c4e1be9ec1
There are a number of times that we loop over NR_MEM_SECTIONS, looking for section_present() on each section. But, when we have very large physical address spaces (large MAX_PHYSMEM_BITS), NR_MEM_SECTIONS becomes very large, making the loops quite long. With MAX_PHYSMEM_BITS=46 and a section size of 128MB, the current loops are 512k iterations, which we barely notice on modern hardware. But, raising MAX_PHYSMEM_BITS higher (like we will see on systems that support 5-level paging) makes this 64x longer and we start to notice, especially on slower systems like simulators. A 10-second delay for 512k iterations is annoying. But, a 640- second delay is crippling. This does not help if we have extremely sparse physical address spaces, but those are quite rare. We expect that most of the "slow" systems where this matters will also be quite small and non-sparse. To fix this, we track the highest section we've ever encountered. This lets us know when we will *never* see another section_present(), and lets us break out of the loops earlier. Doing the whole for_each_present_section_nr() macro is probably overkill, but it will ensure that any future loop iterations that we grow are more likely to be correct. Kirrill said "It shaved almost 40 seconds from boot time in qemu with 5-level paging enabled for me". Link: http://lkml.kernel.org/r/20170504174434.C45A4735@viggo.jf.intel.com Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com> Tested-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org> |
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README
Linux kernel ============ This file was moved to Documentation/admin-guide/README.rst Please notice that there are several guides for kernel developers and users. These guides can be rendered in a number of formats, like HTML and PDF. In order to build the documentation, use ``make htmldocs`` or ``make pdfdocs``. There are various text files in the Documentation/ subdirectory, several of them using the Restructured Text markup notation. See Documentation/00-INDEX for a list of what is contained in each file. Please read the Documentation/process/changes.rst file, as it contains the requirements for building and running the kernel, and information about the problems which may result by upgrading your kernel.