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When you mix native and managed code, you often face the situation that an exception thrown in native code must be handled in managed code and vice versa. In native code, there are two exception models: C++ exception handling and Win32 SEH. In mixed code, you also have to care about managed exceptions. The exception handling architecture in .NET has remarkable similarities to the Win32 SEH model. This enables managed code to catch native C++ exceptions as well as SEH exceptions. In addition to these features, native exceptions can also be mapped to managed exceptions if this is required. Let s start with Win32 SEH exceptions. Even though the following code uses SEH exceptions, it can be compiled with /clr: // ExceptionHandling1.cpp // compile with "cl /clr ExceptionHandling1.cpp" #include <excpt.h> #include <windows.h> // As I will discuss later, #pargma managed is not recommended; it is // only used to show exceptions thrown across managed / unmanaged boundaries // without using two source files

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Large pool: Stores large memory allocations, such as RMAN backup buffers Streams pool: Supports the Oracle Streams feature When you start the Oracle instance, Oracle allocates memory as needed until it reaches the size set in the SGA_TARGET initialization parameter, which sets the limit for the total memory allocation If your total memory allocation is already at the SGA_TARGET limit, you can t dynamically increase memory to any SGA component without decreasing some other component s memory allocation Oracle does allow you to exchange the memory from one dynamically sizable memory component to another For example, you can increase the memory assigned to the buffer cache by taking it from the shared pool If you have certain jobs run only at specified times of the day, you can write a simple script that runs before the job executes and modifies the allocation of memory among the various components.

After the job completes, you can have another script run that changes the memory allocation back to the original settings The next few sections discuss the various components of the SGA You can manage the SGA yourself, by calibrating the memory you make available to the Oracle instance with the changing memory requirements of the running instance However, the best way to manage the SGA is simply by using automatic shared memory management, which I introduce in the Automatic Shared Memory Management section, later in this chapter..

In addition to memory and swap utilization, the script determines the system-load averages, again using the set command to assign values to the positional variables. The /proc/loadavg file contains a single line of space-separated fields, where the first three fields are the 1-minute, 5-minute, and 15-minute load averages of the running system.

The database buffer cache consists of the memory buffers that Oracle uses to hold the data read by the server process from data files on disk in response to user requests. Buffer cache access is, of course, much faster than reading the data from disk storage. When the users modify data, those changes are made in the database buffer cache as well. The buffer cache thus contains both the original blocks read from disk and the changed blocks that have to be written back to disk. You can group the memory buffers in the database buffer cache into three components: Free buffers: These are buffers that do not contain any useful data, and, thus, the database can reuse them to hold new data it reads from disk. Dirty buffers: These contain data that was read from disk and then modified, but hasn t yet been written to the data files on disk. Pinned buffers: These are data buffers that are currently in active use by user sessions.

When a user process requests data, Oracle will first check whether the data is already available in the buffer cache If it is, the server process will read the data from the SGA directly and send it to the user If the data isn t found in the buffer cache, the server process will read the relevant data from the data files on disk and cache it in the database buffer cache Of course, there must be free buffers available in the buffer cache for the data to be read into them If the server process can t find a free buffer after searching through a threshold number of buffers, it asks the database writer process to write some of the dirty buffers to disk, thus freeing them up for writing the new data it wants to read into the buffer cache.