Re: [RFC] Making compound pages mandatory

[Nick Piggin]

Andrew Morton wrote:
> Linus Torvalds <torvalds@xxxxxxxx> wrote:
>
> > On Tue, 16 Nov 2004, David Howells wrote:
> >
> > > Do you have any objection to compound pages being made mandatory, even without
> > > HUGETLB support?
> >
> > I haven't really looked into it, so I cannot make an informed decision.  
> > How big is the overhead? And what's the _point_, since we don't seem to 
> > need them normally, but the code is there for people who _do_ need them? 
>
>
> Yes, it's just the single pointer chase.  Probably that's the common case
> now, because everyone will be enabling hugepages on lots of architectures.
>
> But still, the non-compound code is well tested too, and leaving it in
> place does make a little microoptimisation available to those who want it,
> so I don't see a reason yet to make compound pages compulsory.
>
> So I'd suggest that we make compound pages conditional on a new
> CONFIG_COMPOUND_PAGE and make that equal to HUGETLB_PAGE || !MMU.

Good idea. BTW, any reason why the following (very)micro optimisation
shouldn't go in?

It currently only picks up a couple of things under fs/, but might help
reduce other ifdefery around the place. For example mm.h: page_count and
get_page.



---

 linux-2.6-npiggin/include/linux/page-flags.h |    4 ++++
 1 files changed, 4 insertions(+)

diff -puN include/linux/page-flags.h~mm-page_compound-microopt include/linux/page-flags.h
--- linux-2.6/include/linux/page-flags.h~mm-page_compound-microopt	2004-11-17 14:02:44.000000000 +1100
+++ linux-2.6-npiggin/include/linux/page-flags.h	2004-11-17 14:09:07.000000000 +1100
@@ -286,7 +286,11 @@ extern unsigned long __read_page_state(u
 #define ClearPageReclaim(page)	clear_bit(PG_reclaim, &(page)->flags)
 #define TestClearPageReclaim(page) test_and_clear_bit(PG_reclaim, &(page)->flags)
 
+#ifdef CONFIG_HUGETLB_PAGE
 #define PageCompound(page)	test_bit(PG_compound, &(page)->flags)
+#else
+#define PageCompound(page)	0
+#endif
 #define SetPageCompound(page)	set_bit(PG_compound, &(page)->flags)
 #define ClearPageCompound(page)	clear_bit(PG_compound, &(page)->flags)
 

_