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Solaris™ Internals: Core Kernel Components by Richard McDougall, Jim Mauro

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5.5. Anonymous Memory

At many points we have mentioned anonymous memory. Anonymous memory refers to pages that are not directly associated with a vnode. Such pages are used for a process's heap space, its stack, and copy-on-write pages. In the Solaris kernel, two subsystems are dedicated to managing anonymous memory: the anon layer and the swapfs file system.

The anonymous memory allocated to the heap of a process is a result of a zero-fill-on-demand operation (ZFOD). The ZFOD operation is how we allocate new pages. A ZFOD occurs when we touch a memory address for the first time, and a new page of memory is dynamically allocated at that address. ZFOD is the allocation method used for the heap space and segments created as a map of /dev/zero

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