Max Workers available and yet pending task high
Your numbers don't totally make sense.  In particular, the value for "workersCreated" should not be higher than the value for "maxWorkers."  Here's an example of a normal-looking entry:

&lt;queryProcessing maxWorkers="576" 
                 workersCreated="86" 
                 workersIdle="46" 
                 tasksCompletedWithinInterval="950" 
                 pendingTasks="0" 
                 oldestPendingTaskWaitingTime="0" 
                 hasUnresolvableDeadlockOccurred="0" 
                 hasDeadlockedSchedulersOccurred="0" 
                 trackingNonYieldingScheduler="0x0"&gt;


In your case, you've hit the max worker threads limit (as evidenced by the THREADPOOL waits in sys.dm_os_wait_stats).  Investigating those is a pretty big task, and probably tough to cover in this Q&amp;A format.  This is what's causing tasks to be pending though.  Generally, you'll need to


Fix blocking issues (if that's what's causing threads to tie up schedulers)
Reduce parallelism (by reducing MAXDOP or CTFP) - this is kind of a band-aid fix
Change the app to run less queries at a time


Based on other questions you've asked on the site, I imagine that blocking and long-running transactions are the reason you're hitting this.  You'll need to continue to work with the vendor.



By the way, you can double check that the "max worker threads" setting hasn't been modified by running:

EXEC sp_configure @configname = 'max worker threads';


It should show the run_value as 0 if it hasn't been changed.