ASP.NET Memory: Thou shalt not store UI objects in cache or session scope
ASP.NET Memory: Thou shalt not store UI objects in cache or session scope Surprisingly the issue I wrote about in the event handlers that made the memory balloon (Jan 2006) is something that still happens very frequently, I reference it in cases at least a few times a month. Just this last week I had different variations of it crop up in different cases so in this post I will show a different variation, what to look out for and how to identify it.
Problem description
The issue here is a quick growth in memory, eventually causing OutOfMemory exceptions.
The problem
The problem in this case was that the application stored UI controls in session scope, i.e. Labels, DataGrids etc. in order to keep relevant information about the user throughout the session. The data they want to store is not that big, a subset of a dataset, some user related strings etc. and since it was already nicely contained in the controls there was no reason to extract them into a separate object to store in session scope.
WRONG!
If we look at a dropdown list for example in memory, this is what a dropdown list contains (the Name column contains the member variable names, and the Type column shows the type of the member variable)
0:032> !do 06f4fb14
Name: System.Web.UI.WebControls.DropDownList
MethodTable: 663b95bc
EEClass: 663b952c
Size: 124(0x7c) bytes
(C:\WINDOWS\assembly\GAC_32\System.Web\2.0.0.0__b03f5f7f11d50a3a\System.Web.dll)
Fields:
MT Field Offset Type VT Attr Value Name
790fd8c4 4002144 4 System.String 0 instance 02f1fd64 _id
790fd8c4 4002145 8 System.String 0 instance 02f1fd64 _cachedUniqueID
> 663a66b8 4002146 c ...em.Web.UI.Control 0 instance 06f4f47c _parent
6641d56c 4002147 2c System.Int32 1 instance 6 _controlState
> 6641194c 4002148 10 ...m.Web.UI.StateBag 0 instance 06f51c5c _viewState
663a66b8 4002149 14 ...em.Web.UI.Control 0 instance 06f4dba0 _namingContainer
> 663a40c4 400214a 18 System.Web.UI.Page 0 instance 06f4dba0 _page
6641f33c 400214b 1c ...+OccasionalFields 0 instance 06f53958 _occasionalFields
663b63cc 400214c 20 ...I.TemplateControl 0 instance 00000000 _templateControl
6639b220 400214d 24 ...m.Web.VirtualPath 0 instance 00000000 _templateSourceVirtualDirectory
66417cf0 400214e 28 ...rs.ControlAdapter 0 instance 00000000 _adapter
663bfea4 400214f 30 ...SimpleBitVector32 1 instance 06f4fb44 flags
790fd0f0 400213e d08 System.Object 0 shared static EventDataBinding
>> Domain:Value 00198418:NotInit 001c6fd0:02f06fd0 <<
790fd0f0 400213f d0c System.Object 0 shared static EventInit
>> Domain:Value 00198418:NotInit 001c6fd0:02f06fdc <<
790fd0f0 4002140 d10 System.Object 0 shared static EventLoad
>> Domain:Value 00198418:NotInit 001c6fd0:02f06fe8 <<
790fd0f0 4002141 d14 System.Object 0 shared static EventUnload
>> Domain:Value 00198418:NotInit 001c6fd0:02f06ff4 <<
790fd0f0 4002142 d18 System.Object 0 shared static EventPreRender
>> Domain:Value 00198418:NotInit 001c6fd0:02f07000 <<
790fd0f0 4002143 d1c System.Object 0 shared static EventDisposed
>> Domain:Value 00198418:NotInit 001c6fd0:02f0700c <<
7912d8f8 4002150 d20 System.Object[] 0 shared static automaticIDs
>> Domain:Value 00198418:NotInit 001c6fd0:02f07018 <<
790fd8c4 40025c5 34 System.String 0 instance 00000000 tagName
663e95f0 40025c6 44 System.Int32 1 instance 76 tagKey
663eb1d0 40025c7 38 ...tributeCollection 0 instance 00000000 attrColl
6641194c 40025c8 3c ...m.Web.UI.StateBag 0 instance 00000000 attrState
66401a78 40025c9 40 ...WebControls.Style 0 instance 00000000 controlStyle
663bfea4 40025ca 48 ...SimpleBitVector32 1 instance 06f4fb5c _webControlFlags
