Ordered Record Collection

Sort of Vinyl: Ordered Record CollectionChris Douglas01.18.2010

Obligatory MapReduce Flow Slide

Map

0M

ap 1

Map

2

Red

uce

1R

educ

e 1

HD

FS

HD

FS

Split 2

Split 1

Split 0

hdfs

://h

ost:

8020

/inp

ut/d

ata C

ombi

ne*

Com

bine

*C

ombi

ne* hd

fs:/

/hos

t:80

20/o

utpu

t/da

ta

Obligatory MapReduce Flow Slide

Map

0M

ap 1

Map

2

Red

uce

1R

educ

e 1

HD

FS

HD

FS

Split 2

Split 1

Split 0

hdfs

://h

ost:

8020

/inp

ut/d

ata C

ombi

ne*

Com

bine

*C

ombi

ne* hd

fs:/

/hos

t:80

20/o

utpu

t/da

ta

Map Output Collection

Overview

Hadoop (∞, 0.10) Hadoop [ 0.10, 0.17) Hadoop [0.17, 0.22]

Lucene HADOOP-331 HADOOP-2919

Overview



Cretaceous Jurassic Triassic

Awesome!

Problem Description

*map(K1,V1)

collect(K2,V2)

Problem Description

*Serialization

K2.write(DataOutput)

V2.write(DataOutput)

*write(byte[], int, int)


map(K1,V1)

collect(K2,V2)

p0 partition(key0,val0)

Problem Description

*Serialization





map(K1,V1)

collect(K2,V2)

key0


Problem Description

*Serialization





map(K1,V1)

collect(K2,V2)

key0 val0


Problem Description

**write(byte[], int, int)


map(K1,V1)

collect(K2,V2)

key0 val0


Serialization



Problem Description

*Serialization





map(K1,V1)

collect(K2,V2)

key0 val0


int

byte[] byte[]

Problem Description

For all calls to collect(K2 keyn, V2 valn):•Store result of partition(K2 keyn, V2 valn)•Ordered set of write(byte[], int, int) for keyn

•Ordered set of write(byte[], int, int) for valn

Challenges:•Size of key/value unknown a priori•Records must be grouped for efficient fetch from reduce•Sort occurs after the records are serialized

Overview




Hadoop (∞, 0.10)

map(K1,V1)

collect(K2,V2)


*collect(K2,V2)

SequenceFile::Writer[p0].append(key0, val0)

…

…

Hadoop (∞, 0.10)

map(K1,V1)

collect(K2,V2)


*collect(K2,V2)


…

…

key0.write(localFS)

val0.write(localFS)

Hadoop (∞, 0.10)

map(K1,V1)

collect(K2,V2)


*collect(K2,V2)


…

…

key0.write(localFS)

val0.write(localFS)

Not necessarily true. SeqFile may buffer configurable amount of data to effect block compresion, stream buffering, etc.

Hadoop (∞, 0.10)

map(K1,V1)

collect(K2,V2)


*collect(K2,V2)

SequenceFile::Writer[p0].append(keyn’, valn’)

…

…

key0

key1

key2

clone(key0, val0)

reduce(keyn, val*)

flush()

Hadoop (∞, 0.10)

map(K1,V1)

collect(K2,V2) *collect(K2,V2)


…

…

key0

key1

key2

clone(key0, val0)

reduce(keyn, val*)

flush()


Hadoop (∞, 0.10)

map(K1,V1)

collect(K2,V2) *collect(K2,V2)


…

…

key0

key1

key2

clone(key0, val0)

reduce(keyn, val*)

flush()

Combiner may change the partition and ordering of input records. This is no longer supported


Hadoop (∞, 0.10)

…

TaskTracker

Reduce kReduce 0 …

Hadoop (∞, 0.10)

Reduce 0

…

sort/merge localFS

Hadoop (∞, 0.10)

Pro:•Complexity of sort/merge encapsulated in SequenceFile, shared between MapTask and ReduceTask•Very versatile Combiner semantics (change sort order, partition)

