Issues caused by frequent application restarts are pretty common. If you are interested in what causes application restarts in general, and how you can monitor application restarts and what causes them you should read this post: ASP.NET Case Study: Lost session variables and appdomain recycles
The most recent one was one where the ASP.NET site would respond slowly at regular intervals (read: it grinded to a halt every few hours).
First shot at gathering data
We started off by getting a memory dump with debug diag when the process was responding slowly. Unfortunately at the time that the dump was taken no requests were executing (i.e.
~* e !clrstack showed no managed stacks and
~* kb showed all the threads in their idle states), so we didn’t get much of a clue as to why the process was responding so slowly.
If you are debugging a performance issue it is key to try to get dumps while there are requests executing since memory dumps are just snapshots of the process so other than what is on the heap you really don’t get any history in a memory dump.
Luckily one of my colleagues (who was looking at the issue before me) had noticed that there were an extreme number of exceptions in the dump. Perhaps I shouldn’t call it luck really, if we get memory dumps that at first sight seem to not be all that useful we will still try to poke around in them to see if we can gather any clues at all as to what is going on, in order to make a more targeted approach in the next round of collecting data.
Either case, running
!dumpheap -type *Exception revealed that there had recently been a lot (~1000) of
Exception MethodTable: 791241c0 Exception object: 02d6c4bc Exception type: System.TypeLoadException Message: Could not load type 'System.Web.UI.WebControls.WebParts.Content' from assembly 'System.Web, Version=18.104.22.168, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a'. InnerException: StackTrace (generated): SP IP Function 00000000 00000001 System.Reflection.Assembly._GetType(System.String, Boolean, Boolean) 1D0AE8D8 661680FB System.Web.UI.NamespaceTagNameToTypeMapper.System.Web.UI.ITagNameToTypeMapper.GetControlType(System.String, System.Collections.IDictionary)
So the first working theory was that the exceptions themselves caused the poor performance, or at least had something to do with the poor performance.
Figuring out the cause of the TypeLoadExceptions
This exception tells us that someone is trying to create a
System.Web.UI.WebControls.WebParts.Content object, but it can’t because that type does not exist in System.Web.dll (the assembly that implements the
I looked up the
System.Web.UI.WebControls.WebParts namespace on MSDN and sure enough it didn’t contain any Content class. The only content class I could think of that was even remotely related was the one used for
<asp:content> </asp:content> that you use with master pages.
I started playing around a bit with a simple application with master pages and found that if I browse to my application, and then add a new page with a master page and
asp:content tags I will get this exception when this page compiles.
I know this is a bit of a mental leap, but I figured it probably had something to do with this tag and its namespace mappings so I ran
!dumpheap -stat I found a type called
System.Web.UI.TagNamespaceRegisterEntry. I had 3 of them on the heap for my simple sample each with a tag prefix, namespace and assembly name
0:005> !dumpheap -type System.Web.UI.TagNamespaceRegisterEntry Loading the heap objects into our cache. ------------------------------ Heap 0 Address MT Size 02f04df8 663b6148 28 1 System.Web.UI.TagNamespaceRegisterEntry 02f04e14 663b6148 28 1 System.Web.UI.TagNamespaceRegisterEntry 02f04f3c 663b6148 28 1 System.Web.UI.TagNamespaceRegisterEntry total 3 objects ------------------------------ Heap 1 Address MT Size total 0 objects ------------------------------ total 3 objects Statistics: MT Count TotalSize Class Name 663b6148 3 84 System.Web.UI.TagNamespaceRegisterEntry Total 3 objects, Total size: 84
0:005> !do 02f04df8 Name: System.Web.UI.TagNamespaceRegisterEntry MethodTable: 663b6148 EEClass: 663b60c8 Size: 28(0x1c) bytes GC Generation: 1 (C:\WINDOWS\assembly\GAC_32\System.Web\22.214.171.124__b03f5f7f11d50a3a\System.Web.dll) Fields: MT Field Offset Type VT Attr Value Name 790fd8c4 4002029 4 System.String 0 instance 00000000 _virtualPath 79102290 400202a 8 System.Int32 1 instance 0 _line 790fd8c4 400202b c System.String 0 instance 02f04bc8 _tagPrefix 790fd8c4 400202c 10 System.String 0 instance 02f04be0 _ns 790fd8c4 400202d 14 System.String 0 instance 02f04c24 _assemblyName
I dumped out all 3 of them and found the following mappings
- asp -> System.Web.UI.WebControls.WebParts
- asp -> System.Web.UI.WebControls
- mobile -> System.Web.UI.MobileControls
So, the tag prefix asp: maps to both WebControls.WebParts which explains why it tries to look in the WebParts namespace for Content. Since the process doesn’t fail because of this, but rather just continues happily delivering the right Content object it also means that this exception is benign, i.e. it’ll just try this first, if it fails it continues looking in the WebControls namespace.