790fd0f0 40025cc 4c System.Object 0 instance 00000000 _dataSource
7910be50 40025cd 50 System.Boolean 1 instance 1 _requiresDataBinding
7910be50 40025ce 51 System.Boolean 1 instance 1 _inited
7910be50 40025cf 52 System.Boolean 1 instance 1 _preRendered
7910be50 40025d0 53 System.Boolean 1 instance 0 _requiresBindToNull
7910be50 40025d1 54 System.Boolean 1 instance 0 _throwOnDataPropertyChange
790fd0f0 40025cb ec0 System.Object 0 shared static EventDataBound
>> Domain:Value 00198418:NotInit 001c6fd0:02f209c8 <<
66404954 40025d3 58 ...UI.DataSourceView 0 instance 06f51c90 _currentView
7910be50 40025d4 55 System.Boolean 1 instance 0 _currentViewIsFromDataSourceID
7910be50 40025d5 56 System.Boolean 1 instance 1 _currentViewValid
663c876c 40025d6 5c ...eb.UI.IDataSource 0 instance 06f51c80 _currentDataSource
7910be50 40025d7 57 System.Boolean 1 instance 1 _currentDataSourceValid
663ba948 40025d8 60 ...ceSelectArguments 0 instance 06f539fc _arguments
7910be50 40025d9 64 System.Boolean 1 instance 1 _pagePreLoadFired
7910be50 40025da 65 System.Boolean 1 instance 0 _ignoreDataSourceViewChanged
6641a730 4002653 68 ...istItemCollection 0 instance 06f4fb90 items
79102290 4002654 74 System.Int32 1 instance -1 cachedSelectedIndex
790fd8c4 4002655 6c System.String 0 instance 00000000 cachedSelectedValue
79104368 4002656 70 ...ections.ArrayList 0 instance 06f539b0 cachedSelectedIndices
7910be50 4002657 66 System.Boolean 1 instance 0 _stateLoaded
790fd0f0 4002651 ee0 System.Object 0 shared static EventSelectedIndexChanged
>> Domain:Value 00198418:NotInit 001c6fd0:02f20bfc <<
790fd0f0 4002652 ee4 System.Object 0 shared static EventTextChanged
>> Domain:Value 00198418:NotInit 001c6fd0:02f20c08 <<
So if I have a dropdown list that contain all the US states and their abbreviations, and I want to store this in cache to avoid having to rebuild the list, a logical step to take might be to cache the dropdown list.
However, if I do so, anything that it references will be referenced as long as the dropdown list is in cache, so it can’t be garbage collected. In the case of web controls, they all have a _parent member variable which links back to the parent control, and a _page that links back to the page where the control was originally instantiated. In other words, this means that as long as your control is in cache, you will also be caching the page, any other controls that it uses, any data bindings to data grids etc.
As you can see above you will also be caching the state bag (_viewState), the IDataSource and anything it references, etc. etc. all in all it builds up to quite a bit more data than we originally bargained for, when in reality, what I wanted to store was just the list of states and abbreviations and maybe the selected item (if storing it in session scope).
The resolution
Whenever you store anything in cache or session scope, make sure that you store just what you need and nothing more. If you store an object that is not a simple type like a string or an int, make sure that you know all about that object and it’s member variables so that you know exactly what it is that you will be holding on to. UI objects are never good to store in cache or session scope since they contain quite a bit of extra data about their appearance and location, instead just extracting the ItemsCollection and the selected item would be appropriate in the case above.
How you can identify the problem in a memory dump
You can get a memory dump when memory is high either with Debug Diagnostics, or by running adplus -hang -pn w3wp.exe (from the debugging tools for windows).
