On Fri, Aug 21, 2015 at 09:13:42AM +0200, Michal Hocko wrote:
> On Thu 20-08-15 16:26:04, Andrew Morton wrote:
> > On Wed, 19 Aug 2015 12:21:44 +0300 "Kirill A. Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx> wrote:
> >
> > > The patch halves space occupied by compound_dtor and compound_order in
> > > struct page.
> > >
> > > For compound_order, it's trivial long -> int/short conversion.
> > >
> > > For get_compound_page_dtor(), we now use hardcoded table for destructor
> > > lookup and store its index in the struct page instead of direct pointer
> > > to destructor. It shouldn't be a big trouble to maintain the table: we
> > > have only two destructor and NULL currently.
> > >
> > > This patch free up one word in tail pages for reuse. This is preparation
> > > for the next patch.
> > >
> > > ...
> > >
> > > @@ -145,8 +143,13 @@ struct page {
> > > */
> > > /* First tail page of compound page */
> > > struct {
> > > - compound_page_dtor *compound_dtor;
> > > - unsigned long compound_order;
> > > +#ifdef CONFIG_64BIT
> > > + unsigned int compound_dtor;
> > > + unsigned int compound_order;
> > > +#else
> > > + unsigned short int compound_dtor;
> > > + unsigned short int compound_order;
> > > +#endif
> >
> > 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.
--
Kirill A. Shutemov
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