linux-kernel - Re: [PATCH] x86_64: Make the NUMA hash function nodemap allocation dynamic and remove NODEMAPSIZE

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Message-Id: <200611101043.14749.dada1@cosmosbay.com>
Date:	Fri, 10 Nov 2006 10:43:14 +0100
From:	Eric Dumazet <dada1@...mosbay.com>
To:	Andi Kleen <ak@...e.de>
Cc:	Amul Shah <amul.shah@...sys.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86_64: Make the NUMA hash function nodemap allocation dynamic and remove NODEMAPSIZE

On Friday 10 November 2006 07:48, Andi Kleen wrote:

> Have you checked how much the code .text size changes because
> of the pointer reference? If it's a lot phys_to_nid might need to
> be out of lined.

Here I have also big numbers on pfn_to_page(), on a machine with mapsize=1  
(NUMA kernel, but one node)

oprofile results L1_AND_L2_DTLB_MISSES /usr/src/linux-2.6.18/vmlinux

Counted L1_AND_L2_DTLB_MISSES events (L1 and L2 DTLB misses) with a unit mask 
of 0x00 (No unit mask) count 10000 

ffffffff80258fa0 <pfn_to_page>: /* pfn_to_page total:  48433  0.4914 */

So adding yet another indirection (to a another cache line) might hurt.

Therefore I suggest to use a structure like that :

struct memnode {
 	int shift;
	unsigned int mapsize; /* no need to use 8 bytes here */
	u8 *map;
	u8 embedded_map[64-8]; /* total size = 64 bytes */
 } ____cacheline_aligned;

and make memnode.map point to memnode.embedded_map if mapsize <= 56 ?

This way, most AMD64 dual/quad processors wont waste a full PAGE to store few 
bytes in it, and should use only one cache line.

Thank you

Eric
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