XJoinXJoin: : Getting Fast Answers From Getting Fast Answers From Slow and Bursty NetworksSlow and Bursty Networks

T. UrhanM. J. Franklin

IACS, CSD, University of

Maryland

Presented by: Abdelmounaam

Rezgui

CS-TR-3994

The Problem

How to improve the interactive performance of queries over widely distributed data sources ?

2

RS

Tuples

Tuples

3

The Problem

Source BSource A

Why is the response-time unpredictable ?

• Remote sources

• Intermediate sites

• Communication links

• Overloading

• Congestion

• Failures

are vulnerable

to {

4

Significant and unpredictable delays

Unresponsive and unusable systems

Different classes of delays

• Initial delay: a longer than expected wait to receive the first tuple.

• Slow delivery: data arrive at a fairly constant but slower than expected rate.

• Bursty arrival: bursts of data followed by long periods of no arrivals.

5

Some Join variants

• Nested Loops Join• Block Nested Loops Join• Index Nested Loops Join• Sort-Merge Join• Classic Hash Join• Simple Hash Join• Grace Hash Join• Hybrid Hash Join (HHJ)• TID Hash Join• Symmetric Hash Join (SHJ)• XJoin

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Query Scrambling

reacts to data delivery pbs. by on-the-fly rescheduling of query operators and

restructuring of the query execution plan.

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• improve the response time for the entire query• may slow down the return of some initial results

To be presented on November 22, 1999

Traditional query processing techniques

• Reduce the memory requirements• Reduce Disk I/O

• Delivery of the entire query result (on-line users would like to receive initial results asap.)

• Slow and bursty delivery of data from remote sources can stall query execution.

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XJoin: Fundamental principles

• improves the interactive performance by producing results incrementally (as they become available)

• allows progress to be made even when one or more sources experience delays (delays are exploited to produce more tuples earlier)

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XJoin : The key idea

When inputs are delayed

run a background processing on the previously received results

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• Managing the flow of tuples between memory and secondary storage.

• Controlling the background processing.

• Full answer (all the tuples are produced).

• No duplicate tuples are generated.

XJoin : The challenges

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SHJoin (Symmetric Hash Join)

Hash table 2

Matching

Hash table 1

Source 2Source 112

SHJoin requires:

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Hash tables for both of its inputs be memory resident.

Unacceptable for complex queries.

XJoin

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Partioning:

• each input is partitioned into a number of partitions based on a hash function.

• each partition i of source A, PiA :

PiA = MPiA DPiA

MPiA DPiA =

D I S K Tuple B

hash(Tuple B) = n

SOURCE-B

Memory-resident partitions of source B

. . . . . .k1 n

flu

shDisk-resident

partitions of source B

. . . . . .

Disk-residentpartitions of source A

Memory-resident partitions of source A

. . . . . . . . . . . .1

SOURCE-A

M E

M O

R Y

. . .

n

1n1 k n

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Tuple A

hash(Tuple A) = 1

hash(record B) = j

Partitions of source B

. . . . . . . . .ii

M E

M O

R Y j

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Stage 1: Memory-to-memory Joins

Partitions of source A

j

SOURCE-B

Tuple B

SOURCE-A

Tuple A

hash(record A) = i

. . . . . . . . .

insertinsert probeprobe

Output

Partitions of source BPartitions of source A

M E

M O

R Y

i. . . . . . .

ii

D I

S K

i

Output

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Stage 2: Disk-to-memory Joins

. . . . . . .. . . . . . .. . . . . . .

Partitions of source BPartitions of source A

. . . . .. . . . .. . . . .. . . . .

DPiA MPiB

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Stage 3: Clean-up

• Stage 1 fails to join tuples that were not in the memory at the same time.

• Stage 2 fails to join two tuples if one of them is not in the memory when the other is brought from the disk.

• Stage 3 joins all the partitions (memory-resident and disk-resident portions) of the two sources.

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Handling duplicates

• Timestamps

Tuple X

Tuple X ATS DTS

• Example

Tuple X 99 235

• Counter 51

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Detecting tuples joined in the 1st stage

Tuple A 102 234

Tuple B1 178 198

• Tuples joined in the first stage

DTSATS

Overlapping

Tuple A 102 234

Tuple B2 348 601

• Tuples not joined in the first stage

DTSATS

Non-Overlapping

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Detecting tuples joined in the 2nd stage

Tuple A

DTS

20 340 250 550 300 700100 200

ATS ProbeTSDTSlast

Tuple B

DTS

100 300 800 900500 600

ATS

Overlap

History list for the corresponding partitions

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Optimization 1: Adding a cache

• Stage 2 joins DPiA and MPiB

• Tuples of DPiA are discarded after use.

