> The help option in mm/Kconfig says: > > > help > > SPARSEMEM_VMEMMAP uses a virtually mapped memmap to optimise > > pfn_to_page and page_to_pfn operations. This is the most > > efficient option when sufficient kernel resources are available. > > This doesn't really tell me what it does, what the plusses and minuses are, or > when I would or wouldn't need to select it. I don't have enough information > to make this config selection (what are "sufficient kernel resources"? Would > a 256 meg qemu image qualify?), and the help isn't helping. > > Looking at the commit, the comment is: Hmm. This option make a difference about kernel internal structures. SPARSEMEM_VMEMMAP simplify the calculation of pfn_to_page/page_to_pfn. So, it may make kernel works faster a bit. (I don't remember the number of performance impact.) I guess "sufficient kernel resources" means the space of virtual address. VMEMMAP makes a continuous array of pages on virtual address for all possible physical memory. But it's requires huge space of virtual address. So if your box is 32bit cpu, then SPARSEMEM_VMEMMAP should be off. If you don't worry about internal kernel structures, then you can use default setting. I hope this is your help. > > > SPARSEMEM_VMEMMAP needs to be a selectable config option to support > > building the kernel both with and without sparsemem vmemmap support. This > > selection is desirable for platforms which could be configured one way for > > platform specific builds and the other for multi-platform builds. > > Once again, how does someone configuring a system make a decision? Apparently > you know, but this documentation isn't saying what the decision criteria or > requirements/benefits are. Certainly, this is not good help. Thanks. -- Yasunori Goto -- To unsubscribe from this list: send the line "unsubscribe linux-doc" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
