On Thu, Jan 29, 2009 at 12:50:32PM -0600, Rob Landley wrote: > 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. Hmmm. SPARSEMEM_VMEMMAP enables use of a memmap contigious in the kernel virtual address space, this allows optimised access to the memmap. It does consume more kernel virtual address space and is not suitable for processors with limited virtual address for the kernel (such as i386). > Looking at the commit, the comment is: > > > 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. If you have lots of virtual address space such as on 64 bit processors you should be enabling it. For smaller processors it is appropriate where the memory is mostly contigious physically. Does that help at all? -apw -- 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
