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Date: Mon, 7 Jan 2002 12:25:55 +1100
From: Anton Blanchard <anton@samba.org>
To: Dave Jones <davej@suse.de>
Cc: "David S. Miller" <davem@redhat.com>, linux-kernel@vger.kernel.org
Subject: results: Remove 8 bytes from struct page on 64bit archs
Message-ID: <20020107012555.GA6623@krispykreme>
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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
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