Strange ASP.NET Queue Performance Counters Behavior?
- by LemurTech
We have an ASP.NET 2.0 site running in classic mode. I am seeing very strange behavior in the performance counter values. Perhaps these are bugs (I've been all over Google trying to verify this, without much luck), or perhaps it is just my inexperience with monitoring these things.
This PerfMon graph (http://imgur.com/Jv5io5J) represents a load test where I add up to 350 virtual users to the site, at a rate of about 1/sec, performing relatively simple page browsing. At the end of the test, I gradually taper off the number of users. This is a 4 CPU server. Machine.config settings for are at the defaults.
The solid blue line is ASP.NET Apps v2.x\Requests Executing for the application in question. The profile makes perfect sense, with a quick ramp-up to 32 executing requests (minWorkerThreads x 4CPUs), followed by a slower ramp-up to 48 ((maxWorkerThreads - minWorkerThreads) x 4CPUs).
The solid yellow line is ASP.NET v2.x\Requests Queued. Again, this makes sense: after the initial 32 request threads are activated, the queue begins to build as new thread initialization can't keep pace with incoming requests.
But as executing requests reaches its highest possible value of 48, the counter for ASP.NET Apps v2.x\Requests Queued (green solid line) suddenly springs to life and maintains step with the yellow counter. As far as I can tell, and with no other apps running on the server, these two counters should have had the same values from the start.
One other odd thing: The counter for ASP.NET v2.x\Request Wait Time (dotted yellow line) also does not spring to life until executing requests reaches 48. Shouldn't I be seeing values here from the moment ASP.NET v2.x\Requests Queued begins to build? And likewise, why would ASP.NET Apps v2.x\Request Execution Time (dotted blue) increase significantly only after that peak of 48 is reached? Shouldn't it ramp-up gradually along with queued requests?
