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Wenguang Wang Richard B. BuntDepartment of Computer Science

University of Saskatchewan

November 14, 2000

Simulating DB2 Buffer Pool Management

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Outline

• Background

• Problem

• Methodology

• Simulation results

• Future work

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Background• What is buffer pool

Buffer Pool

Database on Disks

Writes Reads

Applications

Upper layer of DBMSDBMS

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Problem

• Buffer pool management is important to the performance of any DBMS

• The config and tuning of buffer pool is not a easy problem for the database administrator

• The buffer pool management of a DBMS is very complex

• It is hard to study and test the buffer pool management algorithm directly

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Methodology

• Trace-driven simulation provides an effective approach

• Compare to the real DBMS:– Simulator is easier to be controlled

– Simulator requires much lower computing resources (CPU, memory, disk, running time)

– New algorithm is easier to be implemented and tested in the simulator

– Changes that cannot be done or are not easy to do in the real system can be simulated in the simulator

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Methodology

• Create trace-driven simulation tools– Collect trace– Process trace– Develop simulator– Verify simulator

• Perform experiments by the simulator– Understand the effect of buffer pool parameters– Give suggestions to the tuning of buffer pool– Design and test alternate buffer pool algorithms

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System Configuration

• DBMS — IBM DB2– Relational DBMS– Distributed DBMS which supports multiple

nodes. Because buffer pools on different nodes are independent, only the single node DB2 is studied

• Workload — the TPC-C benchmark– An On-Line Transaction Processing benchmark– Many clients send simple queries simultaneously

to the DBMS on the server side– A large amount of data are updated by the queries

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System Configuration (cont.)

• DB2 version 6.1 running on Windows NT Server 4.0

• TPC-C database – Small application: 50-warehouses (5GB data)

spanning over 9 physical disks

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Trace Collection• Trace tools of DB2

• Suspend the TPC-C benchmark periodically to record big enough trace

Buffer Pool

Database on Disks

Writes Reads

Applications

Upper layer of DBMSDBMSTrace point

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Trace Volume

• 60M buffer pool requests

• 200K TPC-C transactions

• Equivalent to 30 minutes TPC-C run when no traces are recorded

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Buffer Pool Simulator

• To simulate the buffer pool management algorithm and the disk activities

• About 8000 lines C++ code

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Architecture of the Simulator

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Clock-Pointer

Page Cleaners

Cleaning pages

DB2 Buffer Pool Algorithm

Clock Algorithm

Threshold: triggers the page cleaning activity

Database on Disks

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Buffer Pool Algorithm (cont.)

Threshold

Asynchronous writesperformed by page

cleaners

Dirty pages

Clean Region

Dirty Region

Expand

BufferPool

Reads

Synchronouswrites

Database on Disks

TPC-C

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Simulator Verification

• Compare the throughput curve

• Compare the run-time statistics– Hit ratio– Dirty page percentage

• Test the effect of parameters– Dirty page threshold– Number of page cleaners

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Simulator Verification— Similar Throughput Curve

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Page Distribution of Buffer Pool

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I/O Activities of the Buffer Pool

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Simulation Results Under Default Configuration

• Page cleaners cannot clean out pages fast enough

under the default configuration (2 page cleaners)

• Too many dirty pages (87%) in the buffer pool

under the default configuration

• The existence of too many dirty pages lowers the

buffer pool hit ratio and performance

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Effect of More Page Cleaners

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IO Activities Under More Page Cleaners

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Effect of Number of Page Cleaners

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Effect of Buffer Pool Parameters

• Threshold cannot affect performance when the number of page cleaners is small

• Setting an appropriate number of page cleaners is important to performance

• Appropriate number of page cleaners are different for different workloads

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Future work• Gain more understanding of the buffer pool

algorithm from the simulator and DB2

• Extend the work to a much larger TPC-C database

• Investigate alternative algorithms of the buffer pool management algorithm which are easier to be managed and tuned

• Test the alternative algorithms first in the simulator and then in the real system

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Questions?