If you look at the log of that test run - its is clear to see that dnn START SCHEDULER ATTEMPT is actually being fired up 3 separate times.
Which is creating three separate worker threads in the system - all attempting to load up the scheduler.

I dont have time at the moment to dig further to understand the ramifications of this in terms of what this means inside the scheduler - its a mess of static objects and instantiated objects that are likely conflicting with each other - so god knows what is really going on ....

HOWEVER - regardless - while I forced this situation to occur by stacking page requests when I knew that the application pool was restarting - in a heavy loaded dnn site - this is ironically something that is likely to happen.

If something causes the app pool to recycle - while people are on the site - then its very possible to get a queue of new page requests hitting the site - trying to get their next page to display.

The same is also likely to occur with sites that are in low demand that dont have keep alive actions - if they are targeted by a web crawler when they are in app pool not running state.
Crawlers send multiple page requests at pretty much the same time.

If a low use site - gets hits by a crawler - it too could have numbers of requests queued up while the site is trying to start up to server the first request.
Westa

And to fix this - what is needed is some sort of lock - that prevents the code running more that once.
Ive added a simple lock command wrapper that puts a hold a new static read only object.

If the file - DNN Platform\Library\Common\Initialize.cs

-------------------------------------------------------
private static readonly object _schedulerStarting = new object();
public static void RunSchedule(HttpRequest request)
{
if (!IsUpgradeOrInstallRequest(request))
{
try
{
lock (_schedulerStarting)
{
if (SchedulingProvider.SchedulerMode == SchedulerMode.REQUEST_METHOD && SchedulingProvider.ReadyForPoll)
{
Logger.Trace("Running Schedule " + (SchedulingProvider.SchedulerMode));
var scheduler = SchedulingProvider.Instance();
var requestScheduleThread = new Thread(scheduler.ExecuteTasks) { IsBackground = true };
requestScheduleThread.Start();
SchedulingProvider.ScheduleLastPolled = DateTime.Now;
}
}
}
catch (Exception exc)
{
Exceptions.LogException(exc);
}
}
}
---------------------------------------------------------------

and the trace result is ...

'w3wp.exe' (Managed (v4.0.30319)): Loaded 'C:\Windows\Microsoft.NET\Framework64\v4.0.30319\Temporary ASP.NET Files\dnn_platform\9a78750d\754f9f06\assembly\dl3\c6c7e936\7315128a_4f33cf01\DotNetNuke.Providers.FiftyOneClientCapabilityProvider.dll', Symbols loaded.
dnn RUN SCHEDULER HTTPREQUEST
dnn START SCHEDULER ATTEMPT
The thread '' (0x3d28) has exited with code 0 (0x0).
'w3wp.exe' (Managed (v4.0.30319)): Loaded 'A_fd13bc11_58bb_40fc_b072_62ec60e80f45'
'w3wp.exe' (Managed (v4.0.30319)): Loaded 'C:\Windows\Microsoft.NET\Framework64\v4.0.30319\Temporary ASP.NET Files\dnn_platform\9a78750d\754f9f06\App_Web_dq0mqzz1.dll', Symbols loaded.
'w3wp.exe' (Managed (v4.0.30319)): Loaded 'C:\Windows\Microsoft.NET\Framework64\v4.0.30319\Temporary ASP.NET Files\dnn_platform\9a78750d\754f9f06\App_Web_2jx2f4cb.dll', Symbols loaded.
'w3wp.exe' (Managed (v4.0.30319)): Loaded 'C:\Windows\Microsoft.NET\Framework64\v4.0.30319\Temporary ASP.NET Files\dnn_platform\9a78750d\754f9f06\App_Web_4jyfvmm4.dll', Symbols loaded.
'w3wp.exe' (Managed (v4.0.30319)): Loaded 'C:\Windows\Microsoft.NET\Framework64\v4.0.30319\Temporary ASP.NET Files\dnn_platform\9a78750d\754f9f06\App_Web_ledl3b5i.dll', Symbols loaded.
'w3wp.exe' (Managed (v4.0.30319)): Loaded 'C:\Windows\Microsoft.NET\Framework64\v4.0.30319\Temporary ASP.NET Files\dnn_platform\9a78750d\754f9f06\App_Web_rymqoiy5.dll', Symbols loaded.
'w3wp.exe' (Managed (v4.0.30319)): Loaded 'C:\Windows\Microsoft.NET\Framework64\v4.0.30319\Temporary ASP.NET Files\dnn_platform\9a78750d\754f9f06\App_Web_imcs5cyf.dll', Symbols loaded.
'w3wp.exe' (Managed (v4.0.30319)): Loaded 'C:\Windows\Microsoft.NET\Framework64\v4.0.30319\Temporary ASP.NET Files\dnn_platform\9a78750d\754f9f06\App_Web_xyg14owe.dll', Symbols loaded.
The thread '' (0x2bfc) has exited with code 0 (0x0).
'w3wp.exe' (Managed (v4.0.30319)): Loaded 'C:\Windows\Microsoft.NET\Framework64\v4.0.30319\Temporary ASP.NET Files\dnn_platform\9a78750d\754f9f06\App_Web_uqqfammj.dll', Symbols loaded.
'w3wp.exe' (Managed (v4.0.30319)): Loaded 'C:\Windows\Microsoft.NET\Framework64\v4.0.30319\Temporary ASP.NET Files\dnn_platform\9a78750d\754f9f06\App_Web_yj3kwkr3.dll', Symbols loaded.
'w3wp.exe' (Managed (v4.0.30319)): Loaded 'C:\Windows\Microsoft.NET\Framework64\v4.0.30319\Temporary ASP.NET Files\dnn_platform\9a78750d\754f9f06\App_Web_01rlzxl3.dll', Symbols loaded.
'w3wp.exe' (Managed (v4.0.30319)): Loaded 'C:\Windows\Microsoft.NET\Framework64\v4.0.30319\Temporary ASP.NET Files\dnn_platform\9a78750d\754f9f06\App_Web_wpwyemun.dll', Symbols loaded.
'w3wp.exe' (Managed (v4.0.30319)): Loaded 'Anonymously Hosted DynamicMethods Assembly'
'w3wp.exe' (Managed (v4.0.30319)): Loaded 'System.Xml.Xsl.CompiledQuery.1'
'w3wp.exe' (Managed (v4.0.30319)): Loaded 'Microsoft.GeneratedCode'
dnn RUN SCHEDULER HTTPREQUEST
dnn RUN SCHEDULER HTTPREQUEST
dnn RUN SCHEDULER HTTPREQUEST
dnn RUN SCHEDULER HTTPREQUEST
dnn RUN SCHEDULER HTTPREQUEST
A first chance exception of type 'System.Threading.ThreadAbortException' occurred in mscorlib.dll

PS - have raised a ticket for this in support - just finished putting it together

https://dnntracker.atlassian.net/brow...

Interestingly just been looking thru logs on one of our low use sites - there is a common thread of what have always been considered to be weird errors in the event log - all relating to visits from web crawlers - in most cases causing a application pool fatal restarts.

Im suspecting now that i understand what is happening that maybe this is at the cause of those issues - god knows what sort of resource overload issues the potential creation of multiple background worker threads is having on those systems.