[Mondrian] Multithreading, Parallel Batch / Query Execution

Ajit Vasudeo Joglekar ajogleka at thoughtworks.com
Wed Apr 18 06:39:18 EDT 2007


Hello,

We are exploring the ways to add parallel query execution in Mondrian. One 
simple prototype we implemented goes through the batches in 
FastBatchingCellReader and runs every batch in a new worker thread. To 
make the ThreadLocals available to the worker thread we changed 
ThreadLocal in RolapStar and other places to InheritableThreadLocal.

The objectives are simple,
- In case an MDX results in more than one sql for fact data, these sqls 
should run in parallel
- No support for explicit transactions / real time updates to the database
- Thread pooling, life cycle management and related things will follow 
later

We used JProfiler to run QueryRunner to identify the biggest bottlenecks 
indicated by thread monitor wait period. QueryRunner was run with 5 
threads, 100 seconds and random queries set to false (5, 100, false). 
Profiling report was generated on the snapshot immediately after 
QueryRunner exited. It was run on a Dell Latitude D620 with Core 2 Duo (2 
Cores), 2 GB RAM, WinXp SP2 32 bit, Sun jdk 1.5.0_07. Mondrian connects to 
Mysql 5 running on the same box.

Monitor Usage Statistics
Session:        QueryRunner
Statistics:     Monitor Usage Statistics Grouped by Monitors
Sorted by:      Block duration

Without parallel batch execution
Monitors                                                Block count Block 
duration
mondrian.rolap.RolapBaseCubeMeasure (id: 11)    3490            9700 ms 
mondrian.rolap.RolapCube (id: 12)                       2426            10 
s 
mondrian.rolap.RolapSchema$Pool (id: 7)         4               15 s 
java.util.HashMap (id: 18)                              11851           35 
s 
mondrian.rolap.SmartMemberReader (id: 25)               24              42 
s 
java.lang.Class (id: 10)                                        129733 139 
s 

With parallel batch execution
Monitors                                                Block count Block 
duration 
mondrian.rolap.RolapCube (id: 11)                       1846            10 
s 
mondrian.rolap.agg.Aggregation (id: 42)                 1023            12 
s 
java.util.HashMap (id: 20)                              4435            19 
s 
mondrian.rolap.RolapSchema$Pool (id: 7)         4               21 s 
mondrian.rolap.SmartMemberReader (id: 23)               35              57 
s 
java.lang.Class (id: 5)                                 91145 106 s 

We are anlyzing these options to reduce lock contentions and improve 
performance
a) Replacing the synchronization with Read and Write RenentrantLocks with 
reduced scope
b) Using ConcurrentHashMap
c) Replace syncronized lazy initialize with static initialize at load time

To verify functional correctness after multithreaded batch execution we 
adapted QueryRunner to do result verification. We observed an issue of 
result set member ordering in case we toggle the 
MondrianProperties.instance().DisableCaching between multiple test runs in 
the same process. Apart from that all the existing test run successfully

We tried out various optimizations with varying degree of success

This is a simplistic approach but seems to be working. 

We will post more on this once we have more statistics to validate the 
benifits of this approach. 
Please let us know if there are any other approaches we should be 
considering.

-Ajit
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