[PATCH 07/21] fat: improve fat_hash()

OGAWA Hirofumi
2008-10-15T14:12:42Z

fat_hash() is using the algorithm known as bad. Instead of it, this
uses hash_32(). The following is the summary of test.

old hash:
hash func (1000 times): 33489 cycles
total inodes in hash table: 70926
largest bucket contains: 696
smallest bucket contains: 54

new hash:
hash func (1000 times): 33129 cycles
total inodes in hash table: 70926
largest bucket contains: 315
smallest bucket contains: 236

Signed-off-by: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
---

fs/fat/fat.h | 1 -
fs/fat/inode.c | 18 +++++++-----------
2 files changed, 7 insertions(+), 12 deletions(-)

diff -puN fs/fat/fat.h~fat_improve-hash fs/fat/fat.h
--- linux-2.6/fs/fat/fat.h~fat_improve-hash 2008-08-25 03:33:31.000000000 +0900
+++ linux-2.6-hirofumi/fs/fat/fat.h 2008-08-25 03:50:16.000000000 +0900
@@ -43,7 +43,6 @@ struct fat_mount_options {

#define FAT_HASH_BITS 8
#define FAT_HASH_SIZE (1UL << FAT_HASH_BITS)
-#define FAT_HASH_MASK (FAT_HASH_SIZE-1)

/*
* MS-DOS file system in-core superblock data
diff -puN fs/fat/inode.c~fat_improve-hash fs/fat/inode.c
--- linux-2.6/fs/fat/inode.c~fat_improve-hash 2008-08-25 03:33:31.000000000 +0900
+++ linux-2.6-hirofumi/fs/fat/inode.c 2008-08-25 03:50:16.000000000 +0900
@@ -26,6 +26,7 @@
#include <linux/uio.h>
#include <linux/writeback.h>
#include <linux/log2.h>
+#include <linux/hash.h>
#include <asm/unaligned.h>
#include "fat.h"

@@ -247,25 +248,21 @@ static void fat_hash_init(struct super_b
INIT_HLIST_HEAD(&sbi->inode_hashtable[i]);
}

-static inline unsigned long fat_hash(struct super_block *sb, loff_t i_pos)
+static inline unsigned long fat_hash(loff_t i_pos)
{
- unsigned long tmp = (unsigned long)i_pos | (unsigned long) sb;
- tmp = tmp + (tmp >> FAT_HASH_BITS) + (tmp >> FAT_HASH_BITS * 2);
- return tmp & FAT_HASH_MASK;
+ return hash_32(i_pos, FAT_HASH_BITS);
}

void fat_attach(struct inode *inode, loff_t i_pos)
{
- struct super_block *sb = inode->i_sb;
- struct msdos_sb_info *sbi = MSDOS_SB(sb);
+ struct msdos_sb_info *sbi = MSDOS_SB(inode->i_sb);
+ struct hlist_head *head = sbi->inode_hashtable + fat_hash(i_pos);

spin_lock(&sbi->inode_hash_lock);
MSDOS_I(inode)->i_pos = i_pos;
- hlist_add_head(&MSDOS_I(inode)->i_fat_hash,
- sbi->inode_hashtable + fat_hash(sb, i_pos));
+ hlist_add_head(&MSDOS_I(inode)->i_fat_hash, head);
spin_unlock(&sbi->inode_hash_lock);
}
-
EXPORT_SYMBOL_GPL(fat_attach);

void fat_detach(struct inode *inode)
@@ -276,13 +273,12 @@ void fat_detach(struct inode *inode)
hlist_del_init(&MSDOS_I(inode)->i_fat_hash);
spin_unlock(&sbi->inode_hash_lock);
}
-
EXPORT_SYMBOL_GPL(fat_detach);

struct inode *fat_iget(struct super_block *sb, loff_t i_pos)
{
struct msdos_sb_info *sbi = MSDOS_SB(sb);
- struct hlist_head *head = sbi->inode_hashtable + fat_hash(sb, i_pos);
+ struct hlist_head *head = sbi->inode_hashtable + fat_hash(i_pos);
struct hlist_node *_p;
struct msdos_inode_info *i;
struct inode *inode = NULL;
_