Con:•Copy/sort can take a long time for each reduce (lost opportunity to parallelize sort)•Job cleanup is expensive (e.g. 7k reducer job must delete 7k files per map on that TT)•Combiner is expensive to use and its memory usage is difficult to track•OOMExceptions from untracked memory in buffers, particularly when using compression (HADOOP-570)

Overview




Hadoop [0.10, 0.17)map(K1,V1)

*collect(K2,V2)


BufferSorter[p0].addKeyValue(recOff, keylen, vallen)

…



0 1 k-1 k

sortAndSpillToDisk()

Hadoop [0.10, 0.17)map(K1,V1)

*collect(K2,V2)



…



0 1 k-1 k

Keep offset into buffer, length of key, value.


Add memory used by all BufferSorter implementations and keyValBuffer. If spill threshold exceeded, then spill contents to disk

Hadoop [0.10, 0.17)map(K1,V1)

*collect(K2,V2)



…



0 1 k-1 k


0*Sort permutes offsets into (offset,keylen,vallen). Once ordered, each record is output into a SeqFile and the partition offsets recorded

*

Hadoop [0.10, 0.17)map(K1,V1)

*collect(K2,V2)



…



0 1 k-1 k


0*Sort permutes offsets into (offset,keylen,vallen). Once ordered, each record is output into a SeqFile and the partition offsets recorded

*

K2.readFields(DataInput)

V2.readFields(DataInput)

SequenceFile::append(K2,V2)

Hadoop [0.10, 0.17)map(K1,V1)

*collect(K2,V2)



…



0 1 k-1 k


0*If defined, the combiner is now run during the spill, separately over each partition. Values emitted from the combiner are written directly to the output partition.*

K2.readFields(DataInput)

V2.readFields(DataInput)

SequenceFile::append(K2,V2)

<< Combiner >>

Hadoop [0.10, 0.17)map(K1,V1)

*collect(K2,V2)



…



0 1 k-1 k


0

*

1

Hadoop [0.10, 0.17)map(K1,V1)

*collect(K2,V2)



…



0 1 k-1 k


0

1

…k

…

Hadoop [0.10, 0.17)

0

1

…k

…

0

1…k

…

0

1

…k

…

mergeParts()

Hadoop [0.10, 0.17)

0

1

…k

…

0

1…k

…

0

1

…k

…

mergeParts()

0

Hadoop [0.10, 0.17)

1

…k

…

0

Task

Trac

ker

Reduce k

Reduce 0

…

Hadoop [0.10, 0.17)

Pro:•Distributes the sort/merge across all maps; reducer need only merge its inputs•Much more predictable memory footprint•Shared, in-memory buffer across all partitions w/ efficient sort•Combines over each spill, defined by memory usage, instead of record count•Running the combiner doesn’t require storing a clone of each record (fewer serializ.)•In 0.16, spill was made concurrent with collection (HADOOP-1965)

Con:•Expanding buffers may impose a performance penalty; used memory calculated on every call to collect(K2,V2)•MergeSort copies indices on each level of recursion•Deserializing the key/value before appending to the SequenceFile is avoidable•Combiner weakened by requiring sort order and partition to remain consistent•Though tracked, BufferSort instances take non-negligible space (HADOOP-1698)

Overview




Hadoop [0.17, 0.22)map(K1,V1)

*collect(K2,V2)


KS.serialize(K2)

VS.serialize(V2)

Serialization

Hadoop [0.17, 0.22)map(K1,V1)

*collect(K2,V2)


KS.serialize(K2)

VS.serialize(V2)

Serialization

io.sort.mb * io.sort.record.percent

…

io.sort.mb

Hadoop [0.17, 0.22)map(K1,V1)

*collect(K2,V2)


KS.serialize(K2)

KS.serialize(V2)

Serialization


…

io.sort.mb

Instead of explicitly tracking space used by record metadata, allocate a configurable amount of space at the beginning of the task

Hadoop [0.17, 0.22)

bufstartbufendbufindexbufmark

kvstartkvend

kvindex


io.s

ort.