However… we still haven’t answered the most important questions
- Why are there so many of them? In my little sample I only get a new one if I modify a page so that it has to be recompiled. If I compile normally I will only get one independently of how many pages I have.
- Why would it be compiling these pages at regular intervals, especially a few hours into the life of the process, all pages should be compiled by then
The only thing that seemed to fit was if perhaps the appdomain was recycling as this would answer both these questions.
Figuring out if the appdomain is recycling
I dumped out the
System.Web.HttpRuntime objects with
!dumpheap -type System.Web.HttpRuntime. There is one per application loaded in the process.
The HttpRuntime object has a member variable called
_firstRequestStartTime and the expected start time should be some time around the start of the process. If the
_firstRequestStartTime is within the last few minutes, that means that the application has restarted very recently.
0:005> !do 0x6eb2d14 Name: System.Web.HttpRuntime MethodTable: 6639a67c EEClass: 6639a60c Size: 152(0x98) bytes GC Generation: 2 (C:\WINDOWS\assembly\GAC_32\System.Web\126.96.36.199__b03f5f7f11d50a3a\System.Web.dll) Fields: MT Field Offset Type VT Attr Value Name 791132b4 40010f8 4 ...amedPermissionSet 0 instance 00000000 _namedPermissionSet 6639b12c 40010f9 8 ...ileChangesMonitor 0 instance 06eb3218 _fcm 6639e2c8 40010fa c ...ing.CacheInternal 0 instance 06eb4d74 _cacheInternal 6639d878 40010fb 10 ...Web.Caching.Cache 0 instance 06eb4c8c _cachePublic 7910be50 40010fc 74 System.Boolean 1 instance 0 _isOnUNCShare 6639ad78 40010fd 14 ...Web.Util.Profiler 0 instance 06eb2dac _profiler 6639ae78 40010fe 18 ...estTimeoutManager 0 instance 06eb2dc8 _timeoutManager 663aebe4 40010ff 1c ....Web.RequestQueue 0 instance 02ed438c _requestQueue 7910be50 4001100 75 System.Boolean 1 instance 0 _apartmentThreading 7910be50 4001101 76 System.Boolean 1 instance 1 _processRequestInApplicationTrust 7910be50 4001102 77 System.Boolean 1 instance 0 _beforeFirstRequest 7910c878 4001103 84 System.DateTime 1 instance 06eb2d98 _firstRequestStartTime 7910be50 4001104 78 System.Boolean 1 instance 1 _firstRequestCompleted 7910be50 4001105 79 System.Boolean 1 instance 0 _userForcedShutdown ...
It’s a bit tricky to convert the time (a value type) to an actual time stamp, but it goes like this.
dq <address of firstrequeststarttime>(dump the content of the start time as a quad word to get the number of ticks.)
