Re: [patch] speed up / fix the new generic semaphore code (fix AIM740% regression with 2.6.26-rc1)

[Linus Torvalds]

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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