Swapping out is useful when the kernel is dangerously low on memory. As the kernel's first defense against critically low memory, it keeps a small reserve of free page frames that can be used only by the most critical functions. This turns out to be essential to avoid system crashes, which might occur when a kernel routine invoked to free resources is unable to obtain the memory area it needs to complete its task. To protect this reserve of free page frames, Linux may perform a swap out on the following occasions:
• By a kernel thread denoted as kswapd that is activated periodically whenever the number of free page frames falls below a predefined threshold.
• When a memory request to the buddy system (see Section 7.1.7) cannot be satisfied because the number of free page frames would fall below a predefined threshold.
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