.formats <quad word>&&0x3FFFFFFFFFFFFFFF(run .formats on the number of ticks with the 2 highest bits filtered out)
0:005> dq 06eb2d98 06eb2d98 48ca8304`65439a9d 00000000`00000000 06eb2da8 6639ad78`00000000 0000000a`02ed4c5c 06eb2db8 01000000`00000000 00000000`00000000 06eb2dc8 06eb2dec`6639ae78 00000000`06eb2f54 06eb2dd8 00000000`00000005 00000000`08f0d180 06eb2de8 7912d8f8`00000000 6639af40`0000000d 06eb2df8 06eb2e44`06eb2e30 06eb2e6c`06eb2e58 06eb2e08 06eb2e94`06eb2e80 06eb2ebc`06eb2ea8
0:005> .formats 48ca8304`65439a9d&0x3FFFFFFFFFFFFFFF Evaluate expression: Hex: 08ca8304`65439a9d Decimal: 633462752501013149 Octal: 0043124060214520715235 Binary: 00001000 11001010 10000011 00000100 01100101 01000011 10011010 10011101 Chars: ....eC.. Time: Tue May 13 13:34:10.101 3608 (GMT+2) Float: low 5.77321e+022 high 1.21882e-033 Double: 2.56941e-266
The first request for this appdomain came in at 13:34:10, now we can compare this to the current time
0:005> .time Debug session time: Tue May 13 13:35:47.210 2008 (GMT+2) System Uptime: 33 days 11:04:15.423 Process Uptime: 0 days 5:42:08.328 ...
And we find that the appdomain restarted less than a minute ago, so it is definitely a case of process restarts.
Why did the appdomain restart?
After adding logging per the ASP.NET Case Study: Lost session variables and appdomain recycles article the following showed up in the event log
_shutDownStack: at System.Environment.GetStackTrace(Exception e, Boolean needFileInfo) at System.Environment.get_StackTrace() at System.Web.Hosting.HostingEnvironment.InitiateShutdownInternal() at System.Web.Hosting.HostingEnvironment.InitiateShutdown() at System.Web.HttpRuntime.ShutdownAppDomain(String stackTrace) at System.Web.HttpRuntime.OnCriticalDirectoryChange(Object sender, FileChangeEvent e) at System.Web.FileChangesMonitor.OnCriticaldirChange(Object sender, FileChangeEvent e) at System.Web.DirectoryMonitor.FireNotifications() at System.Web.Util.WorkItem.CallCallbackWithAssert(WorkItemCallback callback) at System.Web.Util.WorkItem.OnQueueUserWorkItemCompletion(Object state) at System.Threading._ThreadPoolWaitCallback.WaitCallback_Context(Object state) at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state) at System.Threading._ThreadPoolWaitCallback.PerformWaitCallbackInternal(_ThreadPoolWaitCallback tpWaitCallBack) at System.Threading._ThreadPoolWaitCallback.PerformWaitCallback(Object state) _shutDownMessage: Change Notification for critical directories. bin dir change or directory rename HostingEnvironment initiated shutdown HostingEnvironment caused shutdown Change in C:\WINDOWS\Microsoft.NET\Framework\v2.0.50727\Temporary ASP.NET Files\root\5fe0fce6\53087c60\hash\hash.web
The next step was to figure out who was touching the hash.web by monitoring the file with process monitor, and It turned out, it was antivirus scanning the file, which in turn triggered the filechangesmonitor to recycle the appdomain.
Normally I would suggest looking into if it is possible to exclude the Temporary ASP.NET Files directory and subdirectories from automatic virus scanning as well as the web content directories.
In 2.0 it is also possible to disable filechangesmonitoring through the
HKLM\Software\Microsoft\ASP.NET\FCNMode key. This KB article lists the different values that this can be set to, a value of 1 will disable file changes monitoring. The drawback of disabling this is that if you disable it, the application will not restart if you change web.config etc. so you need to carefully consider what you want to set this to.