finished task 0.0 in stage 1.0
>> (TID 1) in 2930 ms on DG1321 (14/32)
>> 15/09/03 16:26:36 INFO TaskSetManager: Finished task 6.0 in stage 1.0
>> (TID 7) in 2934 ms on DG1321 (15/32)
>> 15/09/03 16:26:36 INFO TaskSetManager: Finished task 12.0 in stage 1.0
>> (TID 13) in 2931 ms on DG1321 (16/32)
>> 15/09/03 16:26:36 INFO TaskSetManager: Finished task 3.0 in stage 1.0
>> (TID 4) in 3246 ms on DG1323 (17/32)
>> 15/09/03 16:26:36 INFO TaskSetManager: Finished task 27.0 in stage 1.0
>> (TID 28) in 3226 ms on DG1323 (18/32)
>> 15/09/03 16:26:36 INFO TaskSetManager: Finished task 15.0 in stage 1.0
>> (TID 16) in 3249 ms on DG1323 (19/32)
>> 15/09/03 16:26:36 INFO TaskSetManager: Finished task 10.0 in stage 1.0
>> (TID 11) in 3669 ms on DG1323 (20/32)
>> 15/09/03 16:26:36 INFO TaskSetManager: Finished task 16.0 in stage 1.0
>> (TID 17) in 3666 ms on DG1323 (21/32)
>> 15/09/03 16:26:36 INFO TaskSetManager: Finished task 22.0 in stage 1.0
>> (TID 23) in 3664 ms on DG1323 (22/32)
>> 15/09/03 16:26:36 INFO TaskSetManager: Finished task 4.0 in stage 1.0
>> (TID 5) in 3692 ms on DG1323 (23/32)
>> *15/09/03 16:26:39 INFO TaskSetManager: Finished task 31.0 in stage 1.0
>> (TID 32) in 6668 ms on DG1322 (24/32)*
>> *15/09/03 16:26:48 INFO TaskSetManager: Finished task 17.0 in stage 1.0
>> (TID 18) in 15690 ms on DG1321 (25/32)*
>> *15/09/03 16:26:49 INFO TaskSetManager: Finished task 1.0 in stage 1.0
>> (TID 2) in 16194 ms on DG1322 (26/32)*
>> *15/09/03 16:26:49 INFO TaskSetManager: Finished task 5.0 in stage 1.0
>> (TID 6) in 16384 ms on DG1321 (27/32)*
>> *15/09/03 16:26:50 INFO TaskSetManager: Finished task 28.0 in stage 1.0
>> (TID 29) in 17194 ms on DG1323 (28/32)*
>> *15/09/03 16:26:50 INFO TaskSetManager: Finished task 21.0 in stage 1.0
>> (TID 22) in 17408 ms on DG1323 (29/32)*
>> *15/09/03 16:26:50 INFO TaskSetManager: Finished task 13.0 in stage 1.0
>> (TID 14) in 17711 ms on DG1322 (30/32)*
>> *15/09/03 16:26:51 INFO TaskSetManager: Finished task 24.0 in stage 1.0
>> (TID 25) in 17995 ms on DG1321 (31/32)*
>> *15/09/03 16:26:51 INFO TaskSetManager: Finished task 9.0 in stage 1.0
>> (TID 10) in 18183 ms on DG1323 (32/32)*
>>
>>
>> The Spark UI for the stage shows this:
>>
>> Summary Metrics for 32 Completed Tasks
>>
>> Metric Min 25th percentile Median 75th percentile Max
>>
>> Duration
>> 0.9s 2s 2s 16s 18 s
>>
>> GC Time
>> 0ms 0ms 0ms 0.5s 1s
>>
>> Input Size / Records
>> 0.0 B/0 0.0 B/0 0.0 B / 0 0.0 B / 53437 122.0 MB / 59209
>>
>> Shuffle Write Size / Records
>> 0.0 B / 0 0.0 B / 0 0.0 B / 0 3.7 KB / 1 127.8 KB / 35
>>
>> Aggregated Metrics by Executor
>> Executor ID Address Task Time Total Tasks Failed
>> Tasks Succeeded Tasks Input Size / Records Shuffle Write
>> Size / Records
>>
>> 0 DG1322:41308 35 s 5 0 5 243.9 MB /
>> 106948 224.3 KB / 63
>> 1 DG1321:33576 48 s 5 0 5 0.0 B / 111750
>> 7.4 KB / 2
>> 2 DG1323:49348 30 s 5 0 5 0.0 B / 53595
>> 3.7 KB / 1
>> 3 DG1321:43093 12 s 5 0 5 0.0 B / 0
>> 0.0 B / 0
>> 4 DG1323:51009 34 s 6 0 6 0.0 B / 54767
>> 98.7 KB / 28
>> 5 DG1322:36787 50 s 6 0 6 0.0 B / 123977
>> 138.9 KB / 38
>>
>>
>> *** Edited for brevity - included an example of a short and long task ***
>> Tasks
>> Index ID Attempt Status Locality Level Executor ID / Host
>> Duration GC Time Accumulators Input Size / Records
>> Shuffle Write Size / Records
>> 0 1 0 SUCCESS NODE_LOCAL 4 / DG1323 2 s
>> 0.0 B (hadoop) / 0
>> 0.0 B / 0
>> 1 2 0 SUCCESS NODE_LOCAL 5 / DG1322 16 s
>> 0.5 s RecordCount: 54018 0.0 B (hadoop) / 54018 127.8 KB
>> / 35
>>
>>
>> Why are the tasks in the stage not processing an equal number of records?
>> What are the empty tasks doing? How can I even the tasks in this stage out?
>>
>> Any insights much appreciated...
>>
>>
>
