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Hi, > I'm curious to see how large the tradeoff is with calculating > virtual in page_address(). The overhead there may be larger than > the win we get from better cacheline footprint in struct page Check out: http://samba.org/~anton/linux/struct_page/ "struct page can be hazardous to your health" On this machine (2 way power3) we see a ~5% improvement on dbench by removing ->zone. Quite a nice improvement. When we also remove ->virtual and use pointer arithmetic to do page_address() performance drops back down. The reason for this can be found in: http://samba.org/~anton/linux/struct_page/2.4.18pre1-nopagezone-nopagevirtual/2/6.html Search for divd, remembering the percentage to the left is shifted one instruction down. Yes divides really hurt. Basically the difference is: page_address() using page->virtual: ld 25,112(31) page_address() using pointer arithmetic: .LC1: .quad mem_map .LC2: .quad 0xc000000000000000 ... ld 9,.LC1 li 11,120 ; sizeof(struct page) ld 10,.LC2 ld 0,0(9) subf 0,0,31 divd 0,0,11 sldi 0,0,12 add 25,0,10 Perhaps the compiler should be optimising this better (can we replace the divide?) Anton - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/ | ||||||||||||
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