Througput spikes explained

In previous posts a typical pattern in handling commands was seen. The handling versus reading commands cumulative time consumption showed a pattern as can be seen in the next graph:

mediumheap.png

I had a hunch that this could be related to the client (load generator) behaviour. The client program is written in Java. It runs 128 application threads that continuously communicate with the database. Lots of rows inserted and retrieved. The number of objects created should be considerable. Garbage collection to clean up the Java heap might be the driving force for the above mentioned pattern.

My standard configuration runs with a 2 Giga byte heap. I decided to increase this to 3.2GB, and also increase the Eden size to 1.5GB. For those a little familiar with Java, this is the space in which the objects are created. The test resulted in the following graph:


largeheap.png

Indeed it is the client who is responsible for these spikes. Using a larger heap means more objects can be created in the Java heap before a garbage collection is forced. Also a larger heap holds many more objects translating in longer GC times. Indeed the time spent reading commands has increased. Please note, garbage collections in Eden are "stop the world", meaning the client does not respond to the database process: during a GC the readcommand does take significant more time.

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