Figuring Out Cost Threshold For Parallelism

Grant Fritchey uses R to help him decide on a good cost threshold for parallelism value:

With the Standard Deviation in hand, and a quick rule of thumb that says 68% of all values are going to be within two standard deviations of the data set, I can determine that a value of 16 on my Cost Threshold for Parallelism is going to cover most cases, and will ensure that only a small percentage of queries go parallel on my system, but that those which do go parallel are actually costly queries, not some that just fall outside the default value of 5.

I’ve made a couple of assumptions that are not completely held up by the data. Using the two, or even three, standard deviations to cover just enough of the data isn’t actually supported in this case because I don’t have a normal distribution of data. In fact, the distribution here is quite heavily skewed to one end of the chart. There’s also no data on the frequency of these calls. You may want to add that into your plans for setting your Cost Threshold.

This is a nice start.  If you’re looking for a more experimental analysis, you could try A/B testing (particularly if you have a good sample workload), where you track whatever pertinent counters you need (e.g., query runtime, whether it went parallel, CPU and disk usage) under different cost threshold regimes and do a comparative analysis.

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