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Date: Thu, 8 May 2008 16:07:14 -0700 (PDT)
From: Linus Torvalds <torvalds@linux-foundation.org>
To: Ingo Molnar <mingo@elte.hu>
cc: "Zhang, Yanmin" <yanmin_zhang@linux.intel.com>,
       Andi Kleen <andi@firstfloor.org>, Matthew Wilcox <matthew@wil.cx>,
       LKML <linux-kernel@vger.kernel.org>,
       Alexander Viro <viro@ftp.linux.org.uk>,
       Andrew Morton <akpm@linux-foundation.org>,
       Thomas Gleixner <tglx@linutronix.de>, "H. Peter Anvin" <hpa@zytor.com>,
       Alan Cox <alan@lxorguk.ukuu.org.uk>
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