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" > > <[email protected]> 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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
