linux-kernel - Re: [PATCH 04/20] Convert gfp_zone() to use a table of precalculated values

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Message-ID: <alpine.DEB.1.10.0902231003090.7298@qirst.com>
Date:	Mon, 23 Feb 2009 10:23:52 -0500 (EST)
From:	Christoph Lameter <cl@...ux-foundation.org>
To:	Mel Gorman <mel@....ul.ie>
cc:	Linux Memory Management List <linux-mm@...ck.org>,
	Pekka Enberg <penberg@...helsinki.fi>,
	Rik van Riel <riel@...hat.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Nick Piggin <npiggin@...e.de>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Lin Ming <ming.m.lin@...el.com>,
	Zhang Yanmin <yanmin_zhang@...ux.intel.com>
Subject: Re: [PATCH 04/20] Convert gfp_zone() to use a table of precalculated
 values

On Sun, 22 Feb 2009, Mel Gorman wrote:

> Every page allocation uses gfp_zone() to calcuate what the highest zone
> allowed by a combination of GFP flags is. This is a large number of branches
> to have in a fast path. This patch replaces the branches with a lookup
> table that is calculated at boot-time and stored in the read-mostly section
> so it can be shared. This requires __GFP_MOVABLE to be redefined but it's
> debatable as to whether it should be considered a zone modifier or not.

Are you sure that this is a benefit? Jumps are forward and pretty short
and the compiler is optimizing a branch away in the current code.


0xffffffff8027bde8 <try_to_free_pages+95>:      mov    %esi,-0x58(%rbp)
0xffffffff8027bdeb <try_to_free_pages+98>:      movq   $0xffffffff80278cd0,-0x48(%rbp)
0xffffffff8027bdf3 <try_to_free_pages+106>:     test   $0x1,%r8b
0xffffffff8027bdf7 <try_to_free_pages+110>:     mov    0x620(%rax),%rax
0xffffffff8027bdfe <try_to_free_pages+117>:     mov    %rax,-0x88(%rbp)
0xffffffff8027be05 <try_to_free_pages+124>:     jne    0xffffffff8027be2c <try_to_free_pages+163>
0xffffffff8027be07 <try_to_free_pages+126>:     mov    $0x1,%r14d
0xffffffff8027be0d <try_to_free_pages+132>:     test   $0x4,%r8b
0xffffffff8027be11 <try_to_free_pages+136>:     jne    0xffffffff8027be2c <try_to_free_pages+163>
0xffffffff8027be13 <try_to_free_pages+138>:     xor    %r14d,%r14d
0xffffffff8027be16 <try_to_free_pages+141>:     and    $0x100002,%r8d
0xffffffff8027be1d <try_to_free_pages+148>:     cmp    $0x100002,%r8d
0xffffffff8027be24 <try_to_free_pages+155>:     sete   %r14b
0xffffffff8027be28 <try_to_free_pages+159>:     add    $0x2,%r14d
0xffffffff8027be2c <try_to_free_pages+163>:     mov    %gs:0x8,%rdx

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