SSAS Tabular performance – DefaultSegmentRowCount
I’m currently investigating a poorly performing Tabular model, and came across some interesting test results which seem to contradict the advice in Microsoft’s Performance Tuning of Tabular Models white paper.
- 7.6Gb SSAS tabular cube, running on a 2 x CPU 32 core (Xeon E5-2650 2Ghz, 2 x NUMA nodes) server with 144Gb RAM
- SQL Server 2012 SP1 CU7 Enterprise
- 167m rows of data in primary fact
- 80m distinct CustomerKey values in primary fact
- No cube partitioning
A simple distinct count in DAX of the CustomerKey, with no filtering, is taking 42 seconds on a cold cache. Far too slow for a tabular model. Hence the investigation.
p88 of the Performance Tuning of Tabular Models white paper discusses the DefaultSegmentRowCount, explaining that it defaults to 8m, and that there should be a correlation between the number of cores and the number of segments. [The number of segments calculated as the number of rows divided by the segment size].
It also indicates that a higher segment size may increase compression, and consequently query performance.
Calculating the number of segments for our data set, gives us the following options:
|Segment Size||# Segments|
So, with 32 cores to play with, we should be looking at the default segment size (8m) or maybe reduce it to 4m to get 40 segments. But the extra compression with 16m segment size may be of benefit. So I ran some timing tests on the distinct count measure, and the results are quite interesting.
It clearly shows that in this environment, reducing the DefaultSegmentRowSize property down to 2m improved the query performance (on a cold cache) from 42s down to 27s – 36% improvement. As well as this, processing time was reduced, as was compression.
This setting creates 80 segments, 2.5 times the number of cores available, but achieved the best performance. Note that the server’s ProcessingTimeboxSecPerMRow setting has been set to 0 to allow for maximum compression.
There’s more to this systems’s performance problems than just this, NUMA for a start, but thought I’d throw this out there in case anyone else is blindly following the performance tuning white paper without doing your own experimentation.
Each environment, data set and server spec is different, so if you need to eek out the last ounce of performance, run your own tests on the SSAS settings and see for yourself.