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Subject[PATCH] fs : reorder some 'struct inode' fields to speedup i_size manipulations
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On 32bits SMP platforms, 64bits i_size is protected by a seqcount 
(i_size_seqcount).

When i_size is read or written, i_size_seqcount is read/written as well, so it
make sense to group these two fields together in the same cache line.

Before this patch, accessing i_size needed 3 cache lines (2 for i_size, one
for i_size_seqcount). After, only one cache line is needed/ (dirtied on a
i_size change).

This patch moves i_size_seqcount next to i_size, and also moves i_version to
let offsetof(struct inode, i_size) being 0x40 instead of 0x3c (for 32bits
platforms).

For 64 bits platforms, i_size_seqcount doesnt exist, and the move of a 'long
i_version' should not introduce a new hole because of padding.

Signed-off-by: Eric Dumazet <dada1@cosmosbay.com>
--- linux-2.6.19-rc6/include/linux/fs.h 2006-11-23 10:47:53.000000000 +0100
+++ linux-2.6.19-rc6-ed/include/linux/fs.h 2006-11-23 11:13:36.000000000 +0100
@@ -548,12 +548,15 @@ struct inode {
uid_t i_uid;
gid_t i_gid;
dev_t i_rdev;
+ unsigned long i_version;
loff_t i_size;
+#ifdef __NEED_I_SIZE_ORDERED
+ seqcount_t i_size_seqcount;
+#endif
struct timespec i_atime;
struct timespec i_mtime;
struct timespec i_ctime;
unsigned int i_blkbits;
- unsigned long i_version;
blkcnt_t i_blocks;
unsigned short i_bytes;
spinlock_t i_lock; /* i_blocks, i_bytes, maybe i_size */
@@ -598,9 +601,6 @@ struct inode {
void *i_security;
#endif
void *i_private; /* fs or device private pointer */
-#ifdef __NEED_I_SIZE_ORDERED
- seqcount_t i_size_seqcount;
-#endif
};

/*
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