Messages in this thread | | | Date | Mon, 7 Jan 2002 12:25:55 +1100 | From | Anton Blanchard <> | Subject | results: Remove 8 bytes from struct page on 64bit archs |
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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?)
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