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How many threads is too much for SQL?

Here's a redacted version of a Q&A I had with one of the devs (who is not a DBA) at my consulting company. I thought it was worth sharing.

I got a question on multithreading in SQL 2005. Do you know much about it? Our client would like to know how many threads can be open before performance starts to suffer.

And my response to my colleague:

That's a really hard question to answer since SQL doesn't operate in application "threads". SQL server, with only one processor (and therefore one Scheduler) can handle many many threads from an application. More processors will enable it to more quickly respond to multiple threads, and SQL 2005 detects processor count automatically and scales. "Open threads" is not really something to measure, as a multithreaded application really doesn't correlate directly to SQL Server. Sure, you can see the threads connections open and close, but to measure performance, the count of threads is really not useful.

If they are worried about CPU capacity, there are a number of perfmon counters and internal dynamic management views (DMV's) to look at to see if CPU is a bottleneck.

In Perfmon, you want to be looking at

\System\Processor Queue Length (shouldn't exceed 2x # of CPU's)
\Processor(_Total)\% Processor Time (self-explanatory, lower number the better)
\Process(sqlservr)\% Processor Time (self-explanatory, shouldn't differ from the above if no other major apps are running on the server)

In DMV's, look for

sys.dm_os_wait_stats (You don't want to see a lot of CPU-related wait types)

Here is a good link from MS: http://technet.microsoft.com/en-us/magazine/2007.10.sqlcpu.aspx

There's obviously much more to look at with SQL server, but not knowing my audience I'm not sure how much detail you want. Sorry for the unclear answer but there really is no hard number of application threads to stay under. It depends a lot upon the other subsystems of the server as well (RAM and physical IO). Sounds like they want to do performance tuning. Sounds like an opportunity.

"You always admire what you really don't understand." -Blaise Pascal

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