IQ doesn't "use" threads, per se. IQ allocates threads at startup based on all logical cores (cores and threads per core) on the host. On Many platforms, this can over inflate the host size and cause poor performance. This is why we usually recommend that we set -iqnumbercpus to match the physical core count. This is just a guideline for IQ, though. There are startup parameters, like total threads, that are based on the -iqnumbercpus value. That does not mean that IQ is limited to just that count.
This is where the "IQ doesn't use threads" comes in to play. With -iqnumbercpus set to 4, there would be about 240 threads allocated. When a load or query runs, that query is optimized based on -iqnumbercpus. IQ then turns over the threads to the OS for execution. If a load needs 50 threads to run, then 50 threads are turned over to the OS for execution and scheduling. It could be that the OS runs all 50 simultaneously. It may run just 1. Or something in between.
The net is that just because you set -iqnumbercpus to the physical core count, it doesn't mean that it can't run on more cores and OS threads.
I am completely against leaving Solaris IQ -iqnumbercpus to be determined by the total threads on the host. In some cases, I think that just matching the physical core count is too low. I would start with matching the physical cores of 4 then see how IQ performs. If we don't see the host using all threads (4 cores*8 threads), then increase -iqnumbercpus to 6 or 8 and retest.
Mark
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