If you open it up in windbg, and load sos (.loadby sos mscorwks) these issues are fairly easy to identify.
Running !dumpheap -stat you will see a lot of System.Web.UI items at the bottom of the output in this case.
65412bb4 2,461 3,004,252 System.Data.RBTree`1+Node[[System.Data.DataRow, System.Data]][]
7a75a878 189,495 3,031,920 System.Collections.Specialized.NameObjectCollectionBase+NameObjectEntry
65408b8c 47,914 3,066,496 System.Data.DataRow
663c1f78 80,093 3,524,092 System.Web.UI.WebControls.TableStyle
6640776c 226,668 3,626,688 System.Web.UI.Pair
66412f04 140,180 3,925,040 System.Web.UI.WebControls.ListItem
663eb1d0 278,998 4,463,968 System.Web.UI.AttributeCollection
654359c8 3,203 4,682,276 System.Data.RBTree`1+Node[[System.Int32, mscorlib]][]
663c3f04 397,066 4,764,792 System.Web.UI.WebControls.FontInfo
663bb498 133,543 4,807,548 System.Web.UI.WebControls.TableRow+CellControlCollection
663d5cb0 64,087 5,126,960 System.Web.UI.WebControls.HyperLink
1c1165bc 51,295 5,129,500 SomeControlLibrary.WebControls.SomeControl
79102290 551,137 6,613,644 System.Int32
663d6ad4 78,635 6,919,880 System.Web.UI.WebControls.Table
79104368 312,558 7,501,392 System.Collections.ArrayList
66411ac8 390,338 9,368,112 System.Web.UI.WebControls.Unit
663f2408 112,314 9,434,376 System.Web.UI.WebControls.Image
66401a78 251,127 11,049,588 System.Web.UI.WebControls.Style
663ec8e8 133,546 11,217,864 System.Web.UI.WebControls.TableRow
663dd2b0 259,638 11,424,072 System.Web.UI.WebControls.TableItemStyle
663bd0a0 149,928 11,994,240 System.Web.UI.WebControls.Label
664140a4 263,577 15,814,620 System.Web.UI.LiteralControl
7912d9bc 42,506 16,614,336 System.Collections.Hashtable+bucket[]
663c7308 462,037 16,633,332 System.Web.UI.ControlCollection
6641194c 1,269,095 20,305,520 System.Web.UI.StateBag
663d7328 341,364 28,674,576 System.Web.UI.WebControls.TableCell
7a7580d0 1,494,283 29,885,660 System.Collections.Specialized.HybridDictionary
6641f33c 751,897 33,083,468 System.Web.UI.Control+OccasionalFields
7a75820c 1,309,784 36,673,952 System.Collections.Specialized.ListDictionary
663c1de8 2,427,266 38,836,256 System.Web.UI.StateItem
7912d8f8 1,016,027 47,522,388 System.Object[]
7a7582d8 2,671,736 53,434,720 System.Collections.Specialized.ListDictionary+DictionaryNode
000e1b50 6,519 69,179,268 Free
790fd8c4 1,674,890 213,206,880 System.String
And if you run !dumpheap -type aspx or !dumpheap -type ascx to list all the aspx/ascx pages on the heap you will likely find many of them
0:034> !dumpheap -type aspx
...
Statistics:
MT Count TotalSize Class Name
1d32425c 8 5,088 ASP.register_aspx
1b39666c 175 86,800 ASP.login_aspx
1ceab19c 199 93,928 ASP.products_aspx
1c15b6cc 280 222,880 ASP.home_aspx
1cea17ac 532 261,744 ASP.default_aspx
Total 1,194 objects, Total size: 670,440
The next step is to take any of these pages, preferably the ones that are most plentiful like ASP.default_aspx in this case, and !gcroot some of them to figure out why they are still around
0:034> !dumpheap -mt 1cea17ac
Using our cache to search the heap.
