Messages in this thread | | | Date | Thu, 8 May 2008 16:07:14 -0700 (PDT) | From | Linus Torvalds <> | Subject | Re: [patch] speed up / fix the new generic semaphore code (fix AIM7 40% regression with 2.6.26-rc1) |
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On Thu, 8 May 2008, Linus Torvalds wrote: > > Some of it is that "page_to_pfn(page)", which involves a nasty division > (divide by sizeof(struct page)). It gets turned into that shift and > multiply, but it's still quite expensive with big constants etc.
Btw, sparse will complain about those, because the source code *looks* really cheap.
The normal "page_to_pfn()" looks trivial:
((unsigned long)((page) - mem_map) + ARCH_PFN_OFFSET)
which looks like a trivial subtraction and addition of a constant, but the subtraction is on C pointers, and basically turns into
((unsigned long)page - (unsigned long)mem_map) / sizeof(struct page)
and because "struct page" is not some nice power-of-two in size, that division is rather nasty even though it's a constant size.
Linus
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