.net barcode reader Thread Basics in .NET framework

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consumption) and time (runtime execution performance) overhead associated with them . Let s explore this overhead in more detail now . Every thread has one of each of the following:
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Thread kernel object The OS allocates and initializes one of these data structures for each thread created in the system . The data structure contains a bunch of properties (discussed later in this chapter) that describe the thread . This data structure also contains what is called the thread s context . The context is a block of memory that contains a set of the CPU s registers . When Windows is running on a machine with an x86 CPU, the thread s context uses about 700 bytes of memory . For x64 and IA64 CPUs, the context is about 1,240 and 2,500 bytes of memory, respectively . Thread environment block (TEB) The TEB is a block of memory allocated and initialized in user mode (address space that application code can quickly access) . The TEB consumes 1 page of memory (4 KB on x86 and x64 CPUs, 8 KB on an IA64 CPU) . The TEB contains the head of the thread s exception-handling chain . Each try block that the thread enters inserts a node in the head of this chain; the node is removed from the chain when the thread exists in the try block . In addition, the TEB contains the thread s thread-local storage data as well as some data structures for use by Graphics Device Interface (GDI) and OpenGL graphics . User-mode stack The user-mode stack is used for local variables and arguments passed to methods . It also contains the address indicating what the thread should execute next when the current method returns . By default, Windows allocates 1 MB of memory for each thread s user-mode stack .1 Kernel-mode stack The kernel-mode stack is also used when application code passes arguments to a kernel-mode function in the operating system . For security reasons, Windows copies any arguments passed from user-mode code to the kernel from the thread s user-mode stack to the thread s kernel-mode stack . Once copied, the kernel can verify the arguments values, and since the application code can t access the kernelmode stack, the application can t modify the arguments values after they have been validated and the OS kernel code begins to operate on them . In addition, the kernel calls methods within itself and uses the kernel-mode stack to pass its own arguments, to store a function s local variables, and to store return addresses . The kernel-mode stack is 12 KB when running on a 32-bit Windows system and 24 KB when running on a 64-bit Windows system .
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For native applications, Windows reserves the 1 MB of address space and sparsely commits physical storage to it as the thread actually requires it when growing the stack . However, when managed applications create a thread, the CLR forces Windows to reserve and fully commit the stack immediately, so 1 MB of physical storage is fully allocated as each thread is created . The CLR team did this to make managed code behave more reliably in situations where the system was running low on memory . For example, the CLR and managed applications never have to worry about recovering from insufficient memory when attempting to grow a thread s stack . This was very important for Microsoft SQL Server when executing stored procedures implemented in managed code .
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Part V
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DLL thread-attach and thread-detach notifications Windows has a policy that whenever a thread is created in a process, all DLLs loaded in that process have their DllMain method called, passing a DLL_THREAD_ATTACH flag . Similarly, whenever a thread dies, all DLLs in the process have their DllMain method called, passing it a DLL_THREAD_DETACH flag . Some DLLs need these notifications to perform some special initialization or cleanup for each thread created/destroyed in the process . For example, the C-Runtime library DLL allocates some thread-local storage state that is required should the thread use functions contained within the C-Runtime library .
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In the early days of Windows, many processes had maybe 5 or 6 DLLs loaded into them, but today, some processes have several hundred DLLs loaded into them . Right now, on my machine, Microsoft Office Outlook has about 250 DLLs loaded into its process address space! This means that whenever a new thread is created in Office Outlook, 250 DLL functions must get called before the thread is allowed to do what it was created to do . And these 250 functions must be called again whenever a thread in Outlook dies . Wow this can seriously affect the performance of creating and destroying threads within a process .2 So now, you see all the space and time overhead that is associated with creating a thread, letting it sit around in the system, and destroying it . But the situation gets even worse now we re going to start talking about context switching . A computer with only one CPU in it can do only one thing at a time . Therefore, Windows has to share the actual CPU hardware among all the threads (logical CPUs) that are sitting around in the system . At any given moment in time, Windows assigns one thread to a CPU . That thread is allowed to run for a time-slice (sometimes referred to as a quantum) . When the time-slice expires, Windows context switches to another thread . Every context switch requires that Windows performs the following actions: 1. Save the values in the CPU s registers to the currently running thread s context structure inside the thread s kernel object . 2. Select one thread from the set of existing threads to schedule next . If this thread is owned by another process, then Windows must also switch the virtual address space seen by the CPU before it starts executing any code or touching any data . 3. Load the values in the selected thread s context structure into the CPU s registers . After the context switch is complete, the CPU executes the selected thread until its time-slice expires, and then another context switch happens again . Windows performs context switches about every 30 ms . Context switches are pure overhead; that is, there is no memory or performance benefit that comes from context switches . Windows performs context switching to provide end users with a robust and responsive operating system .
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DLLs produced by C# and most other managed programming languages do not have a DllMain in them at all and so managed DLLs will not receive the DLL_THREAD_ATTACH and DLL_THREAD_DETACH notifications improving performance . In addition, unmanaged DLLs can opt out of these notifications by calling the Win32 DisableThreadLibraryCalls function . Unfortunately, many unmanaged developers are not aware of this function, so they don t call it .
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