![]() ![]() The number of semphores required is much simpler to determine. Generally, the shared pool dominates all other parts of the variable portion, so as a rule of thumb, one can estimate the size as the value of shared_pool_size. There is unfortunately no simple ormula for determining the size of the variable portion. The size of the SGA is the sum of the sizes of the 4 portions. The SGA is broken into 4 sections - the fixed portion, which is constant in size, the variable portion, which varies in size depending on "init.ora" parameters, the redo block buffer, which has its size controlled by log_buffers, and the db block buffer, which has its size controlled by db_block_buffers. This will be the amount of shared memory required. Shared memory required by the Oracle Instance : On instance startup, the first things that the instance does is: -Read the "init.ora" -Start the background processes -Allocate the shared memory and semphores required The size of the SGA will be calculated from various "init.ora" parameters. This mechanism is used by Oracle to maintain concurrency control over the SGA, since it is writeable by all processes attached. wait on turning on a semphore) and post events (i.e. Such behaviour allows semaphores to be used in implementing a post-wait driver - a system where processes can wait for events (i.e. Upon awakening, the process will reattempt to turn the flag on, possibly suceeding or possibly sleeping again. If the flag is already on, processes who try to turn on the flag will sleep until the flag is off. A process can turn on the flag or turn it off. Semaphores can be thought of as flags (hence their name, semaphores). Oracle uses shared memory for implementing the SGA, which needs to be visible to all database sessions. Shared memory is exactly that - a memory region that can shared between different processes. An instance cannot start if it is unable to allocate what it needs. Shared memory and semaphores are two important resources for an Oracle instance on Unix. ![]()
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