From 79046ae07ae21245520ca0aab985ee6678a879f8 Mon Sep 17 00:00:00 2001 From: Andy Whitcroft Date: Wed, 1 Feb 2006 03:05:26 -0800 Subject: [PATCH] GFP_ZONETYPES: add commentry on how to calculate GFP_ZONETYPES define using GFP_ZONEMASK and add commentry Add commentry explaining the optimisation that we can apply to GFP_ZONETYPES when the leftmost bit is a 'loaner', it can only be set in isolation. Signed-off-by: Andy Whitcroft Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- include/linux/mmzone.h | 8 ++++++++ 1 file changed, 8 insertions(+) (limited to 'include') diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h index 93a849f742db..88c30f844abf 100644 --- a/include/linux/mmzone.h +++ b/include/linux/mmzone.h @@ -91,6 +91,14 @@ struct per_cpu_pageset { * be 8 (2 ** 3) zonelists. GFP_ZONETYPES defines the number of possible * combinations of zone modifiers in "zone modifier space". * + * As an optimisation any zone modifier bits which are only valid when + * no other zone modifier bits are set (loners) should be placed in + * the highest order bits of this field. This allows us to reduce the + * extent of the zonelists thus saving space. For example in the case + * of three zone modifier bits, we could require up to eight zonelists. + * If the left most zone modifier is a "loner" then the highest valid + * zonelist would be four allowing us to allocate only five zonelists. + * * NOTE! Make sure this matches the zones in */ #define GFP_ZONEMASK 0x07 -- cgit v1.2.3