This parameter affects whether the kernel will compact memory or direct reclaim to satisfy a high-order allocation. The extfrag/extfrag_index file in debugfs shows what the fragmentation index for each order is in each zone in the system. Values tending towards 0 imply allocations would fail due to lack of memory, values towards 1000 imply failures are due to fragmentation and -1 implies that the allocation will succeed as long as watermarks are met.
The kernel will not compact memory in a zone if the fragmentation index is <= extfrag_threshold. The default value is 500.
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