1 Wenguang WangRichard B. Bunt Department of Computer Science University of Saskatchewan November 14, 2000 Simulating DB2 Buffer Pool Management

2 Outline Background Problem Methodology Simulation results Future work

3 Background What is buffer pool Buffer Pool Database on Disks WritesReads Applications Upper layer of DBMS DBMS

4 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

5 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

6 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

7 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

8 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

9 Trace Collection Trace tools of DB2 Suspend the TPC-C benchmark periodically to record big enough trace Buffer Pool Database on Disks WritesReads Applications Upper layer of DBMS DBMS Trace point

10 Trace Volume 60M buffer pool requests 200K TPC-C transactions Equivalent to 30 minutes TPC-C run when no traces are recorded

11 Buffer Pool Simulator To simulate the buffer pool management algorithm and the disk activities About 8000 lines C++ code

12 Architecture of the Simulator

13 Clock- Pointer Page Cleaners Cleaning pages DB2 Buffer Pool Algorithm Clock Algorithm Threshold: triggers the page cleaning activity Database on Disks

14 Buffer Pool Algorithm (cont.)

15 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

16 Simulator Verification— Similar Throughput Curve

17 Page Distribution of Buffer Pool

18 I/O Activities of the Buffer Pool

19 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

20 Effect of More Page Cleaners

21 IO Activities Under More Page Cleaners

22 Effect of Number of Page Cleaners

23 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

24 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

25 Questions?