mb

map(K1,V1)

*collect(K2,V2)


KS.serialize(K2)

VS.serialize(V2)

Serialization

Hadoop [0.17, 0.22)

bufstartbufendbufindexbufmark

kvstartkvend

kvindex


io.s

ort.

mb

map(K1,V1)

*collect(K2,V2)


KS.serialize(K2)

VS.serialize(V2)

Serialization

kvoffsets kvindices

kvbufferPartition no longer implicitly tracked. Store (partition, keystart,valstart) for every record collected

Hadoop [0.17, 0.22)

bufstartbufend

kvstartkvend

map(K1,V1)

*collect(K2,V2)


KS.serialize(K2)

VS.serialize(V2)

Serialization

bufindexbufmark

kvindex

Hadoop [0.17, 0.22)

bufstartbufend

kvstartkvend

map(K1,V1)

*collect(K2,V2)


KS.serialize(K2)

VS.serialize(V2)

Serialization

bufmark

kvindex

bufindex

Hadoop [0.17, 0.22)

bufstartbufend

kvstartkvend

map(K1,V1)

*collect(K2,V2)


KS.serialize(K2)

VS.serialize(V2)

Serialization

bufmark

kvindex

bufindex

p0

Hadoop [0.17, 0.22)

bufstartbufend

kvstartkvend

map(K1,V1)

*collect(K2,V2)


KS.serialize(K2)

VS.serialize(V2)

Serialization

kvindex

bufindexbufmark

io.sort.spill.percent

Hadoop [0.17, 0.22)

bufstart

kvstart

map(K1,V1)

*collect(K2,V2)


KS.serialize(K2)

VS.serialize(V2)

Serialization

kvendkvindex

bufendbufindexbufmark

Hadoop [0.17, 0.22)

bufstart

kvstart

map(K1,V1)

*collect(K2,V2)


KS.serialize(K2)

VS.serialize(V2)

Serialization

kvend

bufend

kvindex bufindexbufmark

Hadoop [0.17, 0.22)

bufstart

kvstart

map(K1,V1)

*collect(K2,V2)


KS.serialize(K2)

VS.serialize(V2)

Serialization

kvend

bufend

kvindex

bufmarkbufindex

Hadoop [0.17, 0.22)map(K1,V1)

*collect(K2,V2)


KS.serialize(K2)

VS.serialize(V2)

Serialization

kvstartkvend

bufstartbufend

bufmarkbufindex

kvindex

Hadoop [0.17, 0.22)map(K1,V1)

*collect(K2,V2)


KS.serialize(K2)

VS.serialize(V2)

Serialization

kvstartkvend

bufstartbufend

bufmarkbufvoid

bufindexkvindex

RawComparator interface requires that the key be contiguous in the byte[]

Invalid segments in the serialization buffer are marked by bufvoid

Hadoop [0.17, 0.22)map(K1,V1)

*collect(K2,V2)


KS.serialize(K2)

VS.serialize(V2)

Serialization

kvstartkvend

bufstartbufend

bufvoid

bufmarkbufindexkvindex

Hadoop [0.17, 0.22)

Pro:•Predictable memory footprint, collection (though not spill) agnostic to number of reducers. Most memory used for the sort allocated upfront and maintained for the full task duration.•No resizing of buffers, copying of serialized record data or metadata•Uses SequenceFile::appendRaw to avoid deserialization/serialization pass•Effects record compression in-place (removed in 0.18 with improvements to intermediate data format HADOOP-2095)

Other Performance Improvements•Improved performance, no metadata copying using QuickSort (HADOOP-3308)•Caching of spill indices (HADOOP-3638)•Run combiner during the merge (HADOOP-3226)•Improved locking and synchronization (HADOOP-{5664,3617})