The idea: retain some tuples of DPiA (cached)

Could be used by a subsequent run of stage 2

joining DPiB and MPiA

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i . . .. . .i . . .. . .

i . . .. . .i . . .. . . i

CA

CH

E

Partitions of Source B

Partitions of Source A

i . . .. . .i . . .. . .

i . . .. . .i . . .. . . i

CA

CH

E

Partitions of Source B

Partitions of Source A

ME

MO

RY

DIS

K

prob

e

insert

OutputOutputOutput

Partitions of Source B

Partitions of Source A

Second run of stage 2First run of stage 2

prob

eprobe

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Optimization 2: Controlling Stage 2

• Overhead incured by Stage 2 is hidden only when both inputs experience delays

• Reduce the aggressiveness of Stage 2

• Dynamic activation threshold (e. g., 0.01 0.02)

Experiment Environment

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PREDATOR, an Object-Relational DBMS

• Xjoin operator added.

• Query optimizer extended to:

• account for XJoin.

• provide some of the statistics and calculations required by XJoin.

Arrival Patterns

2 have been chosen:

Fig. 1: Bursty arrival.Avg. Rate: 23.5 KB/s

Fig. 2: Fast arrival.Avg. Rate: 129.6 KB/s

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• 100 000 tuple Wisconsin benchmark relations.

• each tuple: 288 bytes

• Unique unclustered integer join attribute

• Result cardinality: 100 000.

• Sun Ultra 5 WS: – Solaris 2.6– 128 MB of real memory– Disk space (approx.): 4 GB– Disk & Memory pages: 8 KB

• Storage manager buffer size: 800 KB

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Results

Experiment 1 Basic performance of XJoin

• Memory space allocated to the join operators: 3 MB.

• Input relations: 28.8 MB each

• Activation threshold (of stage 2): 0.01

• 4 delay scenarios

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Case 1: Slow NetworkBoth sources are slow

• XJoin improves the delivery time of initial answers.

• The reactive background processing is an effective solution to exploit delays.

• The use of cache can further improve performance.

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Case 2: Mixed Network Slow build/Fast probeFast build/Slow probe

• XJoin variants perform better.

• (/Case 1) XJoins with the 2nd Stage perform better.

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• XJoin variants deliver initial results earlier.

• HHJ delivers the 2nd half of the result faster than XJoin-NoCache and XJoin.

• XJoin-No2nd delivers the last 60 % of the result faster than the other XJoin variants.

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Case 3: Fast NetworkBoth sources are fast

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Experiment 2 : Controlling the 2nd stage

• improves inter. perf. with slow and bursty data sources.

• degrades the overall response-time in the case of fast/reliable sources.

Fig. 7: Slow relations. Fig. 8: Fast relations.

• Stage 2 should be employed less aggressively (less often).

• A dynamic activation threshold.

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XJoin-Dyn

• aggressive in the early stages of the query.

• becomes less aggressive as more of the results are produced.

• starts with a low activation treshold (0.01) and then linearly increases it to 0.02.

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Experiment 3 : the effect of memory size

• Recall ! The prime motivation for designing XJoin was the huge memory requirements of the symmetric hash join.

• XJoin reduces the memory requirements but adds overhead (disk I/O & duplicate detection).

• Size of the input relations: 8.6 MB.• 3 different memory allocations:

- 3 MB (neither of the inputs fit into the memory)- 10 MB (one input fits into the memory)- 20 MB (both inputs fit into the memory)

Fig. 9: Slow Network, Varying memory

Fig. 10: Fast Network, Varying memory

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• XJoin performs better both in:

- interactive performance

- completion time.

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Experiment 4 : impact of query complexity

• 2 to 6 relations (1 to 5 joins)• 3 MB to each join operator

Fig. 11. Tuple production rates of XJoin and HHJ (secs)- Slow Network

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Experiment 4 : impact of query complexity

Fig. 12. Tuple production rates of XJoin and HHJ (secs)

- Fast Network

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XJoin delivers the initial results faster

XJoin

An effective query processing technique for providing fast query responses to

users in the presence of slow and bursty remote sources.

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Conclusions

• lowers the memory requirements (partitioning)

• improves the interactive performance.

• reacts to delays and takes advantage of silent periods to produce more tuples faster.

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What de you think about

PJoin A Multithreaded Parallel XJoin Using

the Cilk Language

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Perspectives