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    SubjectRe: [PATCHv3 3/5] mm: pack compound_dtor and compound_order into one word in struct page
    On Fri 21-08-15 13:40:59, Kirill A. Shutemov wrote:
    > On Fri, Aug 21, 2015 at 09:13:42AM +0200, Michal Hocko wrote:
    > > On Thu 20-08-15 16:26:04, Andrew Morton wrote:
    [...]
    > > > Why not use ushort for 64-bit as well?
    > >
    > > Yeah, I have asked the same in the previous round. So I've tried to
    > > compile with ushort. The resulting code was slightly larger
    > > text data bss dec hex filename
    > > 476370 90811 44632 611813 955e5 mm/built-in.o.prev
    > > 476418 90811 44632 611861 95615 mm/built-in.o.after
    > >
    > > E.g. prep_compound_page
    > > before:
    > > 4c6b: c7 47 68 01 00 00 00 movl $0x1,0x68(%rdi)
    > > 4c72: 89 77 6c mov %esi,0x6c(%rdi)
    > > after:
    > > 4c6c: 66 c7 47 68 01 00 movw $0x1,0x68(%rdi)
    > > 4c72: 66 89 77 6a mov %si,0x6a(%rdi)
    > >
    > > which looks very similar to me but I am not an expert here so it might
    > > possible that movw is slower.
    > >
    > > __free_pages_ok
    > > before:
    > > 63af: 8b 77 6c mov 0x6c(%rdi),%esi
    > > after:
    > > 63b1: 0f b7 77 6a movzwl 0x6a(%rdi),%esi
    > >
    > > which looks like a worse code to me. Whether this all is measurable or
    > > worth it I dunno. The ifdef is ugly but maybe the ugliness is a destiny
    > > for struct page.
    >
    > I don't care about the ifdef that much. If you guys prefer to drop it I'm
    > fine with that.

    I can live with it. It makes the struct more complicated which is what
    struck me. If there is a good reason and a better generated code is a
    good one then I do not object but please make it a separate patch so
    that we do not wonder why this has been done in the future.
    --
    Michal Hocko
    SUSE Labs


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