Understanding Linux Network Internals

Memory Optimizations

Unlike user-space code and data, kernel code and data reside permanently in main memory, so it is important to reduce memory waste in every way possible. Initialization code is a good candidate for memory optimization . Given their nature, most initialization routines are executed either just once or not at all, depending on the kernel configuration. For example:

• The module_init routines are executed only once when the associated module is loaded. When the module is statically included in the kernel, the kernel can free the module_init routine right at boot time, after it runs.

• The module_exit routines are never executed when the associated modules are included statically in the kernel. In this case, therefore, there is no need to include module_exit routines into the kernel image (i.e., the routines can be discarded at link time).

The first case is a runtime optimization, and the second one is a link-time optimization.

Code and data that are used only during the boot and are not needed thereafter are placed in one of the memory sections shown in Figure 7-3. Once the kernel has completed the initialization phase, it can discard that entire memory area. This is accomplished by the call to free_init_mem,^[*] as shown in Figure 5-1 in Chapter 5. Different macros are used to place code into the different memory sections of Figure 7-3.

If you look at the example in the earlier section "New Model: Macro-Based Tagging," you can see that the two input routines to module_init ...