Con:•Complexity and new code responsible for several bugs in 0.17•(HADOOP-{3442,3550,3475,3603})•io.sort.record.percent is obscure, critical to performance, and awkward•While predictable, memory usage is arguably too restricted•Really? io.sort.record.percent? (MAPREDUCE-64)

bufstartbufend

bufindexbufmarkequator

kvstartkvendkvindex

kvstart 1048560

kvend 1048560

kvindex 1048560

equator 0

bufstart 0

bufend 0

bufindex 0

bufmark 0

bufvoid 1048576

Hadoop [0.22]

bufstartbufendequator

kvstartkvend

kvindex

bufindexbufmark

kvstart 1048560

kvend 1048560

kvindex 968576

equator 0

bufstart 0

bufend 0

bufindex 300000

bufmark 300000

bufvoid 1048576

Hadoop [0.22]


kvstartkvend

kvindex

bufmark

kvstart 1048560

kvend 1048560

kvindex 968576

equator 0

bufstart 0

bufend 0

bufindex 300030

bufmark 300000

bufvoid 1048576 bufindex

Hadoop [0.22]


kvstartkvend

kvindex

bufmark

kvstart 1048560

kvend 1048560

kvindex 968576

equator 0

bufstart 0

bufend 0

bufindex 300030

bufmark 300000

bufvoid 1048576

Hadoop [0.22]

bufindex


kvstartkvend

kvindex

bufmark

kvstart 1048560

kvend 1048560

kvindex 968576

equator 0

bufstart 0

bufend 0

bufindex 300030

bufmark 300000

bufvoid 1048576bufindex

Hadoop [0.22]


kvstartkvend

kvindex

bufmark

kvstart 1048560

kvend 1048560

kvindex 968576

equator 0

bufstart 0

bufend 0

bufindex 300030

bufmark 300000

bufvoid 1048576bufindex

Hadoop [0.22]

p0kvoffsets and kvindices information interlaced into metadata blocks. The sort is effected in a manner identical to 0.17, but metadata is allocated per-record, rather than a priori (kvoffsets) (kvindices)


kvstartkvend

kvindex

kvstart 1048560

kvend 1048560

kvindex 968560

equator 0

bufstart 0

bufend 0

bufindex 300030

bufmark 300030

bufvoid 1048576bufindexbufmark

Hadoop [0.22]

bufstartkvstart

kvindex

kvstart 1048560

kvend 968576

kvindex 736000

equator 736020

bufstart 0

bufend 300030

bufindex 736020

bufmark 300030

bufvoid 1048576bufend

equatorbufindexbufmark

kvend

Hadoop [0.22]

bufstartkvstart

kvindex

kvstart 1048560

kvend 968576

kvindex 696000

equator 736020

bufstart 0

bufend 300030

bufindex 811020

bufmark 811020

bufvoid 1048576bufend

equator

kvend

bufindexbufmark

Hadoop [0.22]

kvindex

kvstart 968576

kvend 968576

kvindex 696000

equator 736020

bufstart 300030

bufend 300030

bufindex 811020

bufmark 811020

bufvoid 1048576bufstartbufend

equator

kvstartkvend

bufindexbufmark

Hadoop [0.22]

kvindex

kvstart 736000

kvend 736000

kvindex 696000

equator 736020

bufstart 736020

bufend 736020

bufindex 811020

bufmark 811020

bufvoid 1048576

equatorbufstartbufend

kvstartkvend

bufindexbufmark

Hadoop [0.22]


kvstartkvend

kvindex

bufmark

kvstart 1048560

kvend 1048560

kvindex 968576

equator 0

bufstart 0

bufend 0

bufindex 300000

bufmark 300000


Hadoop [0.22]

bufstart

kvstart

kvindex

bufmarkbufendequator

kvstart 1048560

kvend 1048560

kvindex 968576

equator 0

bufstart 0

bufend 0

bufindex 300000

bufmark 300000


kvend

Hadoop [0.22]

kvstartkvend

bufmarkbufstartbufendequator

kvstart 1048560

kvend 1048560

kvindex 968576

equator 0

bufstart 0

bufend 0

bufindex 300000

bufmark 300000

bufvoid 1048576

bufindex

kvindex

Hadoop [0.22]

Questions?

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