Address MT Size Gen
...
040d0eb4 1cea17ac 492 2 ASP.default_aspx
04110d38 1cea17ac 492 2 ASP.default_aspx
0412b918 1cea17ac 492 2 ASP.default_aspx
04201758 1cea17ac 492 2 ASP.default_aspx
04213ea8 1cea17ac 492 2 ASP.default_aspx
0421943c 1cea17ac 492 2 ASP.default_aspx
042496bc 1cea17ac 492 2 ASP.default_aspx
042ae71c 1cea17ac 492 2 ASP.default_aspx
042ec8f4 1cea17ac 492 2 ASP.default_aspx
043310b4 1cea17ac 492 2 ASP.default_aspx
043c7f74 1cea17ac 492 2 ASP.default_aspx
0442d27c 1cea17ac 492 2 ASP.default_aspx
0453cb08 1cea17ac 492 2 ASP.default_aspx
04617168 1cea17ac 492 2 ASP.default_aspx
...
0:034> !gcroot 042ae71c
...
ESP:1fbf894:Root: 109420b0(System.Threading._TimerCallback)->
10942068(System.Threading.TimerCallback)->
10940004(System.Web.Caching.CacheExpires)->
10940024(System.Object[])->
109407b4(System.Web.Caching.ExpiresBucket)->
06f05928(System.Web.Caching.ExpiresPage[])->
06f059ac(System.Web.Caching.ExpiresEntry[])->
04616834(System.Web.Caching.CacheEntry)->
04616804(System.Web.SessionState.InProcSessionState)->
0e474dd4(System.Web.SessionState.SessionStateItemCollection)->
0e474e34(System.Collections.Hashtable)->
0e481110(System.Collections.Hashtable+bucket[])->
04616698(System.Collections.Specialized.NameObjectCollectionBase+NameObjectEntry)->
045ea5d8(System.Web.UI.WebControls.DropDownList)->
042ae71c(ASP.default_aspx)->
If we follow the root chain from the bottom up we see that ASP.default_aspx is a member variable of the DropDownList (the _page member variable) and that this in turn is a member variable of a NameObjectEntry in a hash table in session state. The NameObjectEntry is a key-value pair with the key and the value of a session variable, so through this we can gather that we are storing the DropDownlist in session scope.
If we look at the NameObjectEntry we can also see exactly which session variable it is (LstStates in this case)
0:034> !do 04616698
Name: System.Collections.Specialized.NameObjectCollectionBase+NameObjectEntry
MethodTable: 7a75a878
EEClass: 7a7c51c4
Size: 16(0x10) bytes
GC Generation: 2
(C:\WINDOWS\assembly\GAC_MSIL\System\2.0.0.0__b77a5c561934e089\System.dll)
Fields:
MT Field Offset Type VT Attr Value Name
790fd8c4 400117c 4 System.String 0 instance 04968898 Key
790fd0f0 400117d 8 System.Object 0 instance 045ea5d8 Value
0:034> !do 04968898
Name: System.String
MethodTable: 790fd8c4
EEClass: 790fd824
Size: 38(0x26) bytes
GC Generation: 2
(C:\WINDOWS\assembly\GAC_32\mscorlib\2.0.0.0__b77a5c561934e089\mscorlib.dll)
String: LstStates
Fields:
MT Field Offset Type VT Attr Value Name
79102290 4000096 4 System.Int32 1 instance 11 m_arrayLength
79102290 4000097 8 System.Int32 1 instance 10 m_stringLength
790ff328 4000098 c System.Char 1 instance 4c m_firstChar
790fd8c4 4000099 10 System.String 0 shared static Empty
>> Domain:Value 000db310:790d884c 00109200:790d884c <<
7912dd40 400009a 14 System.Char[] 0 shared static WhitespaceChars
>> Domain:Value 000db310:109203f4 00109200:109242f8 <<
In summary, be careful with what you store away,
Laters, Tess
