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16.17. RDF Performance Tuning

For RDF query performance, we have the following possible questions:

  • Is the Virtuoso process properly configured to handle big data sets?

  • Is the graph always specified?

  • Are public web service endpoints protected against bad queries?

  • Are there patterns where only a predicate is given?

  • Is there a bad query plan because of cost model error?

16.17.1. General

When running with large data sets, one should configure the Virtuoso process to use between 2/3 to 3/5 of system RAM and to stripe storage on all available disks. See NumberOfBuffers , MaxDirtyBuffers , and Striping INI file parameters. 

; default installation
NumberOfBuffers          = 2000
MaxDirtyBuffers          = 1200

Typical sizes for the NumberOfBuffers and MaxDirtyBuffers (3/4 of NumberOfBuffers) parameters in the Virtuoso configuration file (virtuoso.ini) for various memory sizes are as follows, with each buffer consisting of 8K bytes:

Table 16.19. recommended NumberOfBUffers and MaxDirtyBuffers

System RAM NumberOfBuffers MaxDirtyBuffers
2 GB 170000 130000
4 GB 340000 250000
8 GB 680000 500000
16 GB 1360000 1000000
32 GB 2720000 2000000
48 GB 4000000 3000000
64 GB 5450000 4000000

Also, if running with a large database, setting MaxCheckpointRemap to 1/4th of the database size is recommended. This is in pages, 8K per page.

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